Adding upstream version 115.7.0esr.upstream/115.7.0esr

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

44 files changed, 6819 insertions, 0 deletions

diff --git a/third_party/rust/rkv/src/backend.rs b/third_party/rust/rkv/src/backend.rs
new file mode 100644
index 0000000000..63fa08a40c
--- /dev/null
+++ b/third_party/rust/rkv/src/backend.rs
@@ -0,0 +1,40 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+mod common;
+#[cfg(feature = "lmdb")]
+mod impl_lmdb;
+mod impl_safe;
+mod traits;
+
+pub use common::*;
+pub use traits::*;
+
+#[cfg(feature = "lmdb")]
+pub use impl_lmdb::{
+    ArchMigrateError as LmdbArchMigrateError, ArchMigrateResult as LmdbArchMigrateResult,
+    ArchMigrator as LmdbArchMigrator, DatabaseFlagsImpl as LmdbDatabaseFlags,
+    DatabaseImpl as LmdbDatabase, EnvironmentBuilderImpl as Lmdb,
+    EnvironmentFlagsImpl as LmdbEnvironmentFlags, EnvironmentImpl as LmdbEnvironment,
+    ErrorImpl as LmdbError, InfoImpl as LmdbInfo, IterImpl as LmdbIter,
+    RoCursorImpl as LmdbRoCursor, RoTransactionImpl as LmdbRoTransaction,
+    RwCursorImpl as LmdbRwCursor, RwTransactionImpl as LmdbRwTransaction, StatImpl as LmdbStat,
+    WriteFlagsImpl as LmdbWriteFlags,
+};
+
+pub use impl_safe::{
+    DatabaseFlagsImpl as SafeModeDatabaseFlags, DatabaseImpl as SafeModeDatabase,
+    EnvironmentBuilderImpl as SafeMode, EnvironmentFlagsImpl as SafeModeEnvironmentFlags,
+    EnvironmentImpl as SafeModeEnvironment, ErrorImpl as SafeModeError, InfoImpl as SafeModeInfo,
+    IterImpl as SafeModeIter, RoCursorImpl as SafeModeRoCursor,
+    RoTransactionImpl as SafeModeRoTransaction, RwCursorImpl as SafeModeRwCursor,
+    RwTransactionImpl as SafeModeRwTransaction, StatImpl as SafeModeStat,
+    WriteFlagsImpl as SafeModeWriteFlags,
+};
diff --git a/third_party/rust/rkv/src/backend/common.rs b/third_party/rust/rkv/src/backend/common.rs
new file mode 100644
index 0000000000..bea3839d03
--- /dev/null
+++ b/third_party/rust/rkv/src/backend/common.rs
@@ -0,0 +1,44 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+#![allow(non_camel_case_types)]
+
+pub enum EnvironmentFlags {
+    FIXED_MAP,
+    NO_SUB_DIR,
+    WRITE_MAP,
+    READ_ONLY,
+    NO_META_SYNC,
+    NO_SYNC,
+    MAP_ASYNC,
+    NO_TLS,
+    NO_LOCK,
+    NO_READAHEAD,
+    NO_MEM_INIT,
+}
+
+pub enum DatabaseFlags {
+    REVERSE_KEY,
+    #[cfg(feature = "db-dup-sort")]
+    DUP_SORT,
+    #[cfg(feature = "db-dup-sort")]
+    DUP_FIXED,
+    #[cfg(feature = "db-int-key")]
+    INTEGER_KEY,
+    INTEGER_DUP,
+    REVERSE_DUP,
+}
+
+pub enum WriteFlags {
+    NO_OVERWRITE,
+    NO_DUP_DATA,
+    CURRENT,
+    APPEND,
+    APPEND_DUP,
+}
diff --git a/third_party/rust/rkv/src/backend/impl_lmdb.rs b/third_party/rust/rkv/src/backend/impl_lmdb.rs
new file mode 100644
index 0000000000..5364214598
--- /dev/null
+++ b/third_party/rust/rkv/src/backend/impl_lmdb.rs
@@ -0,0 +1,34 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+mod arch_migrator;
+mod arch_migrator_error;
+mod cursor;
+mod database;
+mod environment;
+mod error;
+mod flags;
+mod info;
+mod iter;
+mod stat;
+mod transaction;
+
+pub use arch_migrator::{
+    MigrateError as ArchMigrateError, MigrateResult as ArchMigrateResult, Migrator as ArchMigrator,
+};
+pub use cursor::{RoCursorImpl, RwCursorImpl};
+pub use database::DatabaseImpl;
+pub use environment::{EnvironmentBuilderImpl, EnvironmentImpl};
+pub use error::ErrorImpl;
+pub use flags::{DatabaseFlagsImpl, EnvironmentFlagsImpl, WriteFlagsImpl};
+pub use info::InfoImpl;
+pub use iter::IterImpl;
+pub use stat::StatImpl;
+pub use transaction::{RoTransactionImpl, RwTransactionImpl};
diff --git a/third_party/rust/rkv/src/backend/impl_lmdb/arch_migrator.rs b/third_party/rust/rkv/src/backend/impl_lmdb/arch_migrator.rs
new file mode 100644
index 0000000000..6a91db1a3b
--- /dev/null
+++ b/third_party/rust/rkv/src/backend/impl_lmdb/arch_migrator.rs
@@ -0,0 +1,998 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+#![allow(dead_code)] // TODO: Get rid of unused struct members
+#![allow(clippy::upper_case_acronyms)] // TODO: Consider renaming things like `BRANCH`
+
+//! A utility for migrating data from one LMDB environment to another. Notably, this tool
+//! can migrate data from an enviroment created with a different bit-depth than the
+//! current rkv consumer, which enables the consumer to retrieve data from an environment
+//! that can't be read directly using the rkv APIs.
+//!
+//! The utility supports both 32-bit and 64-bit LMDB source environments, and it
+//! automatically migrates data in both the default database and any named (sub)
+//! databases.  It also migrates the source environment's "map size" and "max DBs"
+//! configuration options to the destination environment.
+//!
+//! The destination environment must be at the rkv consumer's bit depth and should be
+//! empty of data.  It can be an empty directory, in which case the utility will create a
+//! new LMDB environment within the directory.
+//!
+//! The tool currently has these limitations:
+//!
+//! 1. It doesn't support migration from environments created with
+//!     `EnvironmentFlags::NO_SUB_DIR`.  To migrate such an environment, create a
+//!     temporary directory, copy the environment's data file to a file called data.mdb in
+//!     the temporary directory, then migrate the temporary directory as the source
+//!     environment.
+//! 2. It doesn't support migration from databases created with DatabaseFlags::DUP_SORT`
+//!     (with or without `DatabaseFlags::DUP_FIXED`).
+//! 3. It doesn't account for existing data in the destination environment, which means
+//!     that it can overwrite data (causing data loss) or fail to migrate data if the
+//!     destination environment contains existing data.
+//!
+//! ## Basic Usage
+//!
+//! Call `Migrator::new()` with the path to the source environment to create a `Migrator`
+//! instance; then call the instance's `migrate()` method with the path to the destination
+//! environment to migrate data from the source to the destination environment.  For
+//! example, this snippet migrates data from the tests/envs/ref_env_32 environment to a
+//! new environment in a temporary directory:
+//!
+//! ```
+//! use rkv::migrator::LmdbArchMigrator as Migrator;
+//! use std::path::Path;
+//! use tempfile::tempdir;
+//! let mut migrator = Migrator::new(Path::new("tests/envs/ref_env_32")).unwrap();
+//! migrator.migrate(&tempdir().unwrap().path()).unwrap();
+//! ```
+//!
+//! Both `Migrator::new()` and `migrate()` return a `MigrateResult` that is either an
+//! `Ok()` result or an `Err<MigrateError>`, where `MigrateError` is an enum whose
+//! variants identify specific kinds of migration failures.
+
+use std::{
+    collections::{BTreeMap, HashMap},
+    convert::TryFrom,
+    fs::File,
+    io::{Cursor, Read, Seek, SeekFrom, Write},
+    path::{Path, PathBuf},
+    rc::Rc,
+    str,
+};
+
+use bitflags::bitflags;
+use byteorder::{LittleEndian, ReadBytesExt};
+use lmdb::{DatabaseFlags, Environment, Transaction, WriteFlags};
+
+pub use super::arch_migrator_error::MigrateError;
+
+const PAGESIZE: u16 = 4096;
+
+// The magic number is 0xBEEFC0DE, which is 0xDEC0EFBE in little-endian. It appears at
+// offset 12 on 32-bit systems and 16 on 64-bit systems. We don't support big-endian
+// migration, but presumably we could do so by detecting the order of the bytes.
+const MAGIC: [u8; 4] = [0xDE, 0xC0, 0xEF, 0xBE];
+
+pub type MigrateResult<T> = Result<T, MigrateError>;
+
+bitflags! {
+    #[derive(Default)]
+    struct PageFlags: u16 {
+        const BRANCH = 0x01;
+        const LEAF = 0x02;
+        const OVERFLOW = 0x04;
+        const META = 0x08;
+        const DIRTY = 0x10;
+        const LEAF2 = 0x20;
+        const SUBP = 0x40;
+        const LOOSE = 0x4000;
+        const KEEP = 0x8000;
+    }
+}
+
+bitflags! {
+    #[derive(Default)]
+    struct NodeFlags: u16 {
+        const BIGDATA = 0x01;
+        const SUBDATA = 0x02;
+        const DUPDATA = 0x04;
+    }
+}
+
+// The bit depth of the executable that created an LMDB environment. The Migrator
+// determines this automatically based on the location of the magic number in data.mdb.
+#[derive(Clone, Copy, PartialEq)]
+enum Bits {
+    U32,
+    U64,
+}
+
+impl Bits {
+    // The size of usize for the bit-depth represented by the enum variant.
+    fn size(self) -> usize {
+        match self {
+            Bits::U32 => 4,
+            Bits::U64 => 8,
+        }
+    }
+}
+
+// The equivalent of PAGEHDRSZ in LMDB, except that this one varies by bits.
+fn page_header_size(bits: Bits) -> u64 {
+    match bits {
+        Bits::U32 => 12,
+        Bits::U64 => 16,
+    }
+}
+
+// The equivalent of P_INVALID in LMDB, except that this one varies by bits.
+fn validate_page_num(page_num: u64, bits: Bits) -> MigrateResult<()> {
+    let invalid_page_num = match bits {
+        Bits::U32 => u64::from(!0u32),
+        Bits::U64 => !0u64,
+    };
+
+    if page_num == invalid_page_num {
+        return Err(MigrateError::InvalidPageNum);
+    }
+
+    Ok(())
+}
+
+#[derive(Clone, Debug, Default)]
+struct Database {
+    md_pad: u32,
+    md_flags: DatabaseFlags,
+    md_depth: u16,
+    md_branch_pages: u64,
+    md_leaf_pages: u64,
+    md_overflow_pages: u64,
+    md_entries: u64,
+    md_root: u64,
+}
+
+impl Database {
+    fn new(cursor: &mut Cursor<&[u8]>, bits: Bits) -> MigrateResult<Database> {
+        Ok(Database {
+            md_pad: cursor.read_u32::<LittleEndian>()?,
+            md_flags: DatabaseFlags::from_bits(cursor.read_u16::<LittleEndian>()?.into())
+                .ok_or(MigrateError::InvalidDatabaseBits)?,
+            md_depth: cursor.read_u16::<LittleEndian>()?,
+            md_branch_pages: cursor.read_uint::<LittleEndian>(bits.size())?,
+            md_leaf_pages: cursor.read_uint::<LittleEndian>(bits.size())?,
+            md_overflow_pages: cursor.read_uint::<LittleEndian>(bits.size())?,
+            md_entries: cursor.read_uint::<LittleEndian>(bits.size())?,
+            md_root: cursor.read_uint::<LittleEndian>(bits.size())?,
+        })
+    }
+}
+
+#[derive(Debug, Default)]
+struct Databases {
+    free: Database,
+    main: Database,
+}
+
+#[derive(Debug, Default)]
+struct MetaData {
+    mm_magic: u32,
+    mm_version: u32,
+    mm_address: u64,
+    mm_mapsize: u64,
+    mm_dbs: Databases,
+    mm_last_pg: u64,
+    mm_txnid: u64,
+}
+
+#[derive(Debug)]
+enum LeafNode {
+    Regular {
+        mn_lo: u16,
+        mn_hi: u16,
+        mn_flags: NodeFlags,
+        mn_ksize: u16,
+        mv_size: u32,
+        key: Vec<u8>,
+        value: Vec<u8>,
+    },
+    BigData {
+        mn_lo: u16,
+        mn_hi: u16,
+        mn_flags: NodeFlags,
+        mn_ksize: u16,
+        mv_size: u32,
+        key: Vec<u8>,
+        overflow_pgno: u64,
+    },
+    SubData {
+        mn_lo: u16,
+        mn_hi: u16,
+        mn_flags: NodeFlags,
+        mn_ksize: u16,
+        mv_size: u32,
+        key: Vec<u8>,
+        value: Vec<u8>,
+        db: Database,
+    },
+}
+
+#[derive(Debug, Default)]
+struct BranchNode {
+    mp_pgno: u64,
+    mn_ksize: u16,
+    mn_data: Vec<u8>,
+}
+
+#[derive(Debug)]
+enum PageHeader {
+    Regular {
+        mp_pgno: u64,
+        mp_flags: PageFlags,
+        pb_lower: u16,
+        pb_upper: u16,
+    },
+    Overflow {
+        mp_pgno: u64,
+        mp_flags: PageFlags,
+        pb_pages: u32,
+    },
+}
+
+#[derive(Debug)]
+enum Page {
+    META(MetaData),
+    LEAF(Vec<LeafNode>),
+    BRANCH(Vec<BranchNode>),
+}
+
+impl Page {
+    fn new(buf: Vec<u8>, bits: Bits) -> MigrateResult<Page> {
+        let mut cursor = std::io::Cursor::new(&buf[..]);
+
+        match Self::parse_page_header(&mut cursor, bits)? {
+            PageHeader::Regular {
+                mp_flags, pb_lower, ..
+            } => {
+                if mp_flags.contains(PageFlags::LEAF2) || mp_flags.contains(PageFlags::SUBP) {
+                    // We don't yet support DUPFIXED and DUPSORT databases.
+                    return Err(MigrateError::UnsupportedPageHeaderVariant);
+                }
+
+                if mp_flags.contains(PageFlags::META) {
+                    let meta_data = Self::parse_meta_data(&mut cursor, bits)?;
+                    Ok(Page::META(meta_data))
+                } else if mp_flags.contains(PageFlags::LEAF) {
+                    let nodes = Self::parse_leaf_nodes(&mut cursor, pb_lower, bits)?;
+                    Ok(Page::LEAF(nodes))
+                } else if mp_flags.contains(PageFlags::BRANCH) {
+                    let nodes = Self::parse_branch_nodes(&mut cursor, pb_lower, bits)?;
+                    Ok(Page::BRANCH(nodes))
+                } else {
+                    Err(MigrateError::UnexpectedPageHeaderVariant)
+                }
+            }
+            PageHeader::Overflow { .. } => {
+                // There isn't anything to do, nor should we try to instantiate
+                // a page of this type, as we only access them when reading
+                // a value that is too large to fit into a leaf node.
+                Err(MigrateError::UnexpectedPageHeaderVariant)
+            }
+        }
+    }
+
+    fn parse_page_header(cursor: &mut Cursor<&[u8]>, bits: Bits) -> MigrateResult<PageHeader> {
+        let mp_pgno = cursor.read_uint::<LittleEndian>(bits.size())?;
+        let _mp_pad = cursor.read_u16::<LittleEndian>()?;
+        let mp_flags = PageFlags::from_bits(cursor.read_u16::<LittleEndian>()?)
+            .ok_or(MigrateError::InvalidPageBits)?;
+
+        if mp_flags.contains(PageFlags::OVERFLOW) {
+            let pb_pages = cursor.read_u32::<LittleEndian>()?;
+            Ok(PageHeader::Overflow {
+                mp_pgno,
+                mp_flags,
+                pb_pages,
+            })
+        } else {
+            let pb_lower = cursor.read_u16::<LittleEndian>()?;
+            let pb_upper = cursor.read_u16::<LittleEndian>()?;
+            Ok(PageHeader::Regular {
+                mp_pgno,
+                mp_flags,
+                pb_lower,
+                pb_upper,
+            })
+        }
+    }
+
+    fn parse_meta_data(cursor: &mut Cursor<&[u8]>, bits: Bits) -> MigrateResult<MetaData> {
+        cursor.seek(SeekFrom::Start(page_header_size(bits)))?;
+
+        Ok(MetaData {
+            mm_magic: cursor.read_u32::<LittleEndian>()?,
+            mm_version: cursor.read_u32::<LittleEndian>()?,
+            mm_address: cursor.read_uint::<LittleEndian>(bits.size())?,
+            mm_mapsize: cursor.read_uint::<LittleEndian>(bits.size())?,
+            mm_dbs: Databases {
+                free: Database::new(cursor, bits)?,
+                main: Database::new(cursor, bits)?,
+            },
+            mm_last_pg: cursor.read_uint::<LittleEndian>(bits.size())?,
+            mm_txnid: cursor.read_uint::<LittleEndian>(bits.size())?,
+        })
+    }
+
+    fn parse_leaf_nodes(
+        cursor: &mut Cursor<&[u8]>,
+        pb_lower: u16,
+        bits: Bits,
+    ) -> MigrateResult<Vec<LeafNode>> {
+        cursor.set_position(page_header_size(bits));
+        let num_keys = Self::num_keys(pb_lower, bits);
+        let mp_ptrs = Self::parse_mp_ptrs(cursor, num_keys)?;
+
+        let mut leaf_nodes = Vec::with_capacity(num_keys as usize);
+
+        for mp_ptr in mp_ptrs {
+            cursor.set_position(u64::from(mp_ptr));
+            leaf_nodes.push(Self::parse_leaf_node(cursor, bits)?);
+        }
+
+        Ok(leaf_nodes)
+    }
+
+    fn parse_leaf_node(cursor: &mut Cursor<&[u8]>, bits: Bits) -> MigrateResult<LeafNode> {
+        // The order of the mn_lo and mn_hi fields is endian-dependent and would be
+        // reversed in an LMDB environment created on a big-endian system.
+        let mn_lo = cursor.read_u16::<LittleEndian>()?;
+        let mn_hi = cursor.read_u16::<LittleEndian>()?;
+
+        let mn_flags = NodeFlags::from_bits(cursor.read_u16::<LittleEndian>()?)
+            .ok_or(MigrateError::InvalidNodeBits)?;
+        let mn_ksize = cursor.read_u16::<LittleEndian>()?;
+
+        let start = usize::try_from(cursor.position())?;
+        let end = usize::try_from(cursor.position() + u64::from(mn_ksize))?;
+        let key = cursor.get_ref()[start..end].to_vec();
+        cursor.set_position(end as u64);
+
+        let mv_size = Self::leaf_node_size(mn_lo, mn_hi);
+        if mn_flags.contains(NodeFlags::BIGDATA) {
+            let overflow_pgno = cursor.read_uint::<LittleEndian>(bits.size())?;
+            Ok(LeafNode::BigData {
+                mn_lo,
+                mn_hi,
+                mn_flags,
+                mn_ksize,
+                mv_size,
+                key,
+                overflow_pgno,
+            })
+        } else if mn_flags.contains(NodeFlags::SUBDATA) {
+            let start = usize::try_from(cursor.position())?;
+            let end = usize::try_from(cursor.position() + u64::from(mv_size))?;
+            let value = cursor.get_ref()[start..end].to_vec();
+            let mut cursor = std::io::Cursor::new(&value[..]);
+            let db = Database::new(&mut cursor, bits)?;
+            validate_page_num(db.md_root, bits)?;
+            Ok(LeafNode::SubData {
+                mn_lo,
+                mn_hi,
+                mn_flags,
+                mn_ksize,
+                mv_size,
+                key,
+                value,
+                db,
+            })
+        } else {
+            let start = usize::try_from(cursor.position())?;
+            let end = usize::try_from(cursor.position() + u64::from(mv_size))?;
+            let value = cursor.get_ref()[start..end].to_vec();
+            Ok(LeafNode::Regular {
+                mn_lo,
+                mn_hi,
+                mn_flags,
+                mn_ksize,
+                mv_size,
+                key,
+                value,
+            })
+        }
+    }
+
+    fn leaf_node_size(mn_lo: u16, mn_hi: u16) -> u32 {
+        u32::from(mn_lo) + ((u32::from(mn_hi)) << 16)
+    }
+
+    fn parse_branch_nodes(
+        cursor: &mut Cursor<&[u8]>,
+        pb_lower: u16,
+        bits: Bits,
+    ) -> MigrateResult<Vec<BranchNode>> {
+        let num_keys = Self::num_keys(pb_lower, bits);
+        let mp_ptrs = Self::parse_mp_ptrs(cursor, num_keys)?;
+
+        let mut branch_nodes = Vec::with_capacity(num_keys as usize);
+
+        for mp_ptr in mp_ptrs {
+            cursor.set_position(u64::from(mp_ptr));
+            branch_nodes.push(Self::parse_branch_node(cursor, bits)?)
+        }
+
+        Ok(branch_nodes)
+    }
+
+    fn parse_branch_node(cursor: &mut Cursor<&[u8]>, bits: Bits) -> MigrateResult<BranchNode> {
+        // The order of the mn_lo and mn_hi fields is endian-dependent and would be
+        // reversed in an LMDB environment created on a big-endian system.
+        let mn_lo = cursor.read_u16::<LittleEndian>()?;
+        let mn_hi = cursor.read_u16::<LittleEndian>()?;
+
+        let mn_flags = cursor.read_u16::<LittleEndian>()?;
+
+        // Branch nodes overload the mn_lo, mn_hi, and mn_flags fields to store the page
+        // number, so we derive the number from those fields.
+        let mp_pgno = Self::branch_node_page_num(mn_lo, mn_hi, mn_flags, bits);
+
+        let mn_ksize = cursor.read_u16::<LittleEndian>()?;
+
+        let position = cursor.position();
+        let start = usize::try_from(position)?;
+        let end = usize::try_from(position + u64::from(mn_ksize))?;
+        let mn_data = cursor.get_ref()[start..end].to_vec();
+        cursor.set_position(end as u64);
+
+        Ok(BranchNode {
+            mp_pgno,
+            mn_ksize,
+            mn_data,
+        })
+    }
+
+    fn branch_node_page_num(mn_lo: u16, mn_hi: u16, mn_flags: u16, bits: Bits) -> u64 {
+        let mut page_num = u64::from(u32::from(mn_lo) + (u32::from(mn_hi) << 16));
+        if bits == Bits::U64 {
+            page_num += u64::from(mn_flags) << 32;
+        }
+        page_num
+    }
+
+    fn parse_mp_ptrs(cursor: &mut Cursor<&[u8]>, num_keys: u64) -> MigrateResult<Vec<u16>> {
+        let mut mp_ptrs = Vec::with_capacity(num_keys as usize);
+        for _ in 0..num_keys {
+            mp_ptrs.push(cursor.read_u16::<LittleEndian>()?);
+        }
+        Ok(mp_ptrs)
+    }
+
+    fn num_keys(pb_lower: u16, bits: Bits) -> u64 {
+        (u64::from(pb_lower) - page_header_size(bits)) >> 1
+    }
+}
+
+pub struct Migrator {
+    file: File,
+    bits: Bits,
+}
+
+impl Migrator {
+    /// Create a new Migrator for the LMDB environment at the given path. This tries to
+    /// open the data.mdb file in the environment and determine the bit depth of the
+    /// executable that created it, so it can fail and return an Err if the file can't be
+    /// opened or the depth determined.
+    pub fn new(path: &Path) -> MigrateResult<Migrator> {
+        let mut path = PathBuf::from(path);
+        path.push("data.mdb");
+        let mut file = File::open(&path)?;
+
+        file.seek(SeekFrom::Start(page_header_size(Bits::U32)))?;
+        let mut buf = [0; 4];
+        file.read_exact(&mut buf)?;
+
+        let bits = if buf == MAGIC {
+            Bits::U32
+        } else {
+            file.seek(SeekFrom::Start(page_header_size(Bits::U64)))?;
+            file.read_exact(&mut buf)?;
+            if buf == MAGIC {
+                Bits::U64
+            } else {
+                return Err(MigrateError::IndeterminateBitDepth);
+            }
+        };
+
+        Ok(Migrator { file, bits })
+    }
+
+    /// Dump the data in one of the databases in the LMDB environment. If the `database`
+    /// paremeter is None, then we dump the data in the main database.  If it's the name
+    /// of a subdatabase, then we dump the data in that subdatabase.
+    ///
+    /// Note that the output isn't identical to that of the `mdb_dump` utility, since
+    /// `mdb_dump` includes subdatabase key/value pairs when dumping the main database,
+    /// and those values are architecture-dependent, since they contain pointer-sized
+    /// data.
+    ///
+    /// If we wanted to support identical output, we could parameterize inclusion of
+    /// subdatabase pairs in get_pairs() and include them when dumping data, while
+    /// continuing to exclude them when migrating data.
+    pub fn dump<T: Write>(&mut self, database: Option<&str>, mut out: T) -> MigrateResult<()> {
+        let meta_data = self.get_meta_data()?;
+        let root_page_num = meta_data.mm_dbs.main.md_root;
+        let root_page = Rc::new(self.get_page(root_page_num)?);
+
+        let pairs;
+        if let Some(database) = database {
+            let subdbs = self.get_subdbs(root_page)?;
+            let database = subdbs
+                .get(database.as_bytes())
+                .ok_or_else(|| MigrateError::DatabaseNotFound(database.to_string()))?;
+            let root_page_num = database.md_root;
+            let root_page = Rc::new(self.get_page(root_page_num)?);
+            pairs = self.get_pairs(root_page)?;
+        } else {
+            pairs = self.get_pairs(root_page)?;
+        }
+
+        out.write_all(b"VERSION=3\n")?;
+        out.write_all(b"format=bytevalue\n")?;
+        if let Some(database) = database {
+            writeln!(out, "database={database}")?;
+        }
+        out.write_all(b"type=btree\n")?;
+        writeln!(out, "mapsize={}", meta_data.mm_mapsize)?;
+        out.write_all(b"maxreaders=126\n")?;
+        out.write_all(b"db_pagesize=4096\n")?;
+        out.write_all(b"HEADER=END\n")?;
+
+        for (key, value) in pairs {
+            out.write_all(b" ")?;
+            for byte in key {
+                write!(out, "{byte:02x}")?;
+            }
+            out.write_all(b"\n")?;
+            out.write_all(b" ")?;
+            for byte in value {
+                write!(out, "{byte:02x}")?;
+            }
+            out.write_all(b"\n")?;
+        }
+
+        out.write_all(b"DATA=END\n")?;
+
+        Ok(())
+    }
+
+    /// Migrate all data in all of databases in the existing LMDB environment to a new
+    /// environment.  This includes all key/value pairs in the main database that aren't
+    /// metadata about subdatabases and all key/value pairs in all subdatabases.
+    ///
+    /// We also set the map size and maximum databases of the new environment to their
+    /// values for the existing environment.  But we don't set other metadata, and we
+    /// don't check that the new environment is empty before migrating data.
+    ///
+    /// Thus it's possible for this to overwrite existing data or fail to migrate data if
+    /// the new environment isn't empty.  It's the consumer's responsibility to ensure
+    /// that data can be safely migrated to the new environment.  In general, this means
+    /// that environment should be empty.
+    pub fn migrate(&mut self, dest: &Path) -> MigrateResult<()> {
+        let meta_data = self.get_meta_data()?;
+        let root_page_num = meta_data.mm_dbs.main.md_root;
+        validate_page_num(root_page_num, self.bits)?;
+        let root_page = Rc::new(self.get_page(root_page_num)?);
+        let subdbs = self.get_subdbs(Rc::clone(&root_page))?;
+
+        let env = Environment::new()
+            .set_map_size(meta_data.mm_mapsize as usize)
+            .set_max_dbs(subdbs.len() as u32)
+            .open(dest)?;
+
+        // Create the databases before we open a read-write transaction, since database
+        // creation requires its own read-write transaction, which would hang while
+        // awaiting completion of an existing one.
+        env.create_db(None, meta_data.mm_dbs.main.md_flags)?;
+        for (subdb_name, subdb_info) in &subdbs {
+            env.create_db(Some(str::from_utf8(subdb_name)?), subdb_info.md_flags)?;
+        }
+
+        // Now open the read-write transaction that we'll use to migrate all the data.
+        let mut txn = env.begin_rw_txn()?;
+
+        // Migrate the main database.
+        let pairs = self.get_pairs(root_page)?;
+        let db = env.open_db(None)?;
+        for (key, value) in pairs {
+            // If we knew that the target database was empty, we could specify
+            // WriteFlags::APPEND to speed up the migration.
+            txn.put(db, &key, &value, WriteFlags::empty())?;
+        }
+
+        // Migrate subdatabases.
+        for (subdb_name, subdb_info) in &subdbs {
+            let root_page = Rc::new(self.get_page(subdb_info.md_root)?);
+            let pairs = self.get_pairs(root_page)?;
+            let db = env.open_db(Some(str::from_utf8(subdb_name)?))?;
+            for (key, value) in pairs {
+                // If we knew that the target database was empty, we could specify
+                // WriteFlags::APPEND to speed up the migration.
+                txn.put(db, &key, &value, WriteFlags::empty())?;
+            }
+        }
+
+        txn.commit()?;
+
+        Ok(())
+    }
+
+    fn get_subdbs(&mut self, root_page: Rc<Page>) -> MigrateResult<HashMap<Vec<u8>, Database>> {
+        let mut subdbs = HashMap::new();
+        let mut pages = vec![root_page];
+
+        while let Some(page) = pages.pop() {
+            match &*page {
+                Page::BRANCH(nodes) => {
+                    for branch in nodes {
+                        pages.push(Rc::new(self.get_page(branch.mp_pgno)?));
+                    }
+                }
+                Page::LEAF(nodes) => {
+                    for leaf in nodes {
+                        if let LeafNode::SubData { key, db, .. } = leaf {
+                            subdbs.insert(key.to_vec(), db.clone());
+                        };
+                    }
+                }
+                _ => {
+                    return Err(MigrateError::UnexpectedPageVariant);
+                }
+            }
+        }
+
+        Ok(subdbs)
+    }
+
+    fn get_pairs(&mut self, root_page: Rc<Page>) -> MigrateResult<BTreeMap<Vec<u8>, Vec<u8>>> {
+        let mut pairs = BTreeMap::new();
+        let mut pages = vec![root_page];
+
+        while let Some(page) = pages.pop() {
+            match &*page {
+                Page::BRANCH(nodes) => {
+                    for branch in nodes {
+                        pages.push(Rc::new(self.get_page(branch.mp_pgno)?));
+                    }
+                }
+                Page::LEAF(nodes) => {
+                    for leaf in nodes {
+                        match leaf {
+                            LeafNode::Regular { key, value, .. } => {
+                                pairs.insert(key.to_vec(), value.to_vec());
+                            }
+                            LeafNode::BigData {
+                                mv_size,
+                                key,
+                                overflow_pgno,
+                                ..
+                            } => {
+                                // Perhaps we could reduce memory consumption during a
+                                // migration by waiting to read big data until it's time
+                                // to write it to the new database.
+                                let value = self.read_data(
+                                    *overflow_pgno * u64::from(PAGESIZE)
+                                        + page_header_size(self.bits),
+                                    *mv_size as usize,
+                                )?;
+                                pairs.insert(key.to_vec(), value);
+                            }
+                            LeafNode::SubData { .. } => {
+                                // We don't include subdatabase leaves in pairs, since
+                                // there's no architecture-neutral representation of them,
+                                // and in any case they're meta-data that should get
+                                // recreated when we migrate the subdatabases themselves.
+                                //
+                                // If we wanted to create identical dumps to those
+                                // produced by `mdb_dump`, however, we could allow
+                                // consumers to specify that they'd like to include these
+                                // records.
+                            }
+                        };
+                    }
+                }
+                _ => {
+                    return Err(MigrateError::UnexpectedPageVariant);
+                }
+            }
+        }
+
+        Ok(pairs)
+    }
+
+    fn read_data(&mut self, offset: u64, size: usize) -> MigrateResult<Vec<u8>> {
+        self.file.seek(SeekFrom::Start(offset))?;
+        let mut buf: Vec<u8> = vec![0; size];
+        self.file.read_exact(&mut buf[0..size])?;
+        Ok(buf.to_vec())
+    }
+
+    fn get_page(&mut self, page_no: u64) -> MigrateResult<Page> {
+        Page::new(
+            self.read_data(page_no * u64::from(PAGESIZE), usize::from(PAGESIZE))?,
+            self.bits,
+        )
+    }
+
+    fn get_meta_data(&mut self) -> MigrateResult<MetaData> {
+        let (page0, page1) = (self.get_page(0)?, self.get_page(1)?);
+
+        match (page0, page1) {
+            (Page::META(meta0), Page::META(meta1)) => {
+                let meta = if meta1.mm_txnid > meta0.mm_txnid {
+                    meta1
+                } else {
+                    meta0
+                };
+                if meta.mm_magic != 0xBE_EF_C0_DE {
+                    return Err(MigrateError::InvalidMagicNum);
+                }
+                if meta.mm_version != 1 && meta.mm_version != 999 {
+                    return Err(MigrateError::InvalidDataVersion);
+                }
+                Ok(meta)
+            }
+            _ => Err(MigrateError::UnexpectedPageVariant),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use std::{env, fs, mem::size_of};
+
+    use lmdb::{Environment, Error as LmdbError};
+    use tempfile::{tempdir, tempfile};
+
+    fn compare_files(ref_file: &mut File, new_file: &mut File) -> MigrateResult<()> {
+        ref_file.seek(SeekFrom::Start(0))?;
+        new_file.seek(SeekFrom::Start(0))?;
+
+        let ref_buf = &mut [0; 1024];
+        let new_buf = &mut [0; 1024];
+
+        loop {
+            match ref_file.read(ref_buf) {
+                Err(err) => panic!("{}", err),
+                Ok(ref_len) => match new_file.read(new_buf) {
+                    Err(err) => panic!("{}", err),
+                    Ok(new_len) => {
+                        assert_eq!(ref_len, new_len);
+                        if ref_len == 0 {
+                            break;
+                        };
+                        assert_eq!(ref_buf[0..ref_len], new_buf[0..new_len]);
+                    }
+                },
+            }
+        }
+
+        Ok(())
+    }
+
+    #[test]
+    fn test_dump_32() -> MigrateResult<()> {
+        let cwd = env::current_dir()?;
+        let cwd = cwd.to_str().ok_or(MigrateError::StringConversionError)?;
+        let test_env_path: PathBuf = [cwd, "tests", "envs", "ref_env_32"].iter().collect();
+
+        // Dump data from the test env to a new dump file.
+        let mut migrator = Migrator::new(&test_env_path)?;
+        let mut new_dump_file = tempfile()?;
+        migrator.dump(None, &new_dump_file)?;
+
+        // Open the reference dump file.
+        let ref_dump_file_path: PathBuf = [cwd, "tests", "envs", "ref_dump.txt"].iter().collect();
+        let mut ref_dump_file = File::open(ref_dump_file_path)?;
+
+        // Compare the new dump file to the reference dump file.
+        compare_files(&mut ref_dump_file, &mut new_dump_file)?;
+
+        Ok(())
+    }
+
+    #[test]
+    fn test_dump_32_subdb() -> MigrateResult<()> {
+        let cwd = env::current_dir()?;
+        let cwd = cwd.to_str().ok_or(MigrateError::StringConversionError)?;
+        let test_env_path: PathBuf = [cwd, "tests", "envs", "ref_env_32"].iter().collect();
+
+        // Dump data from the test env to a new dump file.
+        let mut migrator = Migrator::new(&test_env_path)?;
+        let mut new_dump_file = tempfile()?;
+        migrator.dump(Some("subdb"), &new_dump_file)?;
+
+        // Open the reference dump file.
+        let ref_dump_file_path: PathBuf = [cwd, "tests", "envs", "ref_dump_subdb.txt"]
+            .iter()
+            .collect();
+        let mut ref_dump_file = File::open(ref_dump_file_path)?;
+
+        // Compare the new dump file to the reference dump file.
+        compare_files(&mut ref_dump_file, &mut new_dump_file)?;
+
+        Ok(())
+    }
+
+    #[test]
+    fn test_dump_64() -> MigrateResult<()> {
+        let cwd = env::current_dir()?;
+        let cwd = cwd.to_str().ok_or(MigrateError::StringConversionError)?;
+        let test_env_path: PathBuf = [cwd, "tests", "envs", "ref_env_64"].iter().collect();
+
+        // Dump data from the test env to a new dump file.
+        let mut migrator = Migrator::new(&test_env_path)?;
+        let mut new_dump_file = tempfile()?;
+        migrator.dump(None, &new_dump_file)?;
+
+        // Open the reference dump file.
+        let ref_dump_file_path: PathBuf = [cwd, "tests", "envs", "ref_dump.txt"].iter().collect();
+        let mut ref_dump_file = File::open(ref_dump_file_path)?;
+
+        // Compare the new dump file to the reference dump file.
+        compare_files(&mut ref_dump_file, &mut new_dump_file)?;
+
+        Ok(())
+    }
+
+    #[test]
+    fn test_dump_64_subdb() -> MigrateResult<()> {
+        let cwd = env::current_dir()?;
+        let cwd = cwd.to_str().ok_or(MigrateError::StringConversionError)?;
+        let test_env_path: PathBuf = [cwd, "tests", "envs", "ref_env_64"].iter().collect();
+
+        // Dump data from the test env to a new dump file.
+        let mut migrator = Migrator::new(&test_env_path)?;
+        let mut new_dump_file = tempfile()?;
+        migrator.dump(Some("subdb"), &new_dump_file)?;
+
+        // Open the reference dump file.
+        let ref_dump_file_path: PathBuf = [cwd, "tests", "envs", "ref_dump_subdb.txt"]
+            .iter()
+            .collect();
+        let mut ref_dump_file = File::open(ref_dump_file_path)?;
+
+        // Compare the new dump file to the reference dump file.
+        compare_files(&mut ref_dump_file, &mut new_dump_file)?;
+
+        Ok(())
+    }
+
+    #[test]
+    fn test_migrate_64() -> MigrateResult<()> {
+        let cwd = env::current_dir()?;
+        let cwd = cwd.to_str().ok_or(MigrateError::StringConversionError)?;
+        let test_env_path: PathBuf = [cwd, "tests", "envs", "ref_env_64"].iter().collect();
+
+        // Migrate data from the old env to a new one.
+        let new_env = tempdir()?;
+        let mut migrator = Migrator::new(&test_env_path)?;
+        migrator.migrate(new_env.path())?;
+
+        // Dump data from the new env to a new dump file.
+        let mut migrator = Migrator::new(new_env.path())?;
+        let mut new_dump_file = tempfile()?;
+        migrator.dump(Some("subdb"), &new_dump_file)?;
+
+        // Open the reference dump file.
+        let ref_dump_file_path: PathBuf = [cwd, "tests", "envs", "ref_dump_subdb.txt"]
+            .iter()
+            .collect();
+        let mut ref_dump_file = File::open(ref_dump_file_path)?;
+
+        // Compare the new dump file to the reference dump file.
+        compare_files(&mut ref_dump_file, &mut new_dump_file)?;
+
+        Ok(())
+    }
+
+    #[test]
+    fn test_migrate_32() -> MigrateResult<()> {
+        let cwd = env::current_dir()?;
+        let cwd = cwd.to_str().ok_or(MigrateError::StringConversionError)?;
+        let test_env_path: PathBuf = [cwd, "tests", "envs", "ref_env_32"].iter().collect();
+
+        // Migrate data from the old env to a new one.
+        let new_env = tempdir()?;
+        let mut migrator = Migrator::new(&test_env_path)?;
+        migrator.migrate(new_env.path())?;
+
+        // Dump data from the new env to a new dump file.
+        let mut migrator = Migrator::new(new_env.path())?;
+        let mut new_dump_file = tempfile()?;
+        migrator.dump(Some("subdb"), &new_dump_file)?;
+
+        // Open the reference dump file.
+        let ref_dump_file_path: PathBuf = [cwd, "tests", "envs", "ref_dump_subdb.txt"]
+            .iter()
+            .collect();
+        let mut ref_dump_file = File::open(ref_dump_file_path)?;
+
+        // Compare the new dump file to the reference dump file.
+        compare_files(&mut ref_dump_file, &mut new_dump_file)?;
+
+        Ok(())
+    }
+
+    #[test]
+    fn test_migrate_and_replace() -> MigrateResult<()> {
+        let test_env_name = match size_of::<usize>() {
+            4 => "ref_env_64",
+            8 => "ref_env_32",
+            _ => panic!("only 32- and 64-bit depths are supported"),
+        };
+
+        let cwd = env::current_dir()?;
+        let cwd = cwd.to_str().ok_or(MigrateError::StringConversionError)?;
+        let test_env_path: PathBuf = [cwd, "tests", "envs", test_env_name].iter().collect();
+
+        let old_env = tempdir()?;
+        fs::copy(
+            test_env_path.join("data.mdb"),
+            old_env.path().join("data.mdb"),
+        )?;
+        fs::copy(
+            test_env_path.join("lock.mdb"),
+            old_env.path().join("lock.mdb"),
+        )?;
+
+        // Confirm that it isn't possible to open the old environment with LMDB.
+        assert_eq!(
+            match Environment::new().open(old_env.path()) {
+                Err(err) => err,
+                _ => panic!("opening the environment should have failed"),
+            },
+            LmdbError::Invalid
+        );
+
+        // Migrate data from the old env to a new one.
+        let new_env = tempdir()?;
+        let mut migrator = Migrator::new(old_env.path())?;
+        migrator.migrate(new_env.path())?;
+
+        // Dump data from the new env to a new dump file.
+        let mut migrator = Migrator::new(new_env.path())?;
+        let mut new_dump_file = tempfile()?;
+        migrator.dump(Some("subdb"), &new_dump_file)?;
+
+        // Open the reference dump file.
+        let ref_dump_file_path: PathBuf = [cwd, "tests", "envs", "ref_dump_subdb.txt"]
+            .iter()
+            .collect();
+        let mut ref_dump_file = File::open(ref_dump_file_path)?;
+
+        // Compare the new dump file to the reference dump file.
+        compare_files(&mut ref_dump_file, &mut new_dump_file)?;
+
+        // Overwrite the old env's files with the new env's files and confirm that it's now
+        // possible to open the old env with LMDB.
+        fs::copy(
+            new_env.path().join("data.mdb"),
+            old_env.path().join("data.mdb"),
+        )?;
+        fs::copy(
+            new_env.path().join("lock.mdb"),
+            old_env.path().join("lock.mdb"),
+        )?;
+        assert!(Environment::new().open(old_env.path()).is_ok());
+
+        Ok(())
+    }
+}
diff --git a/third_party/rust/rkv/src/backend/impl_lmdb/arch_migrator_error.rs b/third_party/rust/rkv/src/backend/impl_lmdb/arch_migrator_error.rs
new file mode 100644
index 0000000000..e23bb49770
--- /dev/null
+++ b/third_party/rust/rkv/src/backend/impl_lmdb/arch_migrator_error.rs
@@ -0,0 +1,79 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+use std::{io, num, str};
+
+use thiserror::Error;
+
+#[derive(Debug, Error)]
+pub enum MigrateError {
+    #[error("database not found: {0:?}")]
+    DatabaseNotFound(String),
+
+    #[error("{0}")]
+    FromString(String),
+
+    #[error("couldn't determine bit depth")]
+    IndeterminateBitDepth,
+
+    #[error("I/O error: {0:?}")]
+    IoError(#[from] io::Error),
+
+    #[error("invalid DatabaseFlags bits")]
+    InvalidDatabaseBits,
+
+    #[error("invalid data version")]
+    InvalidDataVersion,
+
+    #[error("invalid magic number")]
+    InvalidMagicNum,
+
+    #[error("invalid NodeFlags bits")]
+    InvalidNodeBits,
+
+    #[error("invalid PageFlags bits")]
+    InvalidPageBits,
+
+    #[error("invalid page number")]
+    InvalidPageNum,
+
+    #[error("lmdb backend error: {0}")]
+    LmdbError(#[from] lmdb::Error),
+
+    #[error("string conversion error")]
+    StringConversionError,
+
+    #[error("TryFromInt error: {0:?}")]
+    TryFromIntError(#[from] num::TryFromIntError),
+
+    #[error("unexpected Page variant")]
+    UnexpectedPageVariant,
+
+    #[error("unexpected PageHeader variant")]
+    UnexpectedPageHeaderVariant,
+
+    #[error("unsupported PageHeader variant")]
+    UnsupportedPageHeaderVariant,
+
+    #[error("UTF8 error: {0:?}")]
+    Utf8Error(#[from] str::Utf8Error),
+}
+
+impl From<&str> for MigrateError {
+    fn from(e: &str) -> MigrateError {
+        MigrateError::FromString(e.to_string())
+    }
+}
+
+impl From<String> for MigrateError {
+    fn from(e: String) -> MigrateError {
+        MigrateError::FromString(e)
+    }
+}
diff --git a/third_party/rust/rkv/src/backend/impl_lmdb/cursor.rs b/third_party/rust/rkv/src/backend/impl_lmdb/cursor.rs
new file mode 100644
index 0000000000..760abce451
--- /dev/null
+++ b/third_party/rust/rkv/src/backend/impl_lmdb/cursor.rs
@@ -0,0 +1,69 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+use lmdb::Cursor;
+
+use super::IterImpl;
+use crate::backend::traits::BackendRoCursor;
+
+#[derive(Debug)]
+pub struct RoCursorImpl<'c>(pub(crate) lmdb::RoCursor<'c>);
+
+impl<'c> BackendRoCursor<'c> for RoCursorImpl<'c> {
+    type Iter = IterImpl<'c, lmdb::RoCursor<'c>>;
+
+    fn into_iter(self) -> Self::Iter {
+        // We call RoCursor.iter() instead of RoCursor.iter_start() because
+        // the latter panics when there are no items in the store, whereas the
+        // former returns an iterator that yields no items. And since we create
+        // the Cursor and don't change its position, we can be sure that a call
+        // to Cursor.iter() will start at the beginning.
+        IterImpl::new(self.0, lmdb::RoCursor::iter)
+    }
+
+    fn into_iter_from<K>(self, key: K) -> Self::Iter
+    where
+        K: AsRef<[u8]> + 'c,
+    {
+        IterImpl::new(self.0, |cursor| cursor.iter_from(key))
+    }
+
+    fn into_iter_dup_of<K>(self, key: K) -> Self::Iter
+    where
+        K: AsRef<[u8]> + 'c,
+    {
+        IterImpl::new(self.0, |cursor| cursor.iter_dup_of(key))
+    }
+}
+
+#[derive(Debug)]
+pub struct RwCursorImpl<'c>(pub(crate) lmdb::RwCursor<'c>);
+
+impl<'c> BackendRoCursor<'c> for RwCursorImpl<'c> {
+    type Iter = IterImpl<'c, lmdb::RwCursor<'c>>;
+
+    fn into_iter(self) -> Self::Iter {
+        IterImpl::new(self.0, lmdb::RwCursor::iter)
+    }
+
+    fn into_iter_from<K>(self, key: K) -> Self::Iter
+    where
+        K: AsRef<[u8]> + 'c,
+    {
+        IterImpl::new(self.0, |cursor| cursor.iter_from(key))
+    }
+
+    fn into_iter_dup_of<K>(self, key: K) -> Self::Iter
+    where
+        K: AsRef<[u8]> + 'c,
+    {
+        IterImpl::new(self.0, |cursor| cursor.iter_dup_of(key))
+    }
+}
diff --git a/third_party/rust/rkv/src/backend/impl_lmdb/database.rs b/third_party/rust/rkv/src/backend/impl_lmdb/database.rs
new file mode 100644
index 0000000000..8edee5c2c3
--- /dev/null
+++ b/third_party/rust/rkv/src/backend/impl_lmdb/database.rs
@@ -0,0 +1,16 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+use crate::backend::traits::BackendDatabase;
+
+#[derive(Debug, Eq, PartialEq, Copy, Clone)]
+pub struct DatabaseImpl(pub(crate) lmdb::Database);
+
+impl BackendDatabase for DatabaseImpl {}
diff --git a/third_party/rust/rkv/src/backend/impl_lmdb/environment.rs b/third_party/rust/rkv/src/backend/impl_lmdb/environment.rs
new file mode 100644
index 0000000000..e528fad057
--- /dev/null
+++ b/third_party/rust/rkv/src/backend/impl_lmdb/environment.rs
@@ -0,0 +1,299 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+use std::{
+    fs,
+    path::{Path, PathBuf},
+};
+
+use lmdb::Error as LmdbError;
+
+use super::{
+    DatabaseFlagsImpl, DatabaseImpl, EnvironmentFlagsImpl, ErrorImpl, InfoImpl, RoTransactionImpl,
+    RwTransactionImpl, StatImpl,
+};
+use crate::backend::traits::{
+    BackendEnvironment, BackendEnvironmentBuilder, BackendInfo, BackendIter, BackendRoCursor,
+    BackendRoCursorTransaction, BackendStat,
+};
+
+#[derive(Debug, PartialEq, Eq, Copy, Clone)]
+pub struct EnvironmentBuilderImpl {
+    builder: lmdb::EnvironmentBuilder,
+    env_path_type: EnvironmentPathType,
+    env_lock_type: EnvironmentLockType,
+    env_db_type: EnvironmentDefaultDbType,
+    make_dir_if_needed: bool,
+}
+
+impl<'b> BackendEnvironmentBuilder<'b> for EnvironmentBuilderImpl {
+    type Environment = EnvironmentImpl;
+    type Error = ErrorImpl;
+    type Flags = EnvironmentFlagsImpl;
+
+    fn new() -> EnvironmentBuilderImpl {
+        EnvironmentBuilderImpl {
+            builder: lmdb::Environment::new(),
+            env_path_type: EnvironmentPathType::SubDir,
+            env_lock_type: EnvironmentLockType::Lockfile,
+            env_db_type: EnvironmentDefaultDbType::SingleDatabase,
+            make_dir_if_needed: false,
+        }
+    }
+
+    fn set_flags<T>(&mut self, flags: T) -> &mut Self
+    where
+        T: Into<Self::Flags>,
+    {
+        let flags = flags.into();
+        if flags.0 == lmdb::EnvironmentFlags::NO_SUB_DIR {
+            self.env_path_type = EnvironmentPathType::NoSubDir;
+        }
+        if flags.0 == lmdb::EnvironmentFlags::NO_LOCK {
+            self.env_lock_type = EnvironmentLockType::NoLockfile;
+        }
+        self.builder.set_flags(flags.0);
+        self
+    }
+
+    fn set_max_readers(&mut self, max_readers: u32) -> &mut Self {
+        self.builder.set_max_readers(max_readers);
+        self
+    }
+
+    fn set_max_dbs(&mut self, max_dbs: u32) -> &mut Self {
+        if max_dbs > 0 {
+            self.env_db_type = EnvironmentDefaultDbType::MultipleNamedDatabases
+        }
+        self.builder.set_max_dbs(max_dbs);
+        self
+    }
+
+    fn set_map_size(&mut self, size: usize) -> &mut Self {
+        self.builder.set_map_size(size);
+        self
+    }
+
+    fn set_make_dir_if_needed(&mut self, make_dir_if_needed: bool) -> &mut Self {
+        self.make_dir_if_needed = make_dir_if_needed;
+        self
+    }
+
+    fn set_discard_if_corrupted(&mut self, _discard_if_corrupted: bool) -> &mut Self {
+        // Unfortunately, when opening a database, LMDB doesn't handle all the ways it could have
+        // been corrupted. Prefer using the `SafeMode` backend if this is important.
+        unimplemented!();
+    }
+
+    fn open(&self, path: &Path) -> Result<Self::Environment, Self::Error> {
+        match self.env_path_type {
+            EnvironmentPathType::NoSubDir => {
+                if !path.is_file() {
+                    return Err(ErrorImpl::UnsuitableEnvironmentPath(path.into()));
+                }
+            }
+            EnvironmentPathType::SubDir => {
+                if !path.is_dir() {
+                    if !self.make_dir_if_needed {
+                        return Err(ErrorImpl::UnsuitableEnvironmentPath(path.into()));
+                    }
+                    fs::create_dir_all(path)?;
+                }
+            }
+        }
+
+        self.builder
+            .open(path)
+            .map_err(ErrorImpl::LmdbError)
+            .and_then(|lmdbenv| {
+                EnvironmentImpl::new(
+                    path,
+                    self.env_path_type,
+                    self.env_lock_type,
+                    self.env_db_type,
+                    lmdbenv,
+                )
+            })
+    }
+}
+
+#[derive(Debug, PartialEq, Eq, Copy, Clone)]
+pub enum EnvironmentPathType {
+    SubDir,
+    NoSubDir,
+}
+
+#[derive(Debug, PartialEq, Eq, Copy, Clone)]
+pub enum EnvironmentLockType {
+    Lockfile,
+    NoLockfile,
+}
+
+#[derive(Debug, PartialEq, Eq, Copy, Clone)]
+pub enum EnvironmentDefaultDbType {
+    SingleDatabase,
+    MultipleNamedDatabases,
+}
+
+#[derive(Debug)]
+pub struct EnvironmentImpl {
+    path: PathBuf,
+    env_path_type: EnvironmentPathType,
+    env_lock_type: EnvironmentLockType,
+    env_db_type: EnvironmentDefaultDbType,
+    lmdbenv: lmdb::Environment,
+}
+
+impl EnvironmentImpl {
+    pub(crate) fn new(
+        path: &Path,
+        env_path_type: EnvironmentPathType,
+        env_lock_type: EnvironmentLockType,
+        env_db_type: EnvironmentDefaultDbType,
+        lmdbenv: lmdb::Environment,
+    ) -> Result<EnvironmentImpl, ErrorImpl> {
+        Ok(EnvironmentImpl {
+            path: path.to_path_buf(),
+            env_path_type,
+            env_lock_type,
+            env_db_type,
+            lmdbenv,
+        })
+    }
+}
+
+impl<'e> BackendEnvironment<'e> for EnvironmentImpl {
+    type Database = DatabaseImpl;
+    type Error = ErrorImpl;
+    type Flags = DatabaseFlagsImpl;
+    type Info = InfoImpl;
+    type RoTransaction = RoTransactionImpl<'e>;
+    type RwTransaction = RwTransactionImpl<'e>;
+    type Stat = StatImpl;
+
+    fn get_dbs(&self) -> Result<Vec<Option<String>>, Self::Error> {
+        if self.env_db_type == EnvironmentDefaultDbType::SingleDatabase {
+            return Ok(vec![None]);
+        }
+        let db = self
+            .lmdbenv
+            .open_db(None)
+            .map(DatabaseImpl)
+            .map_err(ErrorImpl::LmdbError)?;
+        let reader = self.begin_ro_txn()?;
+        let cursor = reader.open_ro_cursor(&db)?;
+        let mut iter = cursor.into_iter();
+        let mut store = vec![];
+        while let Some(result) = iter.next() {
+            let (key, _) = result?;
+            let name = String::from_utf8(key.to_owned())
+                .map_err(|_| ErrorImpl::LmdbError(lmdb::Error::Corrupted))?;
+            store.push(Some(name));
+        }
+        Ok(store)
+    }
+
+    fn open_db(&self, name: Option<&str>) -> Result<Self::Database, Self::Error> {
+        self.lmdbenv
+            .open_db(name)
+            .map(DatabaseImpl)
+            .map_err(ErrorImpl::LmdbError)
+    }
+
+    fn create_db(
+        &self,
+        name: Option<&str>,
+        flags: Self::Flags,
+    ) -> Result<Self::Database, Self::Error> {
+        self.lmdbenv
+            .create_db(name, flags.0)
+            .map(DatabaseImpl)
+            .map_err(ErrorImpl::LmdbError)
+    }
+
+    fn begin_ro_txn(&'e self) -> Result<Self::RoTransaction, Self::Error> {
+        self.lmdbenv
+            .begin_ro_txn()
+            .map(RoTransactionImpl)
+            .map_err(ErrorImpl::LmdbError)
+    }
+
+    fn begin_rw_txn(&'e self) -> Result<Self::RwTransaction, Self::Error> {
+        self.lmdbenv
+            .begin_rw_txn()
+            .map(RwTransactionImpl)
+            .map_err(ErrorImpl::LmdbError)
+    }
+
+    fn sync(&self, force: bool) -> Result<(), Self::Error> {
+        self.lmdbenv.sync(force).map_err(ErrorImpl::LmdbError)
+    }
+
+    fn stat(&self) -> Result<Self::Stat, Self::Error> {
+        self.lmdbenv
+            .stat()
+            .map(StatImpl)
+            .map_err(ErrorImpl::LmdbError)
+    }
+
+    fn info(&self) -> Result<Self::Info, Self::Error> {
+        self.lmdbenv
+            .info()
+            .map(InfoImpl)
+            .map_err(ErrorImpl::LmdbError)
+    }
+
+    fn freelist(&self) -> Result<usize, Self::Error> {
+        self.lmdbenv.freelist().map_err(ErrorImpl::LmdbError)
+    }
+
+    fn load_ratio(&self) -> Result<Option<f32>, Self::Error> {
+        let stat = self.stat()?;
+        let info = self.info()?;
+        let freelist = self.freelist()?;
+
+        let last_pgno = info.last_pgno() + 1; // pgno is 0 based.
+        let total_pgs = info.map_size() / stat.page_size();
+        if freelist > last_pgno {
+            return Err(ErrorImpl::LmdbError(LmdbError::Corrupted));
+        }
+        let used_pgs = last_pgno - freelist;
+        Ok(Some(used_pgs as f32 / total_pgs as f32))
+    }
+
+    fn set_map_size(&self, size: usize) -> Result<(), Self::Error> {
+        self.lmdbenv
+            .set_map_size(size)
+            .map_err(ErrorImpl::LmdbError)
+    }
+
+    fn get_files_on_disk(&self) -> Vec<PathBuf> {
+        let mut store = vec![];
+
+        if self.env_path_type == EnvironmentPathType::NoSubDir {
+            // The option NO_SUB_DIR could change the default directory layout; therefore this should
+            // probably return the path used to create environment, along with the custom lockfile
+            // when available.
+            unimplemented!();
+        }
+
+        let mut db_filename = self.path.clone();
+        db_filename.push("data.mdb");
+        store.push(db_filename);
+
+        if self.env_lock_type == EnvironmentLockType::Lockfile {
+            let mut lock_filename = self.path.clone();
+            lock_filename.push("lock.mdb");
+            store.push(lock_filename);
+        }
+
+        store
+    }
+}
diff --git a/third_party/rust/rkv/src/backend/impl_lmdb/error.rs b/third_party/rust/rkv/src/backend/impl_lmdb/error.rs
new file mode 100644
index 0000000000..a2fd8a7859
--- /dev/null
+++ b/third_party/rust/rkv/src/backend/impl_lmdb/error.rs
@@ -0,0 +1,57 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+use std::{fmt, io, path::PathBuf};
+
+use crate::{backend::traits::BackendError, error::StoreError};
+
+#[derive(Debug)]
+pub enum ErrorImpl {
+    LmdbError(lmdb::Error),
+    UnsuitableEnvironmentPath(PathBuf),
+    IoError(io::Error),
+}
+
+impl BackendError for ErrorImpl {}
+
+impl fmt::Display for ErrorImpl {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            ErrorImpl::LmdbError(e) => e.fmt(fmt),
+            ErrorImpl::UnsuitableEnvironmentPath(_) => write!(fmt, "UnsuitableEnvironmentPath"),
+            ErrorImpl::IoError(e) => e.fmt(fmt),
+        }
+    }
+}
+
+impl Into<StoreError> for ErrorImpl {
+    fn into(self) -> StoreError {
+        match self {
+            ErrorImpl::LmdbError(lmdb::Error::Corrupted) => StoreError::DatabaseCorrupted,
+            ErrorImpl::LmdbError(lmdb::Error::NotFound) => StoreError::KeyValuePairNotFound,
+            ErrorImpl::LmdbError(lmdb::Error::BadValSize) => StoreError::KeyValuePairBadSize,
+            ErrorImpl::LmdbError(lmdb::Error::Invalid) => StoreError::FileInvalid,
+            ErrorImpl::LmdbError(lmdb::Error::MapFull) => StoreError::MapFull,
+            ErrorImpl::LmdbError(lmdb::Error::DbsFull) => StoreError::DbsFull,
+            ErrorImpl::LmdbError(lmdb::Error::ReadersFull) => StoreError::ReadersFull,
+            ErrorImpl::LmdbError(error) => StoreError::LmdbError(error),
+            ErrorImpl::UnsuitableEnvironmentPath(path) => {
+                StoreError::UnsuitableEnvironmentPath(path)
+            }
+            ErrorImpl::IoError(error) => StoreError::IoError(error),
+        }
+    }
+}
+
+impl From<io::Error> for ErrorImpl {
+    fn from(e: io::Error) -> ErrorImpl {
+        ErrorImpl::IoError(e)
+    }
+}
diff --git a/third_party/rust/rkv/src/backend/impl_lmdb/flags.rs b/third_party/rust/rkv/src/backend/impl_lmdb/flags.rs
new file mode 100644
index 0000000000..7a16bb7c46
--- /dev/null
+++ b/third_party/rust/rkv/src/backend/impl_lmdb/flags.rs
@@ -0,0 +1,123 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+use crate::backend::{
+    common::{DatabaseFlags, EnvironmentFlags, WriteFlags},
+    traits::{BackendDatabaseFlags, BackendEnvironmentFlags, BackendFlags, BackendWriteFlags},
+};
+
+#[derive(Debug, Eq, PartialEq, Copy, Clone, Default)]
+pub struct EnvironmentFlagsImpl(pub(crate) lmdb::EnvironmentFlags);
+
+impl BackendFlags for EnvironmentFlagsImpl {
+    fn empty() -> EnvironmentFlagsImpl {
+        EnvironmentFlagsImpl(lmdb::EnvironmentFlags::empty())
+    }
+}
+
+impl BackendEnvironmentFlags for EnvironmentFlagsImpl {
+    fn set(&mut self, flag: EnvironmentFlags, value: bool) {
+        self.0.set(flag.into(), value)
+    }
+}
+
+impl Into<EnvironmentFlagsImpl> for EnvironmentFlags {
+    fn into(self) -> EnvironmentFlagsImpl {
+        EnvironmentFlagsImpl(self.into())
+    }
+}
+
+impl Into<lmdb::EnvironmentFlags> for EnvironmentFlags {
+    fn into(self) -> lmdb::EnvironmentFlags {
+        match self {
+            EnvironmentFlags::FIXED_MAP => lmdb::EnvironmentFlags::FIXED_MAP,
+            EnvironmentFlags::NO_SUB_DIR => lmdb::EnvironmentFlags::NO_SUB_DIR,
+            EnvironmentFlags::WRITE_MAP => lmdb::EnvironmentFlags::WRITE_MAP,
+            EnvironmentFlags::READ_ONLY => lmdb::EnvironmentFlags::READ_ONLY,
+            EnvironmentFlags::NO_META_SYNC => lmdb::EnvironmentFlags::NO_META_SYNC,
+            EnvironmentFlags::NO_SYNC => lmdb::EnvironmentFlags::NO_SYNC,
+            EnvironmentFlags::MAP_ASYNC => lmdb::EnvironmentFlags::MAP_ASYNC,
+            EnvironmentFlags::NO_TLS => lmdb::EnvironmentFlags::NO_TLS,
+            EnvironmentFlags::NO_LOCK => lmdb::EnvironmentFlags::NO_LOCK,
+            EnvironmentFlags::NO_READAHEAD => lmdb::EnvironmentFlags::NO_READAHEAD,
+            EnvironmentFlags::NO_MEM_INIT => lmdb::EnvironmentFlags::NO_MEM_INIT,
+        }
+    }
+}
+
+#[derive(Debug, Eq, PartialEq, Copy, Clone, Default)]
+pub struct DatabaseFlagsImpl(pub(crate) lmdb::DatabaseFlags);
+
+impl BackendFlags for DatabaseFlagsImpl {
+    fn empty() -> DatabaseFlagsImpl {
+        DatabaseFlagsImpl(lmdb::DatabaseFlags::empty())
+    }
+}
+
+impl BackendDatabaseFlags for DatabaseFlagsImpl {
+    fn set(&mut self, flag: DatabaseFlags, value: bool) {
+        self.0.set(flag.into(), value)
+    }
+}
+
+impl Into<DatabaseFlagsImpl> for DatabaseFlags {
+    fn into(self) -> DatabaseFlagsImpl {
+        DatabaseFlagsImpl(self.into())
+    }
+}
+
+impl Into<lmdb::DatabaseFlags> for DatabaseFlags {
+    fn into(self) -> lmdb::DatabaseFlags {
+        match self {
+            DatabaseFlags::REVERSE_KEY => lmdb::DatabaseFlags::REVERSE_KEY,
+            #[cfg(feature = "db-dup-sort")]
+            DatabaseFlags::DUP_SORT => lmdb::DatabaseFlags::DUP_SORT,
+            #[cfg(feature = "db-dup-sort")]
+            DatabaseFlags::DUP_FIXED => lmdb::DatabaseFlags::DUP_FIXED,
+            #[cfg(feature = "db-int-key")]
+            DatabaseFlags::INTEGER_KEY => lmdb::DatabaseFlags::INTEGER_KEY,
+            DatabaseFlags::INTEGER_DUP => lmdb::DatabaseFlags::INTEGER_DUP,
+            DatabaseFlags::REVERSE_DUP => lmdb::DatabaseFlags::REVERSE_DUP,
+        }
+    }
+}
+
+#[derive(Debug, Eq, PartialEq, Copy, Clone, Default)]
+pub struct WriteFlagsImpl(pub(crate) lmdb::WriteFlags);
+
+impl BackendFlags for WriteFlagsImpl {
+    fn empty() -> WriteFlagsImpl {
+        WriteFlagsImpl(lmdb::WriteFlags::empty())
+    }
+}
+
+impl BackendWriteFlags for WriteFlagsImpl {
+    fn set(&mut self, flag: WriteFlags, value: bool) {
+        self.0.set(flag.into(), value)
+    }
+}
+
+impl Into<WriteFlagsImpl> for WriteFlags {
+    fn into(self) -> WriteFlagsImpl {
+        WriteFlagsImpl(self.into())
+    }
+}
+
+impl Into<lmdb::WriteFlags> for WriteFlags {
+    fn into(self) -> lmdb::WriteFlags {
+        match self {
+            WriteFlags::NO_OVERWRITE => lmdb::WriteFlags::NO_OVERWRITE,
+            WriteFlags::NO_DUP_DATA => lmdb::WriteFlags::NO_DUP_DATA,
+            WriteFlags::CURRENT => lmdb::WriteFlags::CURRENT,
+            WriteFlags::APPEND => lmdb::WriteFlags::APPEND,
+            WriteFlags::APPEND_DUP => lmdb::WriteFlags::APPEND_DUP,
+        }
+    }
+}
diff --git a/third_party/rust/rkv/src/backend/impl_lmdb/info.rs b/third_party/rust/rkv/src/backend/impl_lmdb/info.rs
new file mode 100644
index 0000000000..6188065c07
--- /dev/null
+++ b/third_party/rust/rkv/src/backend/impl_lmdb/info.rs
@@ -0,0 +1,35 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+use crate::backend::traits::BackendInfo;
+
+pub struct InfoImpl(pub(crate) lmdb::Info);
+
+impl BackendInfo for InfoImpl {
+    fn map_size(&self) -> usize {
+        self.0.map_size()
+    }
+
+    fn last_pgno(&self) -> usize {
+        self.0.last_pgno()
+    }
+
+    fn last_txnid(&self) -> usize {
+        self.0.last_txnid()
+    }
+
+    fn max_readers(&self) -> usize {
+        self.0.max_readers() as usize
+    }
+
+    fn num_readers(&self) -> usize {
+        self.0.num_readers() as usize
+    }
+}
diff --git a/third_party/rust/rkv/src/backend/impl_lmdb/iter.rs b/third_party/rust/rkv/src/backend/impl_lmdb/iter.rs
new file mode 100644
index 0000000000..519d361d4f
--- /dev/null
+++ b/third_party/rust/rkv/src/backend/impl_lmdb/iter.rs
@@ -0,0 +1,41 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+use super::ErrorImpl;
+use crate::backend::traits::BackendIter;
+
+pub struct IterImpl<'i, C> {
+    // LMDB semantics dictate that a cursor must be valid for the entire lifetime
+    // of an iterator. In other words, cursors must not be dropped while an
+    // iterator built from it is alive. Unfortunately, the LMDB crate API does
+    // not express this through the type system, so we must enforce it somehow.
+    #[allow(dead_code)]
+    cursor: C,
+    iter: lmdb::Iter<'i>,
+}
+
+impl<'i, C> IterImpl<'i, C> {
+    pub(crate) fn new(
+        mut cursor: C,
+        to_iter: impl FnOnce(&mut C) -> lmdb::Iter<'i>,
+    ) -> IterImpl<'i, C> {
+        let iter = to_iter(&mut cursor);
+        IterImpl { cursor, iter }
+    }
+}
+
+impl<'i, C> BackendIter<'i> for IterImpl<'i, C> {
+    type Error = ErrorImpl;
+
+    #[allow(clippy::type_complexity)]
+    fn next(&mut self) -> Option<Result<(&'i [u8], &'i [u8]), Self::Error>> {
+        self.iter.next().map(|e| e.map_err(ErrorImpl::LmdbError))
+    }
+}
diff --git a/third_party/rust/rkv/src/backend/impl_lmdb/stat.rs b/third_party/rust/rkv/src/backend/impl_lmdb/stat.rs
new file mode 100644
index 0000000000..b0de8c5051
--- /dev/null
+++ b/third_party/rust/rkv/src/backend/impl_lmdb/stat.rs
@@ -0,0 +1,39 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+use crate::backend::traits::BackendStat;
+
+pub struct StatImpl(pub(crate) lmdb::Stat);
+
+impl BackendStat for StatImpl {
+    fn page_size(&self) -> usize {
+        self.0.page_size() as usize
+    }
+
+    fn depth(&self) -> usize {
+        self.0.depth() as usize
+    }
+
+    fn branch_pages(&self) -> usize {
+        self.0.branch_pages()
+    }
+
+    fn leaf_pages(&self) -> usize {
+        self.0.leaf_pages()
+    }
+
+    fn overflow_pages(&self) -> usize {
+        self.0.overflow_pages()
+    }
+
+    fn entries(&self) -> usize {
+        self.0.entries()
+    }
+}
diff --git a/third_party/rust/rkv/src/backend/impl_lmdb/transaction.rs b/third_party/rust/rkv/src/backend/impl_lmdb/transaction.rs
new file mode 100644
index 0000000000..21752763fd
--- /dev/null
+++ b/third_party/rust/rkv/src/backend/impl_lmdb/transaction.rs
@@ -0,0 +1,107 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+use lmdb::Transaction;
+
+use super::{DatabaseImpl, ErrorImpl, RoCursorImpl, WriteFlagsImpl};
+use crate::backend::traits::{
+    BackendRoCursorTransaction, BackendRoTransaction, BackendRwCursorTransaction,
+    BackendRwTransaction,
+};
+
+#[derive(Debug)]
+pub struct RoTransactionImpl<'t>(pub(crate) lmdb::RoTransaction<'t>);
+
+impl<'t> BackendRoTransaction for RoTransactionImpl<'t> {
+    type Database = DatabaseImpl;
+    type Error = ErrorImpl;
+
+    fn get(&self, db: &Self::Database, key: &[u8]) -> Result<&[u8], Self::Error> {
+        self.0.get(db.0, &key).map_err(ErrorImpl::LmdbError)
+    }
+
+    fn abort(self) {
+        self.0.abort()
+    }
+}
+
+impl<'t> BackendRoCursorTransaction<'t> for RoTransactionImpl<'t> {
+    type RoCursor = RoCursorImpl<'t>;
+
+    fn open_ro_cursor(&'t self, db: &Self::Database) -> Result<Self::RoCursor, Self::Error> {
+        self.0
+            .open_ro_cursor(db.0)
+            .map(RoCursorImpl)
+            .map_err(ErrorImpl::LmdbError)
+    }
+}
+
+#[derive(Debug)]
+pub struct RwTransactionImpl<'t>(pub(crate) lmdb::RwTransaction<'t>);
+
+impl<'t> BackendRwTransaction for RwTransactionImpl<'t> {
+    type Database = DatabaseImpl;
+    type Error = ErrorImpl;
+    type Flags = WriteFlagsImpl;
+
+    fn get(&self, db: &Self::Database, key: &[u8]) -> Result<&[u8], Self::Error> {
+        self.0.get(db.0, &key).map_err(ErrorImpl::LmdbError)
+    }
+
+    fn put(
+        &mut self,
+        db: &Self::Database,
+        key: &[u8],
+        value: &[u8],
+        flags: Self::Flags,
+    ) -> Result<(), Self::Error> {
+        self.0
+            .put(db.0, &key, &value, flags.0)
+            .map_err(ErrorImpl::LmdbError)
+    }
+
+    #[cfg(not(feature = "db-dup-sort"))]
+    fn del(&mut self, db: &Self::Database, key: &[u8]) -> Result<(), Self::Error> {
+        self.0.del(db.0, &key, None).map_err(ErrorImpl::LmdbError)
+    }
+
+    #[cfg(feature = "db-dup-sort")]
+    fn del(
+        &mut self,
+        db: &Self::Database,
+        key: &[u8],
+        value: Option<&[u8]>,
+    ) -> Result<(), Self::Error> {
+        self.0.del(db.0, &key, value).map_err(ErrorImpl::LmdbError)
+    }
+
+    fn clear_db(&mut self, db: &Self::Database) -> Result<(), Self::Error> {
+        self.0.clear_db(db.0).map_err(ErrorImpl::LmdbError)
+    }
+
+    fn commit(self) -> Result<(), Self::Error> {
+        self.0.commit().map_err(ErrorImpl::LmdbError)
+    }
+
+    fn abort(self) {
+        self.0.abort()
+    }
+}
+
+impl<'t> BackendRwCursorTransaction<'t> for RwTransactionImpl<'t> {
+    type RoCursor = RoCursorImpl<'t>;
+
+    fn open_ro_cursor(&'t self, db: &Self::Database) -> Result<Self::RoCursor, Self::Error> {
+        self.0
+            .open_ro_cursor(db.0)
+            .map(RoCursorImpl)
+            .map_err(ErrorImpl::LmdbError)
+    }
+}
diff --git a/third_party/rust/rkv/src/backend/impl_safe.rs b/third_party/rust/rkv/src/backend/impl_safe.rs
new file mode 100644
index 0000000000..66ca012973
--- /dev/null
+++ b/third_party/rust/rkv/src/backend/impl_safe.rs
@@ -0,0 +1,30 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+mod cursor;
+mod database;
+mod environment;
+mod error;
+mod flags;
+mod info;
+mod iter;
+mod snapshot;
+mod stat;
+mod transaction;
+
+pub use cursor::{RoCursorImpl, RwCursorImpl};
+pub use database::DatabaseImpl;
+pub use environment::{EnvironmentBuilderImpl, EnvironmentImpl};
+pub use error::ErrorImpl;
+pub use flags::{DatabaseFlagsImpl, EnvironmentFlagsImpl, WriteFlagsImpl};
+pub use info::InfoImpl;
+pub use iter::IterImpl;
+pub use stat::StatImpl;
+pub use transaction::{RoTransactionImpl, RwTransactionImpl};
diff --git a/third_party/rust/rkv/src/backend/impl_safe/cursor.rs b/third_party/rust/rkv/src/backend/impl_safe/cursor.rs
new file mode 100644
index 0000000000..c3bfefad94
--- /dev/null
+++ b/third_party/rust/rkv/src/backend/impl_safe/cursor.rs
@@ -0,0 +1,98 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+use super::{snapshot::Snapshot, IterImpl};
+use crate::backend::traits::BackendRoCursor;
+
+#[derive(Debug)]
+pub struct RoCursorImpl<'c>(pub(crate) &'c Snapshot);
+
+#[cfg(not(feature = "db-dup-sort"))]
+impl<'c> BackendRoCursor<'c> for RoCursorImpl<'c> {
+    type Iter = IterImpl<'c>;
+
+    fn into_iter(self) -> Self::Iter {
+        IterImpl(Box::new(self.0.iter()))
+    }
+
+    fn into_iter_from<K>(self, key: K) -> Self::Iter
+    where
+        K: AsRef<[u8]> + 'c,
+    {
+        IterImpl(Box::new(
+            self.0.iter().skip_while(move |&(k, _)| k < key.as_ref()),
+        ))
+    }
+
+    fn into_iter_dup_of<K>(self, key: K) -> Self::Iter
+    where
+        K: AsRef<[u8]> + 'c,
+    {
+        IterImpl(Box::new(
+            self.0.iter().filter(move |&(k, _)| k == key.as_ref()),
+        ))
+    }
+}
+
+#[cfg(feature = "db-dup-sort")]
+impl<'c> BackendRoCursor<'c> for RoCursorImpl<'c> {
+    type Iter = IterImpl<'c>;
+
+    fn into_iter(self) -> Self::Iter {
+        let flattened = self
+            .0
+            .iter()
+            .flat_map(|(key, values)| values.map(move |value| (key, value)));
+        IterImpl(Box::new(flattened))
+    }
+
+    fn into_iter_from<K>(self, key: K) -> Self::Iter
+    where
+        K: AsRef<[u8]> + 'c,
+    {
+        let skipped = self.0.iter().skip_while(move |&(k, _)| k < key.as_ref());
+        let flattened = skipped.flat_map(|(key, values)| values.map(move |value| (key, value)));
+        IterImpl(Box::new(flattened))
+    }
+
+    fn into_iter_dup_of<K>(self, key: K) -> Self::Iter
+    where
+        K: AsRef<[u8]> + 'c,
+    {
+        let filtered = self.0.iter().filter(move |&(k, _)| k == key.as_ref());
+        let flattened = filtered.flat_map(|(key, values)| values.map(move |value| (key, value)));
+        IterImpl(Box::new(flattened))
+    }
+}
+
+#[derive(Debug)]
+pub struct RwCursorImpl<'c>(&'c mut Snapshot);
+
+impl<'c> BackendRoCursor<'c> for RwCursorImpl<'c> {
+    type Iter = IterImpl<'c>;
+
+    fn into_iter(self) -> Self::Iter {
+        unimplemented!()
+    }
+
+    fn into_iter_from<K>(self, _key: K) -> Self::Iter
+    where
+        K: AsRef<[u8]> + 'c,
+    {
+        unimplemented!()
+    }
+
+    fn into_iter_dup_of<K>(self, _key: K) -> Self::Iter
+    where
+        K: AsRef<[u8]> + 'c,
+    {
+        unimplemented!()
+    }
+}
diff --git a/third_party/rust/rkv/src/backend/impl_safe/database.rs b/third_party/rust/rkv/src/backend/impl_safe/database.rs
new file mode 100644
index 0000000000..f8eb4aef76
--- /dev/null
+++ b/third_party/rust/rkv/src/backend/impl_safe/database.rs
@@ -0,0 +1,41 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+use id_arena::Id;
+use serde_derive::{Deserialize, Serialize};
+
+use super::{snapshot::Snapshot, DatabaseFlagsImpl};
+use crate::backend::traits::BackendDatabase;
+
+#[derive(Debug, Eq, PartialEq, Copy, Clone, Hash)]
+pub struct DatabaseImpl(pub(crate) Id<Database>);
+
+impl BackendDatabase for DatabaseImpl {}
+
+#[derive(Debug, Serialize, Deserialize)]
+pub struct Database {
+    snapshot: Snapshot,
+}
+
+impl Database {
+    pub(crate) fn new(flags: Option<DatabaseFlagsImpl>, snapshot: Option<Snapshot>) -> Database {
+        Database {
+            snapshot: snapshot.unwrap_or_else(|| Snapshot::new(flags)),
+        }
+    }
+
+    pub(crate) fn snapshot(&self) -> Snapshot {
+        self.snapshot.clone()
+    }
+
+    pub(crate) fn replace(&mut self, snapshot: Snapshot) -> Snapshot {
+        std::mem::replace(&mut self.snapshot, snapshot)
+    }
+}
diff --git a/third_party/rust/rkv/src/backend/impl_safe/environment.rs b/third_party/rust/rkv/src/backend/impl_safe/environment.rs
new file mode 100644
index 0000000000..17cfb87f1e
--- /dev/null
+++ b/third_party/rust/rkv/src/backend/impl_safe/environment.rs
@@ -0,0 +1,332 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+use std::{
+    borrow::Cow,
+    collections::HashMap,
+    fs,
+    ops::DerefMut,
+    path::{Path, PathBuf},
+    sync::{Arc, RwLock, RwLockReadGuard, RwLockWriteGuard},
+};
+
+use id_arena::Arena;
+use log::warn;
+
+use super::{
+    database::Database, DatabaseFlagsImpl, DatabaseImpl, EnvironmentFlagsImpl, ErrorImpl, InfoImpl,
+    RoTransactionImpl, RwTransactionImpl, StatImpl,
+};
+use crate::backend::traits::{BackendEnvironment, BackendEnvironmentBuilder};
+
+const DEFAULT_DB_FILENAME: &str = "data.safe.bin";
+
+type DatabaseArena = Arena<Database>;
+type DatabaseNameMap = HashMap<Option<String>, DatabaseImpl>;
+
+#[derive(Debug, PartialEq, Eq, Copy, Clone)]
+pub struct EnvironmentBuilderImpl {
+    flags: EnvironmentFlagsImpl,
+    max_readers: Option<usize>,
+    max_dbs: Option<usize>,
+    map_size: Option<usize>,
+    make_dir_if_needed: bool,
+    discard_if_corrupted: bool,
+}
+
+impl<'b> BackendEnvironmentBuilder<'b> for EnvironmentBuilderImpl {
+    type Environment = EnvironmentImpl;
+    type Error = ErrorImpl;
+    type Flags = EnvironmentFlagsImpl;
+
+    fn new() -> EnvironmentBuilderImpl {
+        EnvironmentBuilderImpl {
+            flags: EnvironmentFlagsImpl::empty(),
+            max_readers: None,
+            max_dbs: None,
+            map_size: None,
+            make_dir_if_needed: false,
+            discard_if_corrupted: false,
+        }
+    }
+
+    fn set_flags<T>(&mut self, flags: T) -> &mut Self
+    where
+        T: Into<Self::Flags>,
+    {
+        self.flags = flags.into();
+        self
+    }
+
+    fn set_max_readers(&mut self, max_readers: u32) -> &mut Self {
+        self.max_readers = Some(max_readers as usize);
+        self
+    }
+
+    fn set_max_dbs(&mut self, max_dbs: u32) -> &mut Self {
+        self.max_dbs = Some(max_dbs as usize);
+        self
+    }
+
+    fn set_map_size(&mut self, map_size: usize) -> &mut Self {
+        self.map_size = Some(map_size);
+        self
+    }
+
+    fn set_make_dir_if_needed(&mut self, make_dir_if_needed: bool) -> &mut Self {
+        self.make_dir_if_needed = make_dir_if_needed;
+        self
+    }
+
+    fn set_discard_if_corrupted(&mut self, discard_if_corrupted: bool) -> &mut Self {
+        self.discard_if_corrupted = discard_if_corrupted;
+        self
+    }
+
+    fn open(&self, path: &Path) -> Result<Self::Environment, Self::Error> {
+        // Technically NO_SUB_DIR should change these checks here, but they're both currently
+        // unimplemented with this storage backend.
+        if !path.is_dir() {
+            if !self.make_dir_if_needed {
+                return Err(ErrorImpl::UnsuitableEnvironmentPath(path.into()));
+            }
+            fs::create_dir_all(path)?;
+        }
+        let mut env = EnvironmentImpl::new(
+            path,
+            self.flags,
+            self.max_readers,
+            self.max_dbs,
+            self.map_size,
+        )?;
+        env.read_from_disk(self.discard_if_corrupted)?;
+        Ok(env)
+    }
+}
+
+#[derive(Debug)]
+pub(crate) struct EnvironmentDbs {
+    pub(crate) arena: DatabaseArena,
+    pub(crate) name_map: DatabaseNameMap,
+}
+
+#[derive(Debug)]
+pub(crate) struct EnvironmentDbsRefMut<'a> {
+    pub(crate) arena: &'a mut DatabaseArena,
+    pub(crate) name_map: &'a mut DatabaseNameMap,
+}
+
+impl<'a> From<&'a mut EnvironmentDbs> for EnvironmentDbsRefMut<'a> {
+    fn from(dbs: &mut EnvironmentDbs) -> EnvironmentDbsRefMut {
+        EnvironmentDbsRefMut {
+            arena: &mut dbs.arena,
+            name_map: &mut dbs.name_map,
+        }
+    }
+}
+
+#[derive(Debug)]
+pub struct EnvironmentImpl {
+    path: PathBuf,
+    max_dbs: usize,
+    dbs: RwLock<EnvironmentDbs>,
+    ro_txns: Arc<()>,
+    rw_txns: Arc<()>,
+}
+
+impl EnvironmentImpl {
+    fn serialize(&self) -> Result<Vec<u8>, ErrorImpl> {
+        let dbs = self.dbs.read().map_err(|_| ErrorImpl::EnvPoisonError)?;
+        let data: HashMap<_, _> = dbs
+            .name_map
+            .iter()
+            .map(|(name, id)| (name, &dbs.arena[id.0]))
+            .collect();
+        Ok(bincode::serialize(&data)?)
+    }
+
+    fn deserialize(
+        bytes: &[u8],
+        discard_if_corrupted: bool,
+    ) -> Result<(DatabaseArena, DatabaseNameMap), ErrorImpl> {
+        let mut arena = DatabaseArena::new();
+        let mut name_map = HashMap::new();
+        let data: HashMap<_, _> = match bincode::deserialize(bytes) {
+            Err(_) if discard_if_corrupted => Ok(HashMap::new()),
+            result => result,
+        }?;
+        for (name, db) in data {
+            name_map.insert(name, DatabaseImpl(arena.alloc(db)));
+        }
+        Ok((arena, name_map))
+    }
+}
+
+impl EnvironmentImpl {
+    pub(crate) fn new(
+        path: &Path,
+        flags: EnvironmentFlagsImpl,
+        max_readers: Option<usize>,
+        max_dbs: Option<usize>,
+        map_size: Option<usize>,
+    ) -> Result<EnvironmentImpl, ErrorImpl> {
+        if !flags.is_empty() {
+            warn!("Ignoring `flags={:?}`", flags);
+        }
+        if let Some(max_readers) = max_readers {
+            warn!("Ignoring `max_readers={}`", max_readers);
+        }
+        if let Some(map_size) = map_size {
+            warn!("Ignoring `map_size={}`", map_size);
+        }
+
+        Ok(EnvironmentImpl {
+            path: path.to_path_buf(),
+            max_dbs: max_dbs.unwrap_or(std::usize::MAX),
+            dbs: RwLock::new(EnvironmentDbs {
+                arena: DatabaseArena::new(),
+                name_map: HashMap::new(),
+            }),
+            ro_txns: Arc::new(()),
+            rw_txns: Arc::new(()),
+        })
+    }
+
+    pub(crate) fn read_from_disk(&mut self, discard_if_corrupted: bool) -> Result<(), ErrorImpl> {
+        let mut path = Cow::from(&self.path);
+        if fs::metadata(&path)?.is_dir() {
+            path.to_mut().push(DEFAULT_DB_FILENAME);
+        };
+        if fs::metadata(&path).is_err() {
+            return Ok(());
+        };
+        let (arena, name_map) = Self::deserialize(&fs::read(&path)?, discard_if_corrupted)?;
+        self.dbs = RwLock::new(EnvironmentDbs { arena, name_map });
+        Ok(())
+    }
+
+    pub(crate) fn write_to_disk(&self) -> Result<(), ErrorImpl> {
+        let mut path = Cow::from(&self.path);
+        if fs::metadata(&path)?.is_dir() {
+            path.to_mut().push(DEFAULT_DB_FILENAME);
+        };
+
+        // Write to a temp file first.
+        let tmp_path = path.with_extension("tmp");
+        fs::write(&tmp_path, self.serialize()?)?;
+
+        // Atomically move that file to the database file.
+        fs::rename(tmp_path, path)?;
+        Ok(())
+    }
+
+    pub(crate) fn dbs(&self) -> Result<RwLockReadGuard<EnvironmentDbs>, ErrorImpl> {
+        self.dbs.read().map_err(|_| ErrorImpl::EnvPoisonError)
+    }
+
+    pub(crate) fn dbs_mut(&self) -> Result<RwLockWriteGuard<EnvironmentDbs>, ErrorImpl> {
+        self.dbs.write().map_err(|_| ErrorImpl::EnvPoisonError)
+    }
+}
+
+impl<'e> BackendEnvironment<'e> for EnvironmentImpl {
+    type Database = DatabaseImpl;
+    type Error = ErrorImpl;
+    type Flags = DatabaseFlagsImpl;
+    type Info = InfoImpl;
+    type RoTransaction = RoTransactionImpl<'e>;
+    type RwTransaction = RwTransactionImpl<'e>;
+    type Stat = StatImpl;
+
+    fn get_dbs(&self) -> Result<Vec<Option<String>>, Self::Error> {
+        let dbs = self.dbs.read().map_err(|_| ErrorImpl::EnvPoisonError)?;
+        Ok(dbs.name_map.keys().map(|key| key.to_owned()).collect())
+    }
+
+    fn open_db(&self, name: Option<&str>) -> Result<Self::Database, Self::Error> {
+        if Arc::strong_count(&self.ro_txns) > 1 {
+            return Err(ErrorImpl::DbsIllegalOpen);
+        }
+        // TOOD: don't reallocate `name`.
+        let key = name.map(String::from);
+        let dbs = self.dbs.read().map_err(|_| ErrorImpl::EnvPoisonError)?;
+        let db = dbs.name_map.get(&key).ok_or(ErrorImpl::DbNotFoundError)?;
+        Ok(*db)
+    }
+
+    fn create_db(
+        &self,
+        name: Option<&str>,
+        flags: Self::Flags,
+    ) -> Result<Self::Database, Self::Error> {
+        if Arc::strong_count(&self.ro_txns) > 1 {
+            return Err(ErrorImpl::DbsIllegalOpen);
+        }
+        // TOOD: don't reallocate `name`.
+        let key = name.map(String::from);
+        let mut dbs = self.dbs.write().map_err(|_| ErrorImpl::EnvPoisonError)?;
+        if dbs.name_map.keys().filter_map(|k| k.as_ref()).count() >= self.max_dbs && name.is_some() {
+            return Err(ErrorImpl::DbsFull);
+        }
+        let parts = EnvironmentDbsRefMut::from(dbs.deref_mut());
+        let arena = parts.arena;
+        let name_map = parts.name_map;
+        let id = name_map
+            .entry(key)
+            .or_insert_with(|| DatabaseImpl(arena.alloc(Database::new(Some(flags), None))));
+        Ok(*id)
+    }
+
+    fn begin_ro_txn(&'e self) -> Result<Self::RoTransaction, Self::Error> {
+        RoTransactionImpl::new(self, self.ro_txns.clone())
+    }
+
+    fn begin_rw_txn(&'e self) -> Result<Self::RwTransaction, Self::Error> {
+        RwTransactionImpl::new(self, self.rw_txns.clone())
+    }
+
+    fn sync(&self, force: bool) -> Result<(), Self::Error> {
+        warn!("Ignoring `force={}`", force);
+        self.write_to_disk()
+    }
+
+    fn stat(&self) -> Result<Self::Stat, Self::Error> {
+        Ok(StatImpl)
+    }
+
+    fn info(&self) -> Result<Self::Info, Self::Error> {
+        Ok(InfoImpl)
+    }
+
+    fn freelist(&self) -> Result<usize, Self::Error> {
+        unimplemented!()
+    }
+
+    fn load_ratio(&self) -> Result<Option<f32>, Self::Error> {
+        warn!("`load_ratio()` is irrelevant for this storage backend.");
+        Ok(None)
+    }
+
+    fn set_map_size(&self, size: usize) -> Result<(), Self::Error> {
+        warn!(
+            "`set_map_size({})` is ignored by this storage backend.",
+            size
+        );
+        Ok(())
+    }
+
+    fn get_files_on_disk(&self) -> Vec<PathBuf> {
+        // Technically NO_SUB_DIR and NO_LOCK should change this output, but
+        // they're both currently unimplemented with this storage backend.
+        let mut db_filename = self.path.clone();
+        db_filename.push(DEFAULT_DB_FILENAME);
+        vec![db_filename]
+    }
+}
diff --git a/third_party/rust/rkv/src/backend/impl_safe/error.rs b/third_party/rust/rkv/src/backend/impl_safe/error.rs
new file mode 100644
index 0000000000..f086cdc954
--- /dev/null
+++ b/third_party/rust/rkv/src/backend/impl_safe/error.rs
@@ -0,0 +1,79 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+use std::{fmt, io, path::PathBuf};
+
+use bincode::Error as BincodeError;
+
+use crate::{backend::traits::BackendError, error::StoreError};
+
+#[derive(Debug)]
+pub enum ErrorImpl {
+    KeyValuePairNotFound,
+    EnvPoisonError,
+    DbsFull,
+    DbsIllegalOpen,
+    DbNotFoundError,
+    DbIsForeignError,
+    UnsuitableEnvironmentPath(PathBuf),
+    IoError(io::Error),
+    BincodeError(BincodeError),
+}
+
+impl BackendError for ErrorImpl {}
+
+impl fmt::Display for ErrorImpl {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            ErrorImpl::KeyValuePairNotFound => write!(fmt, "KeyValuePairNotFound (safe mode)"),
+            ErrorImpl::EnvPoisonError => write!(fmt, "EnvPoisonError (safe mode)"),
+            ErrorImpl::DbsFull => write!(fmt, "DbsFull (safe mode)"),
+            ErrorImpl::DbsIllegalOpen => write!(fmt, "DbIllegalOpen (safe mode)"),
+            ErrorImpl::DbNotFoundError => write!(fmt, "DbNotFoundError (safe mode)"),
+            ErrorImpl::DbIsForeignError => write!(fmt, "DbIsForeignError (safe mode)"),
+            ErrorImpl::UnsuitableEnvironmentPath(_) => {
+                write!(fmt, "UnsuitableEnvironmentPath (safe mode)")
+            }
+            ErrorImpl::IoError(e) => e.fmt(fmt),
+            ErrorImpl::BincodeError(e) => e.fmt(fmt),
+        }
+    }
+}
+
+impl Into<StoreError> for ErrorImpl {
+    fn into(self) -> StoreError {
+        // The `StoreError::KeyValuePairBadSize` error is unused, because this
+        // backend supports keys and values of arbitrary sizes.
+        // The `StoreError::MapFull` and `StoreError::ReadersFull` are
+        // unimplemented yet, but they should be in the future.
+        match self {
+            ErrorImpl::KeyValuePairNotFound => StoreError::KeyValuePairNotFound,
+            ErrorImpl::BincodeError(_) => StoreError::FileInvalid,
+            ErrorImpl::DbsFull => StoreError::DbsFull,
+            ErrorImpl::UnsuitableEnvironmentPath(path) => {
+                StoreError::UnsuitableEnvironmentPath(path)
+            }
+            ErrorImpl::IoError(error) => StoreError::IoError(error),
+            _ => StoreError::SafeModeError(self),
+        }
+    }
+}
+
+impl From<io::Error> for ErrorImpl {
+    fn from(e: io::Error) -> ErrorImpl {
+        ErrorImpl::IoError(e)
+    }
+}
+
+impl From<BincodeError> for ErrorImpl {
+    fn from(e: BincodeError) -> ErrorImpl {
+        ErrorImpl::BincodeError(e)
+    }
+}
diff --git a/third_party/rust/rkv/src/backend/impl_safe/flags.rs b/third_party/rust/rkv/src/backend/impl_safe/flags.rs
new file mode 100644
index 0000000000..a0d623614e
--- /dev/null
+++ b/third_party/rust/rkv/src/backend/impl_safe/flags.rs
@@ -0,0 +1,124 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+use bitflags::bitflags;
+use serde_derive::{Deserialize, Serialize};
+
+use crate::backend::{
+    common::{DatabaseFlags, EnvironmentFlags, WriteFlags},
+    traits::{BackendDatabaseFlags, BackendEnvironmentFlags, BackendFlags, BackendWriteFlags},
+};
+
+bitflags! {
+    #[derive(Default, Serialize, Deserialize)]
+    pub struct EnvironmentFlagsImpl: u32 {
+        const NIL = 0b0000_0000;
+    }
+}
+
+impl BackendFlags for EnvironmentFlagsImpl {
+    fn empty() -> EnvironmentFlagsImpl {
+        EnvironmentFlagsImpl::empty()
+    }
+}
+
+impl BackendEnvironmentFlags for EnvironmentFlagsImpl {
+    fn set(&mut self, flag: EnvironmentFlags, value: bool) {
+        self.set(flag.into(), value)
+    }
+}
+
+impl Into<EnvironmentFlagsImpl> for EnvironmentFlags {
+    fn into(self) -> EnvironmentFlagsImpl {
+        match self {
+            EnvironmentFlags::FIXED_MAP => unimplemented!(),
+            EnvironmentFlags::NO_SUB_DIR => unimplemented!(),
+            EnvironmentFlags::WRITE_MAP => unimplemented!(),
+            EnvironmentFlags::READ_ONLY => unimplemented!(),
+            EnvironmentFlags::NO_META_SYNC => unimplemented!(),
+            EnvironmentFlags::NO_SYNC => unimplemented!(),
+            EnvironmentFlags::MAP_ASYNC => unimplemented!(),
+            EnvironmentFlags::NO_TLS => unimplemented!(),
+            EnvironmentFlags::NO_LOCK => unimplemented!(),
+            EnvironmentFlags::NO_READAHEAD => unimplemented!(),
+            EnvironmentFlags::NO_MEM_INIT => unimplemented!(),
+        }
+    }
+}
+
+bitflags! {
+    #[derive(Default, Serialize, Deserialize)]
+    pub struct DatabaseFlagsImpl: u32 {
+        const NIL = 0b0000_0000;
+        #[cfg(feature = "db-dup-sort")]
+        const DUP_SORT = 0b0000_0001;
+        #[cfg(feature = "db-int-key")]
+        const INTEGER_KEY = 0b0000_0010;
+    }
+}
+
+impl BackendFlags for DatabaseFlagsImpl {
+    fn empty() -> DatabaseFlagsImpl {
+        DatabaseFlagsImpl::empty()
+    }
+}
+
+impl BackendDatabaseFlags for DatabaseFlagsImpl {
+    fn set(&mut self, flag: DatabaseFlags, value: bool) {
+        self.set(flag.into(), value)
+    }
+}
+
+impl Into<DatabaseFlagsImpl> for DatabaseFlags {
+    fn into(self) -> DatabaseFlagsImpl {
+        match self {
+            DatabaseFlags::REVERSE_KEY => unimplemented!(),
+            #[cfg(feature = "db-dup-sort")]
+            DatabaseFlags::DUP_SORT => DatabaseFlagsImpl::DUP_SORT,
+            #[cfg(feature = "db-dup-sort")]
+            DatabaseFlags::DUP_FIXED => unimplemented!(),
+            #[cfg(feature = "db-int-key")]
+            DatabaseFlags::INTEGER_KEY => DatabaseFlagsImpl::INTEGER_KEY,
+            DatabaseFlags::INTEGER_DUP => unimplemented!(),
+            DatabaseFlags::REVERSE_DUP => unimplemented!(),
+        }
+    }
+}
+
+bitflags! {
+    #[derive(Default, Serialize, Deserialize)]
+    pub struct WriteFlagsImpl: u32 {
+        const NIL = 0b0000_0000;
+    }
+}
+
+impl BackendFlags for WriteFlagsImpl {
+    fn empty() -> WriteFlagsImpl {
+        WriteFlagsImpl::empty()
+    }
+}
+
+impl BackendWriteFlags for WriteFlagsImpl {
+    fn set(&mut self, flag: WriteFlags, value: bool) {
+        self.set(flag.into(), value)
+    }
+}
+
+impl Into<WriteFlagsImpl> for WriteFlags {
+    fn into(self) -> WriteFlagsImpl {
+        match self {
+            WriteFlags::NO_OVERWRITE => unimplemented!(),
+            WriteFlags::NO_DUP_DATA => unimplemented!(),
+            WriteFlags::CURRENT => unimplemented!(),
+            WriteFlags::APPEND => unimplemented!(),
+            WriteFlags::APPEND_DUP => unimplemented!(),
+        }
+    }
+}
diff --git a/third_party/rust/rkv/src/backend/impl_safe/info.rs b/third_party/rust/rkv/src/backend/impl_safe/info.rs
new file mode 100644
index 0000000000..18f0f51da3
--- /dev/null
+++ b/third_party/rust/rkv/src/backend/impl_safe/info.rs
@@ -0,0 +1,35 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+use crate::backend::traits::BackendInfo;
+
+pub struct InfoImpl;
+
+impl BackendInfo for InfoImpl {
+    fn map_size(&self) -> usize {
+        unimplemented!()
+    }
+
+    fn last_pgno(&self) -> usize {
+        unimplemented!()
+    }
+
+    fn last_txnid(&self) -> usize {
+        unimplemented!()
+    }
+
+    fn max_readers(&self) -> usize {
+        unimplemented!()
+    }
+
+    fn num_readers(&self) -> usize {
+        unimplemented!()
+    }
+}
diff --git a/third_party/rust/rkv/src/backend/impl_safe/iter.rs b/third_party/rust/rkv/src/backend/impl_safe/iter.rs
new file mode 100644
index 0000000000..a784c00a5b
--- /dev/null
+++ b/third_party/rust/rkv/src/backend/impl_safe/iter.rs
@@ -0,0 +1,24 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+use super::ErrorImpl;
+use crate::backend::traits::BackendIter;
+
+// FIXME: Use generics instead.
+pub struct IterImpl<'i>(pub(crate) Box<dyn Iterator<Item = (&'i [u8], &'i [u8])> + 'i>);
+
+impl<'i> BackendIter<'i> for IterImpl<'i> {
+    type Error = ErrorImpl;
+
+    #[allow(clippy::type_complexity)]
+    fn next(&mut self) -> Option<Result<(&'i [u8], &'i [u8]), Self::Error>> {
+        self.0.next().map(Ok)
+    }
+}
diff --git a/third_party/rust/rkv/src/backend/impl_safe/snapshot.rs b/third_party/rust/rkv/src/backend/impl_safe/snapshot.rs
new file mode 100644
index 0000000000..95f5690d34
--- /dev/null
+++ b/third_party/rust/rkv/src/backend/impl_safe/snapshot.rs
@@ -0,0 +1,141 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+use std::{
+    collections::{BTreeMap, BTreeSet},
+    sync::Arc,
+};
+
+use serde_derive::{Deserialize, Serialize};
+
+use super::DatabaseFlagsImpl;
+
+type Key = Box<[u8]>;
+type Value = Box<[u8]>;
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct Snapshot {
+    flags: DatabaseFlagsImpl,
+    #[cfg(not(feature = "db-dup-sort"))]
+    map: Arc<BTreeMap<Key, Value>>,
+    #[cfg(feature = "db-dup-sort")]
+    map: Arc<BTreeMap<Key, BTreeSet<Value>>>,
+}
+
+impl Snapshot {
+    pub(crate) fn new(flags: Option<DatabaseFlagsImpl>) -> Snapshot {
+        Snapshot {
+            flags: flags.unwrap_or_default(),
+            map: Default::default(),
+        }
+    }
+
+    pub(crate) fn flags(&self) -> &DatabaseFlagsImpl {
+        &self.flags
+    }
+
+    pub(crate) fn clear(&mut self) {
+        self.map = Default::default();
+    }
+}
+
+#[cfg(not(feature = "db-dup-sort"))]
+impl Snapshot {
+    pub(crate) fn get(&self, key: &[u8]) -> Option<&[u8]> {
+        self.map.get(key).map(|value| value.as_ref())
+    }
+
+    pub(crate) fn put(&mut self, key: &[u8], value: &[u8]) {
+        let map = Arc::make_mut(&mut self.map);
+        map.insert(Box::from(key), Box::from(value));
+    }
+
+    pub(crate) fn del(&mut self, key: &[u8]) -> Option<()> {
+        let map = Arc::make_mut(&mut self.map);
+        map.remove(key).map(|_| ())
+    }
+
+    pub(crate) fn iter(&self) -> impl Iterator<Item = (&[u8], &[u8])> {
+        self.map
+            .iter()
+            .map(|(key, value)| (key.as_ref(), value.as_ref()))
+    }
+}
+
+#[cfg(feature = "db-dup-sort")]
+impl Snapshot {
+    pub(crate) fn get(&self, key: &[u8]) -> Option<&[u8]> {
+        self.map
+            .get(key)
+            .and_then(|v| v.iter().next())
+            .map(|v| v.as_ref())
+    }
+
+    pub(crate) fn put(&mut self, key: &[u8], value: &[u8]) {
+        let map = Arc::make_mut(&mut self.map);
+        match map.get_mut(key) {
+            None => {
+                let mut values = BTreeSet::new();
+                values.insert(Box::from(value));
+                map.insert(Box::from(key), values);
+            }
+            Some(values) => {
+                values.clear();
+                values.insert(Box::from(value));
+            }
+        }
+    }
+
+    pub(crate) fn del(&mut self, key: &[u8]) -> Option<()> {
+        let map = Arc::make_mut(&mut self.map);
+        match map.get_mut(key) {
+            None => None,
+            Some(values) => {
+                let was_empty = values.is_empty();
+                values.clear();
+                Some(()).filter(|_| !was_empty)
+            }
+        }
+    }
+
+    pub(crate) fn iter(&self) -> impl Iterator<Item = (&[u8], impl Iterator<Item = &[u8]>)> {
+        self.map
+            .iter()
+            .map(|(key, values)| (key.as_ref(), values.iter().map(|value| value.as_ref())))
+    }
+}
+
+#[cfg(feature = "db-dup-sort")]
+impl Snapshot {
+    pub(crate) fn put_dup(&mut self, key: &[u8], value: &[u8]) {
+        let map = Arc::make_mut(&mut self.map);
+        match map.get_mut(key) {
+            None => {
+                let mut values = BTreeSet::new();
+                values.insert(Box::from(value));
+                map.insert(Box::from(key), values);
+            }
+            Some(values) => {
+                values.insert(Box::from(value));
+            }
+        }
+    }
+
+    pub(crate) fn del_exact(&mut self, key: &[u8], value: &[u8]) -> Option<()> {
+        let map = Arc::make_mut(&mut self.map);
+        match map.get_mut(key) {
+            None => None,
+            Some(values) => {
+                let was_removed = values.remove(value);
+                Some(()).filter(|_| was_removed)
+            }
+        }
+    }
+}
diff --git a/third_party/rust/rkv/src/backend/impl_safe/stat.rs b/third_party/rust/rkv/src/backend/impl_safe/stat.rs
new file mode 100644
index 0000000000..c117b56833
--- /dev/null
+++ b/third_party/rust/rkv/src/backend/impl_safe/stat.rs
@@ -0,0 +1,39 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+use crate::backend::traits::BackendStat;
+
+pub struct StatImpl;
+
+impl BackendStat for StatImpl {
+    fn page_size(&self) -> usize {
+        unimplemented!()
+    }
+
+    fn depth(&self) -> usize {
+        unimplemented!()
+    }
+
+    fn branch_pages(&self) -> usize {
+        unimplemented!()
+    }
+
+    fn leaf_pages(&self) -> usize {
+        unimplemented!()
+    }
+
+    fn overflow_pages(&self) -> usize {
+        unimplemented!()
+    }
+
+    fn entries(&self) -> usize {
+        unimplemented!()
+    }
+}
diff --git a/third_party/rust/rkv/src/backend/impl_safe/transaction.rs b/third_party/rust/rkv/src/backend/impl_safe/transaction.rs
new file mode 100644
index 0000000000..d37352a42d
--- /dev/null
+++ b/third_party/rust/rkv/src/backend/impl_safe/transaction.rs
@@ -0,0 +1,208 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+#![allow(dead_code)] // TODO: Get rid of unused struct members
+
+use std::{collections::HashMap, sync::Arc};
+
+use super::{
+    snapshot::Snapshot, DatabaseImpl, EnvironmentImpl, ErrorImpl, RoCursorImpl, WriteFlagsImpl,
+};
+use crate::backend::traits::{
+    BackendRoCursorTransaction, BackendRoTransaction, BackendRwCursorTransaction,
+    BackendRwTransaction,
+};
+
+#[derive(Debug)]
+pub struct RoTransactionImpl<'t> {
+    env: &'t EnvironmentImpl,
+    snapshots: HashMap<DatabaseImpl, Snapshot>,
+    idx: Arc<()>,
+}
+
+impl<'t> RoTransactionImpl<'t> {
+    pub(crate) fn new(
+        env: &'t EnvironmentImpl,
+        idx: Arc<()>,
+    ) -> Result<RoTransactionImpl<'t>, ErrorImpl> {
+        let snapshots = env
+            .dbs()?
+            .arena
+            .iter()
+            .map(|(id, db)| (DatabaseImpl(id), db.snapshot()))
+            .collect();
+        Ok(RoTransactionImpl {
+            env,
+            snapshots,
+            idx,
+        })
+    }
+}
+
+impl<'t> BackendRoTransaction for RoTransactionImpl<'t> {
+    type Database = DatabaseImpl;
+    type Error = ErrorImpl;
+
+    fn get(&self, db: &Self::Database, key: &[u8]) -> Result<&[u8], Self::Error> {
+        let snapshot = self.snapshots.get(db).ok_or(ErrorImpl::DbIsForeignError)?;
+        snapshot.get(key).ok_or(ErrorImpl::KeyValuePairNotFound)
+    }
+
+    fn abort(self) {
+        // noop
+    }
+}
+
+impl<'t> BackendRoCursorTransaction<'t> for RoTransactionImpl<'t> {
+    type RoCursor = RoCursorImpl<'t>;
+
+    fn open_ro_cursor(&'t self, db: &Self::Database) -> Result<Self::RoCursor, Self::Error> {
+        let snapshot = self.snapshots.get(db).ok_or(ErrorImpl::DbIsForeignError)?;
+        Ok(RoCursorImpl(snapshot))
+    }
+}
+
+#[derive(Debug)]
+pub struct RwTransactionImpl<'t> {
+    env: &'t EnvironmentImpl,
+    snapshots: HashMap<DatabaseImpl, Snapshot>,
+    idx: Arc<()>,
+}
+
+impl<'t> RwTransactionImpl<'t> {
+    pub(crate) fn new(
+        env: &'t EnvironmentImpl,
+        idx: Arc<()>,
+    ) -> Result<RwTransactionImpl<'t>, ErrorImpl> {
+        let snapshots = env
+            .dbs()?
+            .arena
+            .iter()
+            .map(|(id, db)| (DatabaseImpl(id), db.snapshot()))
+            .collect();
+        Ok(RwTransactionImpl {
+            env,
+            snapshots,
+            idx,
+        })
+    }
+}
+
+impl<'t> BackendRwTransaction for RwTransactionImpl<'t> {
+    type Database = DatabaseImpl;
+    type Error = ErrorImpl;
+    type Flags = WriteFlagsImpl;
+
+    fn get(&self, db: &Self::Database, key: &[u8]) -> Result<&[u8], Self::Error> {
+        let snapshot = self.snapshots.get(db).ok_or(ErrorImpl::DbIsForeignError)?;
+        snapshot.get(key).ok_or(ErrorImpl::KeyValuePairNotFound)
+    }
+
+    #[cfg(not(feature = "db-dup-sort"))]
+    fn put(
+        &mut self,
+        db: &Self::Database,
+        key: &[u8],
+        value: &[u8],
+        _flags: Self::Flags,
+    ) -> Result<(), Self::Error> {
+        let snapshot = self
+            .snapshots
+            .get_mut(db)
+            .ok_or_else(|| ErrorImpl::DbIsForeignError)?;
+        snapshot.put(key, value);
+        Ok(())
+    }
+
+    #[cfg(feature = "db-dup-sort")]
+    fn put(
+        &mut self,
+        db: &Self::Database,
+        key: &[u8],
+        value: &[u8],
+        _flags: Self::Flags,
+    ) -> Result<(), Self::Error> {
+        use super::DatabaseFlagsImpl;
+        let snapshot = self
+            .snapshots
+            .get_mut(db)
+            .ok_or(ErrorImpl::DbIsForeignError)?;
+        if snapshot.flags().contains(DatabaseFlagsImpl::DUP_SORT) {
+            snapshot.put_dup(key, value);
+        } else {
+            snapshot.put(key, value);
+        }
+        Ok(())
+    }
+
+    #[cfg(not(feature = "db-dup-sort"))]
+    fn del(&mut self, db: &Self::Database, key: &[u8]) -> Result<(), Self::Error> {
+        let snapshot = self
+            .snapshots
+            .get_mut(db)
+            .ok_or_else(|| ErrorImpl::DbIsForeignError)?;
+        let deleted = snapshot.del(key);
+        Ok(deleted.ok_or_else(|| ErrorImpl::KeyValuePairNotFound)?)
+    }
+
+    #[cfg(feature = "db-dup-sort")]
+    fn del(
+        &mut self,
+        db: &Self::Database,
+        key: &[u8],
+        value: Option<&[u8]>,
+    ) -> Result<(), Self::Error> {
+        use super::DatabaseFlagsImpl;
+        let snapshot = self
+            .snapshots
+            .get_mut(db)
+            .ok_or(ErrorImpl::DbIsForeignError)?;
+        let deleted = match (value, snapshot.flags()) {
+            (Some(value), flags) if flags.contains(DatabaseFlagsImpl::DUP_SORT) => {
+                snapshot.del_exact(key, value)
+            }
+            _ => snapshot.del(key),
+        };
+        deleted.ok_or(ErrorImpl::KeyValuePairNotFound)
+    }
+
+    fn clear_db(&mut self, db: &Self::Database) -> Result<(), Self::Error> {
+        let snapshot = self
+            .snapshots
+            .get_mut(db)
+            .ok_or(ErrorImpl::DbIsForeignError)?;
+        snapshot.clear();
+        Ok(())
+    }
+
+    fn commit(self) -> Result<(), Self::Error> {
+        let mut dbs = self.env.dbs_mut()?;
+
+        for (id, snapshot) in self.snapshots {
+            let db = dbs.arena.get_mut(id.0).ok_or(ErrorImpl::DbIsForeignError)?;
+            db.replace(snapshot);
+        }
+
+        drop(dbs);
+        self.env.write_to_disk()
+    }
+
+    fn abort(self) {
+        // noop
+    }
+}
+
+impl<'t> BackendRwCursorTransaction<'t> for RwTransactionImpl<'t> {
+    type RoCursor = RoCursorImpl<'t>;
+
+    fn open_ro_cursor(&'t self, db: &Self::Database) -> Result<Self::RoCursor, Self::Error> {
+        let snapshot = self.snapshots.get(db).ok_or(ErrorImpl::DbIsForeignError)?;
+        Ok(RoCursorImpl(snapshot))
+    }
+}
diff --git a/third_party/rust/rkv/src/backend/traits.rs b/third_party/rust/rkv/src/backend/traits.rs
new file mode 100644
index 0000000000..1dda15a85c
--- /dev/null
+++ b/third_party/rust/rkv/src/backend/traits.rs
@@ -0,0 +1,202 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+use std::{
+    fmt::{Debug, Display},
+    path::{Path, PathBuf},
+};
+
+use crate::{
+    backend::common::{DatabaseFlags, EnvironmentFlags, WriteFlags},
+    error::StoreError,
+};
+
+pub trait BackendError: Debug + Display + Into<StoreError> {}
+
+pub trait BackendDatabase: Debug + Eq + PartialEq + Copy + Clone {}
+
+pub trait BackendFlags: Debug + Eq + PartialEq + Copy + Clone + Default {
+    fn empty() -> Self;
+}
+
+pub trait BackendEnvironmentFlags: BackendFlags {
+    fn set(&mut self, flag: EnvironmentFlags, value: bool);
+}
+
+pub trait BackendDatabaseFlags: BackendFlags {
+    fn set(&mut self, flag: DatabaseFlags, value: bool);
+}
+
+pub trait BackendWriteFlags: BackendFlags {
+    fn set(&mut self, flag: WriteFlags, value: bool);
+}
+
+pub trait BackendStat {
+    fn page_size(&self) -> usize;
+
+    fn depth(&self) -> usize;
+
+    fn branch_pages(&self) -> usize;
+
+    fn leaf_pages(&self) -> usize;
+
+    fn overflow_pages(&self) -> usize;
+
+    fn entries(&self) -> usize;
+}
+
+pub trait BackendInfo {
+    fn map_size(&self) -> usize;
+
+    fn last_pgno(&self) -> usize;
+
+    fn last_txnid(&self) -> usize;
+
+    fn max_readers(&self) -> usize;
+
+    fn num_readers(&self) -> usize;
+}
+
+pub trait BackendEnvironmentBuilder<'b>: Debug + Eq + PartialEq + Copy + Clone {
+    type Error: BackendError;
+    type Environment: BackendEnvironment<'b>;
+    type Flags: BackendEnvironmentFlags;
+
+    fn new() -> Self;
+
+    fn set_flags<T>(&mut self, flags: T) -> &mut Self
+    where
+        T: Into<Self::Flags>;
+
+    fn set_max_dbs(&mut self, max_dbs: u32) -> &mut Self;
+
+    fn set_max_readers(&mut self, max_readers: u32) -> &mut Self;
+
+    fn set_map_size(&mut self, size: usize) -> &mut Self;
+
+    fn set_make_dir_if_needed(&mut self, make_dir_if_needed: bool) -> &mut Self;
+
+    fn set_discard_if_corrupted(&mut self, discard_if_corrupted: bool) -> &mut Self;
+
+    fn open(&self, path: &Path) -> Result<Self::Environment, Self::Error>;
+}
+
+pub trait BackendEnvironment<'e>: Debug {
+    type Error: BackendError;
+    type Database: BackendDatabase;
+    type Flags: BackendDatabaseFlags;
+    type Stat: BackendStat;
+    type Info: BackendInfo;
+    type RoTransaction: BackendRoCursorTransaction<'e, Database = Self::Database>;
+    type RwTransaction: BackendRwCursorTransaction<'e, Database = Self::Database>;
+
+    fn get_dbs(&self) -> Result<Vec<Option<String>>, Self::Error>;
+
+    fn open_db(&self, name: Option<&str>) -> Result<Self::Database, Self::Error>;
+
+    fn create_db(
+        &self,
+        name: Option<&str>,
+        flags: Self::Flags,
+    ) -> Result<Self::Database, Self::Error>;
+
+    fn begin_ro_txn(&'e self) -> Result<Self::RoTransaction, Self::Error>;
+
+    fn begin_rw_txn(&'e self) -> Result<Self::RwTransaction, Self::Error>;
+
+    fn sync(&self, force: bool) -> Result<(), Self::Error>;
+
+    fn stat(&self) -> Result<Self::Stat, Self::Error>;
+
+    fn info(&self) -> Result<Self::Info, Self::Error>;
+
+    fn freelist(&self) -> Result<usize, Self::Error>;
+
+    fn load_ratio(&self) -> Result<Option<f32>, Self::Error>;
+
+    fn set_map_size(&self, size: usize) -> Result<(), Self::Error>;
+
+    fn get_files_on_disk(&self) -> Vec<PathBuf>;
+}
+
+pub trait BackendRoTransaction: Debug {
+    type Error: BackendError;
+    type Database: BackendDatabase;
+
+    fn get(&self, db: &Self::Database, key: &[u8]) -> Result<&[u8], Self::Error>;
+
+    fn abort(self);
+}
+
+pub trait BackendRwTransaction: Debug {
+    type Error: BackendError;
+    type Database: BackendDatabase;
+    type Flags: BackendWriteFlags;
+
+    fn get(&self, db: &Self::Database, key: &[u8]) -> Result<&[u8], Self::Error>;
+
+    fn put(
+        &mut self,
+        db: &Self::Database,
+        key: &[u8],
+        value: &[u8],
+        flags: Self::Flags,
+    ) -> Result<(), Self::Error>;
+
+    #[cfg(not(feature = "db-dup-sort"))]
+    fn del(&mut self, db: &Self::Database, key: &[u8]) -> Result<(), Self::Error>;
+
+    #[cfg(feature = "db-dup-sort")]
+    fn del(
+        &mut self,
+        db: &Self::Database,
+        key: &[u8],
+        value: Option<&[u8]>,
+    ) -> Result<(), Self::Error>;
+
+    fn clear_db(&mut self, db: &Self::Database) -> Result<(), Self::Error>;
+
+    fn commit(self) -> Result<(), Self::Error>;
+
+    fn abort(self);
+}
+
+pub trait BackendRoCursorTransaction<'t>: BackendRoTransaction {
+    type RoCursor: BackendRoCursor<'t>;
+
+    fn open_ro_cursor(&'t self, db: &Self::Database) -> Result<Self::RoCursor, Self::Error>;
+}
+
+pub trait BackendRwCursorTransaction<'t>: BackendRwTransaction {
+    type RoCursor: BackendRoCursor<'t>;
+
+    fn open_ro_cursor(&'t self, db: &Self::Database) -> Result<Self::RoCursor, Self::Error>;
+}
+
+pub trait BackendRoCursor<'c>: Debug {
+    type Iter: BackendIter<'c>;
+
+    fn into_iter(self) -> Self::Iter;
+
+    fn into_iter_from<K>(self, key: K) -> Self::Iter
+    where
+        K: AsRef<[u8]> + 'c;
+
+    fn into_iter_dup_of<K>(self, key: K) -> Self::Iter
+    where
+        K: AsRef<[u8]> + 'c;
+}
+
+pub trait BackendIter<'i> {
+    type Error: BackendError;
+
+    #[allow(clippy::type_complexity)]
+    fn next(&mut self) -> Option<Result<(&'i [u8], &'i [u8]), Self::Error>>;
+}
diff --git a/third_party/rust/rkv/src/bin/dump.rs b/third_party/rust/rkv/src/bin/dump.rs
new file mode 100644
index 0000000000..c0e12b986f
--- /dev/null
+++ b/third_party/rust/rkv/src/bin/dump.rs
@@ -0,0 +1,47 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+use std::{env::args, io, path::Path};
+
+use rkv::migrator::{LmdbArchMigrateError, LmdbArchMigrator};
+
+fn main() -> Result<(), LmdbArchMigrateError> {
+    let mut cli_args = args();
+    let mut db_name = None;
+    let mut env_path = None;
+
+    // The first arg is the name of the program, which we can ignore.
+    cli_args.next();
+
+    while let Some(arg) = cli_args.next() {
+        if &arg[0..1] == "-" {
+            match &arg[1..] {
+                "s" => {
+                    db_name = match cli_args.next() {
+                        None => return Err("-s must be followed by database name".into()),
+                        Some(str) => Some(str),
+                    };
+                }
+                str => return Err(format!("arg -{str} not recognized").into()),
+            }
+        } else {
+            if env_path.is_some() {
+                return Err("must provide only one path to the LMDB environment".into());
+            }
+            env_path = Some(arg);
+        }
+    }
+
+    let env_path = env_path.ok_or("must provide a path to the LMDB environment")?;
+    let mut migrator = LmdbArchMigrator::new(Path::new(&env_path))?;
+    migrator.dump(db_name.as_deref(), io::stdout()).unwrap();
+
+    Ok(())
+}
diff --git a/third_party/rust/rkv/src/bin/rand.rs b/third_party/rust/rkv/src/bin/rand.rs
new file mode 100644
index 0000000000..56d578c22e
--- /dev/null
+++ b/third_party/rust/rkv/src/bin/rand.rs
@@ -0,0 +1,106 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+//! A command-line utility to create an LMDB environment containing random data.
+//! It requires one flag, `-s path/to/environment`, which specifies the location
+//! where the tool should create the environment.  Optionally, you may specify
+//! the number of key/value pairs to create via the `-n <number>` flag
+//! (for which the default value is 50).
+
+use std::{env::args, fs, fs::File, io::Read, path::Path};
+
+use rkv::{
+    backend::{BackendEnvironmentBuilder, Lmdb},
+    Rkv, StoreOptions, Value,
+};
+
+fn main() {
+    let mut args = args();
+    let mut database = None;
+    let mut path = None;
+    let mut num_pairs = 50;
+
+    // The first arg is the name of the program, which we can ignore.
+    args.next();
+
+    while let Some(arg) = args.next() {
+        if &arg[0..1] == "-" {
+            match &arg[1..] {
+                "s" => {
+                    database = match args.next() {
+                        None => panic!("-s must be followed by database arg"),
+                        Some(str) => Some(str),
+                    };
+                }
+                "n" => {
+                    num_pairs = match args.next() {
+                        None => panic!("-s must be followed by number of pairs"),
+                        Some(str) => str.parse().expect("number"),
+                    };
+                }
+                str => panic!("arg -{} not recognized", str),
+            }
+        } else {
+            if path.is_some() {
+                panic!("must provide only one path to the LMDB environment");
+            }
+            path = Some(arg);
+        }
+    }
+
+    if path.is_none() {
+        panic!("must provide a path to the LMDB environment");
+    }
+
+    let path = path.unwrap();
+    fs::create_dir_all(&path).expect("dir created");
+
+    let mut builder = Rkv::environment_builder::<Lmdb>();
+    builder.set_max_dbs(2);
+    // Allocate enough map to accommodate the largest random collection.
+    // We currently do this by allocating twice the maximum possible size
+    // of the pairs (assuming maximum key and value sizes).
+    builder.set_map_size((511 + 65535) * num_pairs * 2);
+    let rkv = Rkv::from_builder(Path::new(&path), builder).expect("Rkv");
+    let store = rkv
+        .open_single(database.as_deref(), StoreOptions::create())
+        .expect("opened");
+    let mut writer = rkv.write().expect("writer");
+
+    // Generate random values for the number of keys and key/value lengths.
+    // On Linux, "Just use /dev/urandom!" <https://www.2uo.de/myths-about-urandom/>.
+    // On macOS it doesn't matter (/dev/random and /dev/urandom are identical).
+    let mut random = File::open("/dev/urandom").unwrap();
+    let mut nums = [0u8; 4];
+    random.read_exact(&mut nums).unwrap();
+
+    // Generate 0–255 pairs.
+    for _ in 0..num_pairs {
+        // Generate key and value lengths.  The key must be 1–511 bytes long.
+        // The value length can be 0 and is essentially unbounded; we generate
+        // value lengths of 0–0xffff (65535).
+        // NB: the modulus method for generating a random number within a range
+        // introduces distribution skew, but we don't need it to be perfect.
+        let key_len = ((u16::from(nums[0]) + (u16::from(nums[1]) << 8)) % 511 + 1) as usize;
+        let value_len = (u16::from(nums[2]) + (u16::from(nums[3]) << 8)) as usize;
+
+        let mut key: Vec<u8> = vec![0; key_len];
+        random.read_exact(&mut key[0..key_len]).unwrap();
+
+        let mut value: Vec<u8> = vec![0; value_len];
+        random.read_exact(&mut value[0..value_len]).unwrap();
+
+        store
+            .put(&mut writer, key, &Value::Blob(&value))
+            .expect("wrote");
+    }
+
+    writer.commit().expect("committed");
+}
diff --git a/third_party/rust/rkv/src/env.rs b/third_party/rust/rkv/src/env.rs
new file mode 100644
index 0000000000..32e4fe5035
--- /dev/null
+++ b/third_party/rust/rkv/src/env.rs
@@ -0,0 +1,320 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+use std::{
+    fs,
+    os::raw::c_uint,
+    path::{Path, PathBuf},
+};
+
+#[cfg(any(feature = "db-dup-sort", feature = "db-int-key"))]
+use crate::backend::{BackendDatabaseFlags, DatabaseFlags};
+use crate::{
+    backend::{
+        BackendEnvironment, BackendEnvironmentBuilder, BackendRoCursorTransaction,
+        BackendRwCursorTransaction, SafeModeError,
+    },
+    error::{CloseError, StoreError},
+    readwrite::{Reader, Writer},
+    store::{single::SingleStore, CloseOptions, Options as StoreOptions},
+};
+
+#[cfg(feature = "db-dup-sort")]
+use crate::store::multi::MultiStore;
+
+#[cfg(feature = "db-int-key")]
+use crate::store::integer::IntegerStore;
+#[cfg(feature = "db-int-key")]
+use crate::store::keys::PrimitiveInt;
+
+#[cfg(all(feature = "db-dup-sort", feature = "db-int-key"))]
+use crate::store::integermulti::MultiIntegerStore;
+
+pub static DEFAULT_MAX_DBS: c_uint = 5;
+
+/// Wrapper around an `Environment` (e.g. such as an `LMDB` or `SafeMode` environment).
+#[derive(Debug)]
+pub struct Rkv<E> {
+    _path: PathBuf,
+    env: E,
+}
+
+/// Static methods.
+impl<'e, E> Rkv<E>
+where
+    E: BackendEnvironment<'e>,
+{
+    pub fn environment_builder<B>() -> B
+    where
+        B: BackendEnvironmentBuilder<'e, Environment = E>,
+    {
+        B::new()
+    }
+
+    /// Return a new Rkv environment that supports up to `DEFAULT_MAX_DBS` open databases.
+    #[allow(clippy::new_ret_no_self)]
+    pub fn new<B>(path: &Path) -> Result<Rkv<E>, StoreError>
+    where
+        B: BackendEnvironmentBuilder<'e, Environment = E>,
+    {
+        Rkv::with_capacity::<B>(path, DEFAULT_MAX_DBS)
+    }
+
+    /// Return a new Rkv environment that supports the specified number of open databases.
+    pub fn with_capacity<B>(path: &Path, max_dbs: c_uint) -> Result<Rkv<E>, StoreError>
+    where
+        B: BackendEnvironmentBuilder<'e, Environment = E>,
+    {
+        let mut builder = B::new();
+        builder.set_max_dbs(max_dbs);
+
+        // Future: set flags, maximum size, etc. here if necessary.
+        Rkv::from_builder(path, builder)
+    }
+
+    /// Return a new Rkv environment from the provided builder.
+    pub fn from_builder<B>(path: &Path, builder: B) -> Result<Rkv<E>, StoreError>
+    where
+        B: BackendEnvironmentBuilder<'e, Environment = E>,
+    {
+        Ok(Rkv {
+            _path: path.into(),
+            env: builder.open(path).map_err(|e| e.into())?,
+        })
+    }
+}
+
+/// Store creation methods.
+impl<'e, E> Rkv<E>
+where
+    E: BackendEnvironment<'e>,
+{
+    /// Return all created databases.
+    pub fn get_dbs(&self) -> Result<Vec<Option<String>>, StoreError> {
+        self.env.get_dbs().map_err(|e| e.into())
+    }
+
+    /// Create or Open an existing database in (&[u8] -> Single Value) mode.
+    /// Note: that create=true cannot be called concurrently with other operations so if
+    /// you are sure that the database exists, call this with create=false.
+    pub fn open_single<'s, T>(
+        &self,
+        name: T,
+        opts: StoreOptions<E::Flags>,
+    ) -> Result<SingleStore<E::Database>, StoreError>
+    where
+        T: Into<Option<&'s str>>,
+    {
+        self.open(name, opts).map(SingleStore::new)
+    }
+
+    /// Create or Open an existing database in (Integer -> Single Value) mode.
+    /// Note: that create=true cannot be called concurrently with other operations so if
+    /// you are sure that the database exists, call this with create=false.
+    #[cfg(feature = "db-int-key")]
+    pub fn open_integer<'s, T, K>(
+        &self,
+        name: T,
+        mut opts: StoreOptions<E::Flags>,
+    ) -> Result<IntegerStore<E::Database, K>, StoreError>
+    where
+        K: PrimitiveInt,
+        T: Into<Option<&'s str>>,
+    {
+        opts.flags.set(DatabaseFlags::INTEGER_KEY, true);
+        self.open(name, opts).map(IntegerStore::new)
+    }
+
+    /// Create or Open an existing database in (&[u8] -> Multiple Values) mode.
+    /// Note: that create=true cannot be called concurrently with other operations so if
+    /// you are sure that the database exists, call this with create=false.
+    #[cfg(feature = "db-dup-sort")]
+    pub fn open_multi<'s, T>(
+        &self,
+        name: T,
+        mut opts: StoreOptions<E::Flags>,
+    ) -> Result<MultiStore<E::Database>, StoreError>
+    where
+        T: Into<Option<&'s str>>,
+    {
+        opts.flags.set(DatabaseFlags::DUP_SORT, true);
+        self.open(name, opts).map(MultiStore::new)
+    }
+
+    /// Create or Open an existing database in (Integer -> Multiple Values) mode.
+    /// Note: that create=true cannot be called concurrently with other operations so if
+    /// you are sure that the database exists, call this with create=false.
+    #[cfg(all(feature = "db-dup-sort", feature = "db-int-key"))]
+    pub fn open_multi_integer<'s, T, K>(
+        &self,
+        name: T,
+        mut opts: StoreOptions<E::Flags>,
+    ) -> Result<MultiIntegerStore<E::Database, K>, StoreError>
+    where
+        K: PrimitiveInt,
+        T: Into<Option<&'s str>>,
+    {
+        opts.flags.set(DatabaseFlags::INTEGER_KEY, true);
+        opts.flags.set(DatabaseFlags::DUP_SORT, true);
+        self.open(name, opts).map(MultiIntegerStore::new)
+    }
+
+    fn open<'s, T>(&self, name: T, opts: StoreOptions<E::Flags>) -> Result<E::Database, StoreError>
+    where
+        T: Into<Option<&'s str>>,
+    {
+        if opts.create {
+            self.env
+                .create_db(name.into(), opts.flags)
+                .map_err(|e| match e.into() {
+                    #[cfg(feature = "lmdb")]
+                    StoreError::LmdbError(lmdb::Error::BadRslot) => {
+                        StoreError::open_during_transaction()
+                    }
+                    StoreError::SafeModeError(SafeModeError::DbsIllegalOpen) => {
+                        StoreError::open_during_transaction()
+                    }
+                    e => e,
+                })
+        } else {
+            self.env.open_db(name.into()).map_err(|e| match e.into() {
+                #[cfg(feature = "lmdb")]
+                StoreError::LmdbError(lmdb::Error::BadRslot) => {
+                    StoreError::open_during_transaction()
+                }
+                StoreError::SafeModeError(SafeModeError::DbsIllegalOpen) => {
+                    StoreError::open_during_transaction()
+                }
+                e => e,
+            })
+        }
+    }
+}
+
+/// Read and write accessors.
+impl<'e, E> Rkv<E>
+where
+    E: BackendEnvironment<'e>,
+{
+    /// Create a read transaction.  There can be multiple concurrent readers for an
+    /// environment, up to the maximum specified by LMDB (default 126), and you can open
+    /// readers while a write transaction is active.
+    pub fn read<T>(&'e self) -> Result<Reader<T>, StoreError>
+    where
+        E: BackendEnvironment<'e, RoTransaction = T>,
+        T: BackendRoCursorTransaction<'e, Database = E::Database>,
+    {
+        Ok(Reader::new(self.env.begin_ro_txn().map_err(|e| e.into())?))
+    }
+
+    /// Create a write transaction.  There can be only one write transaction active at any
+    /// given time, so trying to create a second one will block until the first is
+    /// committed or aborted.
+    pub fn write<T>(&'e self) -> Result<Writer<T>, StoreError>
+    where
+        E: BackendEnvironment<'e, RwTransaction = T>,
+        T: BackendRwCursorTransaction<'e, Database = E::Database>,
+    {
+        Ok(Writer::new(self.env.begin_rw_txn().map_err(|e| e.into())?))
+    }
+}
+
+/// Other environment methods.
+impl<'e, E> Rkv<E>
+where
+    E: BackendEnvironment<'e>,
+{
+    /// Flush the data buffers to disk. This call is only useful, when the environment was
+    /// open with either `NO_SYNC`, `NO_META_SYNC` or `MAP_ASYNC` (see below). The call is
+    /// not valid if the environment was opened with `READ_ONLY`.
+    ///
+    /// Data is always written to disk when `transaction.commit()` is called, but the
+    /// operating system may keep it buffered. LMDB always flushes the OS buffers upon
+    /// commit as well, unless the environment was opened with `NO_SYNC` or in part
+    /// `NO_META_SYNC`.
+    ///
+    /// `force`: if true, force a synchronous flush. Otherwise if the environment has the
+    /// `NO_SYNC` flag set the flushes will be omitted, and with `MAP_ASYNC` they will
+    /// be asynchronous.
+    pub fn sync(&self, force: bool) -> Result<(), StoreError> {
+        self.env.sync(force).map_err(|e| e.into())
+    }
+
+    /// Retrieve statistics about this environment.
+    ///
+    /// It includes:
+    ///   * Page size in bytes
+    ///   * B-tree depth
+    ///   * Number of internal (non-leaf) pages
+    ///   * Number of leaf pages
+    ///   * Number of overflow pages
+    ///   * Number of data entries
+    pub fn stat(&self) -> Result<E::Stat, StoreError> {
+        self.env.stat().map_err(|e| e.into())
+    }
+
+    /// Retrieve information about this environment.
+    ///
+    /// It includes:
+    ///   * Map size in bytes
+    ///   * The last used page number
+    ///   * The last transaction ID
+    ///   * Max number of readers allowed
+    ///   * Number of readers in use
+    pub fn info(&self) -> Result<E::Info, StoreError> {
+        self.env.info().map_err(|e| e.into())
+    }
+
+    /// Retrieve the load ratio (# of used pages / total pages) about this environment.
+    ///
+    /// With the formular: (last_page_no - freelist_pages) / total_pages.
+    /// A value of `None` means that the backend doesn't ever need to be resized.
+    pub fn load_ratio(&self) -> Result<Option<f32>, StoreError> {
+        self.env.load_ratio().map_err(|e| e.into())
+    }
+
+    /// Sets the size of the memory map to use for the environment.
+    ///
+    /// This can be used to resize the map when the environment is already open. You can
+    /// also use `Rkv::environment_builder()` to set the map size during the `Rkv`
+    /// initialization.
+    ///
+    /// Note:
+    ///
+    /// * No active transactions allowed when performing resizing in this process. It's up
+    ///   to the consumer to enforce that.
+    ///
+    /// * The size should be a multiple of the OS page size. Any attempt to set a size
+    ///   smaller than the space already consumed by the environment will be silently
+    ///   changed to the current size of the used space.
+    ///
+    /// * In the multi-process case, once a process resizes the map, other processes need
+    ///   to either re-open the environment, or call set_map_size with size 0 to update
+    ///   the environment. Otherwise, new transaction creation will fail with
+    ///   `LmdbError::MapResized`.
+    pub fn set_map_size(&self, size: usize) -> Result<(), StoreError> {
+        self.env.set_map_size(size).map_err(Into::into)
+    }
+
+    /// Closes this environment and optionally deletes all its files from disk. Doesn't
+    /// delete the folder used when opening the environment.
+    pub fn close(self, options: CloseOptions) -> Result<(), CloseError> {
+        let files = self.env.get_files_on_disk();
+        drop(self);
+
+        if options.delete {
+            for file in files {
+                fs::remove_file(file)?;
+            }
+        }
+
+        Ok(())
+    }
+}
diff --git a/third_party/rust/rkv/src/error.rs b/third_party/rust/rkv/src/error.rs
new file mode 100644
index 0000000000..f50dda73e4
--- /dev/null
+++ b/third_party/rust/rkv/src/error.rs
@@ -0,0 +1,150 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+use std::{io, path::PathBuf, sync, thread, thread::ThreadId};
+
+use thiserror::Error;
+
+pub use crate::backend::SafeModeError;
+use crate::value::Type;
+
+#[derive(Debug, Error)]
+pub enum DataError {
+    #[error("unknown type tag: {0}")]
+    UnknownType(u8),
+
+    #[error("unexpected type tag: expected {expected}, got {actual}")]
+    UnexpectedType { expected: Type, actual: Type },
+
+    #[error("empty data; expected tag")]
+    Empty,
+
+    #[error("invalid value for type {value_type}: {err}")]
+    DecodingError {
+        value_type: Type,
+        err: Box<bincode::ErrorKind>,
+    },
+
+    #[error("couldn't encode value: {0}")]
+    EncodingError(#[from] Box<bincode::ErrorKind>),
+
+    #[error("invalid uuid bytes")]
+    InvalidUuid,
+}
+
+#[derive(Debug, Error)]
+pub enum StoreError {
+    #[error("manager poisoned")]
+    ManagerPoisonError,
+
+    #[error("database corrupted")]
+    DatabaseCorrupted,
+
+    #[error("key/value pair not found")]
+    KeyValuePairNotFound,
+
+    #[error("unsupported size of key/DB name/data")]
+    KeyValuePairBadSize,
+
+    #[error("file is not a valid database")]
+    FileInvalid,
+
+    #[error("environment mapsize reached")]
+    MapFull,
+
+    #[error("environment maxdbs reached")]
+    DbsFull,
+
+    #[error("environment maxreaders reached")]
+    ReadersFull,
+
+    #[error("I/O error: {0:?}")]
+    IoError(#[from] io::Error),
+
+    #[error("environment path does not exist or not the right type: {0:?}")]
+    UnsuitableEnvironmentPath(PathBuf),
+
+    #[error("data error: {0:?}")]
+    DataError(#[from] DataError),
+
+    #[cfg(feature = "lmdb")]
+    #[error("lmdb backend error: {0}")]
+    LmdbError(lmdb::Error),
+
+    #[error("safe mode backend error: {0}")]
+    SafeModeError(SafeModeError),
+
+    #[error("read transaction already exists in thread {0:?}")]
+    ReadTransactionAlreadyExists(ThreadId),
+
+    #[error("attempted to open DB during transaction in thread {0:?}")]
+    OpenAttemptedDuringTransaction(ThreadId),
+}
+
+impl StoreError {
+    pub fn open_during_transaction() -> StoreError {
+        StoreError::OpenAttemptedDuringTransaction(thread::current().id())
+    }
+
+    pub fn read_transaction_already_exists() -> StoreError {
+        StoreError::ReadTransactionAlreadyExists(thread::current().id())
+    }
+}
+
+impl<T> From<sync::PoisonError<T>> for StoreError {
+    fn from(_: sync::PoisonError<T>) -> StoreError {
+        StoreError::ManagerPoisonError
+    }
+}
+
+#[derive(Debug, Error)]
+pub enum CloseError {
+    #[error("manager poisoned")]
+    ManagerPoisonError,
+
+    #[error("close attempted while manager has an environment still open")]
+    EnvironmentStillOpen,
+
+    #[error("close attempted while an environment not known to the manager is still open")]
+    UnknownEnvironmentStillOpen,
+
+    #[error("I/O error: {0:?}")]
+    IoError(#[from] io::Error),
+}
+
+impl<T> From<sync::PoisonError<T>> for CloseError {
+    fn from(_: sync::PoisonError<T>) -> CloseError {
+        CloseError::ManagerPoisonError
+    }
+}
+
+#[derive(Debug, Error)]
+pub enum MigrateError {
+    #[error("store error: {0}")]
+    StoreError(#[from] StoreError),
+
+    #[error("close error: {0}")]
+    CloseError(#[from] CloseError),
+
+    #[error("manager poisoned")]
+    ManagerPoisonError,
+
+    #[error("source is empty")]
+    SourceEmpty,
+
+    #[error("destination is not empty")]
+    DestinationNotEmpty,
+}
+
+impl<T> From<sync::PoisonError<T>> for MigrateError {
+    fn from(_: sync::PoisonError<T>) -> MigrateError {
+        MigrateError::ManagerPoisonError
+    }
+}
diff --git a/third_party/rust/rkv/src/helpers.rs b/third_party/rust/rkv/src/helpers.rs
new file mode 100644
index 0000000000..89dcdf8dd7
--- /dev/null
+++ b/third_party/rust/rkv/src/helpers.rs
@@ -0,0 +1,43 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+use std::{
+    io,
+    path::{Path, PathBuf},
+};
+
+use url::Url;
+
+use crate::{error::StoreError, value::Value};
+
+pub(crate) fn read_transform(value: Result<&[u8], StoreError>) -> Result<Value, StoreError> {
+    match value {
+        Ok(bytes) => Value::from_tagged_slice(bytes).map_err(StoreError::DataError),
+        Err(e) => Err(e),
+    }
+}
+
+// Workaround the UNC path on Windows, see https://github.com/rust-lang/rust/issues/42869.
+// Otherwise, `Env::from_builder()` will panic with error_no(123).
+pub(crate) fn canonicalize_path<'p, P>(path: P) -> io::Result<PathBuf>
+where
+    P: Into<&'p Path>,
+{
+    let canonical = path.into().canonicalize()?;
+
+    Ok(if cfg!(target_os = "windows") {
+        let map_err = |_| io::Error::new(io::ErrorKind::Other, "path canonicalization error");
+        Url::from_file_path(&canonical)
+            .and_then(|url| url.to_file_path())
+            .map_err(map_err)?
+    } else {
+        canonical
+    })
+}
diff --git a/third_party/rust/rkv/src/lib.rs b/third_party/rust/rkv/src/lib.rs
new file mode 100644
index 0000000000..0c8951e191
--- /dev/null
+++ b/third_party/rust/rkv/src/lib.rs
@@ -0,0 +1,236 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+#![allow(clippy::from_over_into)] // TODO: `Into` implementations in [safe/lmdb]/flags.rs
+
+//! A simple, humane, typed key-value storage solution. It supports multiple backend
+//! engines with varying guarantees, such as [LMDB](http://www.lmdb.tech/doc/) for
+//! performance, or "SafeMode" for reliability.
+//!
+//! It aims to achieve the following:
+//!
+//! - Avoid sharp edges (e.g., obscure error codes for common situations).
+//! - Correctly restrict access to one handle per process via a
+//!   [Manager](struct.Manager.html).
+//! - Use Rust's type system to make single-typed key stores safe and ergonomic.
+//! - Encode and decode values via [bincode](https://docs.rs/bincode/)/[serde](https://docs.rs/serde/)
+//!   and type tags, achieving platform-independent storage and input/output flexibility.
+//!
+//! It exposes these primary abstractions:
+//!
+//! - [Manager](struct.Manager.html): a singleton that controls access to environments
+//! - [Rkv](struct.Rkv.html): an environment contains a set of key/value databases
+//! - [SingleStore](store/single/struct.SingleStore.html): a database contains a set of
+//!   key/value pairs
+//!
+//! Keys can be anything that implements `AsRef<[u8]>` or integers
+//! (when accessing an [IntegerStore](store/integer/struct.IntegerStore.html)).
+//!
+//! Values can be any of the types defined by the [Value](value/enum.Value.html) enum,
+//! including:
+//!
+//! - booleans (`Value::Bool`)
+//! - integers (`Value::I64`, `Value::U64`)
+//! - floats (`Value::F64`)
+//! - strings (`Value::Str`)
+//! - blobs (`Value::Blob`)
+//!
+//! See [Value](value/enum.Value.html) for the complete list of supported types.
+//!
+//! ## Basic Usage
+//! ```
+//! use rkv::{Manager, Rkv, SingleStore, Value, StoreOptions};
+//! use rkv::backend::{SafeMode, SafeModeEnvironment};
+//! use std::fs;
+//! use tempfile::Builder;
+//!
+//! // First determine the path to the environment, which is represented on disk as a
+//! // directory containing two files:
+//! //
+//! //   * a data file containing the key/value stores
+//! //   * a lock file containing metadata about current transactions
+//! //
+//! // In this example, we use the `tempfile` crate to create the directory.
+//! //
+//! let root = Builder::new().prefix("simple-db").tempdir().unwrap();
+//! fs::create_dir_all(root.path()).unwrap();
+//! let path = root.path();
+//!
+//! // The `Manager` enforces that each process opens the same environment at most once by
+//! // caching a handle to each environment that it opens. Use it to retrieve the handle
+//! // to an opened environment—or create one if it hasn't already been opened:
+//! let mut manager = Manager::<SafeModeEnvironment>::singleton().write().unwrap();
+//! let created_arc = manager.get_or_create(path, Rkv::new::<SafeMode>).unwrap();
+//! let env = created_arc.read().unwrap();
+//!
+//! // Then you can use the environment handle to get a handle to a datastore:
+//! let store = env.open_single("mydb", StoreOptions::create()).unwrap();
+//!
+//! {
+//!     // Use a write transaction to mutate the store via a `Writer`. There can be only
+//!     // one writer for a given environment, so opening a second one will block until
+//!     // the first completes.
+//!     let mut writer = env.write().unwrap();
+//!
+//!     // Keys are `AsRef<[u8]>`, while values are `Value` enum instances. Use the `Blob`
+//!     // variant to store arbitrary collections of bytes. Putting data returns a
+//!     // `Result<(), StoreError>`, where StoreError is an enum identifying the reason
+//!     // for a failure.
+//!     store.put(&mut writer, "int", &Value::I64(1234)).unwrap();
+//!     store.put(&mut writer, "uint", &Value::U64(1234_u64)).unwrap();
+//!     store.put(&mut writer, "float", &Value::F64(1234.0.into())).unwrap();
+//!     store.put(&mut writer, "instant", &Value::Instant(1528318073700)).unwrap();
+//!     store.put(&mut writer, "boolean", &Value::Bool(true)).unwrap();
+//!     store.put(&mut writer, "string", &Value::Str("Héllo, wörld!")).unwrap();
+//!     store.put(&mut writer, "json", &Value::Json(r#"{"foo":"bar", "number": 1}"#)).unwrap();
+//!     store.put(&mut writer, "blob", &Value::Blob(b"blob")).unwrap();
+//!
+//!     // You must commit a write transaction before the writer goes out of scope, or the
+//!     // transaction will abort and the data won't persist.
+//!     writer.commit().unwrap();
+//! }
+//!
+//! {
+//!     // Use a read transaction to query the store via a `Reader`. There can be multiple
+//!     // concurrent readers for a store, and readers never block on a writer nor other
+//!     // readers.
+//!     let reader = env.read().expect("reader");
+//!
+//!     // Keys are `AsRef<u8>`, and the return value is `Result<Option<Value>, StoreError>`.
+//!     println!("Get int {:?}", store.get(&reader, "int").unwrap());
+//!     println!("Get uint {:?}", store.get(&reader, "uint").unwrap());
+//!     println!("Get float {:?}", store.get(&reader, "float").unwrap());
+//!     println!("Get instant {:?}", store.get(&reader, "instant").unwrap());
+//!     println!("Get boolean {:?}", store.get(&reader, "boolean").unwrap());
+//!     println!("Get string {:?}", store.get(&reader, "string").unwrap());
+//!     println!("Get json {:?}", store.get(&reader, "json").unwrap());
+//!     println!("Get blob {:?}", store.get(&reader, "blob").unwrap());
+//!
+//!     // Retrieving a non-existent value returns `Ok(None)`.
+//!     println!("Get non-existent value {:?}", store.get(&reader, "non-existent").unwrap());
+//!
+//!     // A read transaction will automatically close once the reader goes out of scope,
+//!     // so isn't necessary to close it explicitly, although you can do so by calling
+//!     // `Reader.abort()`.
+//! }
+//!
+//! {
+//!     // Aborting a write transaction rolls back the change(s).
+//!     let mut writer = env.write().unwrap();
+//!     store.put(&mut writer, "foo", &Value::Str("bar")).unwrap();
+//!     writer.abort();
+//!     let reader = env.read().expect("reader");
+//!     println!("It should be None! ({:?})", store.get(&reader, "foo").unwrap());
+//! }
+//!
+//! {
+//!     // Explicitly aborting a transaction is not required unless an early abort is
+//!     // desired, since both read and write transactions will implicitly be aborted once
+//!     // they go out of scope.
+//!     {
+//!         let mut writer = env.write().unwrap();
+//!         store.put(&mut writer, "foo", &Value::Str("bar")).unwrap();
+//!     }
+//!     let reader = env.read().expect("reader");
+//!     println!("It should be None! ({:?})", store.get(&reader, "foo").unwrap());
+//! }
+//!
+//! {
+//!     // Deleting a key/value pair also requires a write transaction.
+//!     let mut writer = env.write().unwrap();
+//!     store.put(&mut writer, "foo", &Value::Str("bar")).unwrap();
+//!     store.put(&mut writer, "bar", &Value::Str("baz")).unwrap();
+//!     store.delete(&mut writer, "foo").unwrap();
+//!
+//!     // A write transaction also supports reading, and the version of the store that it
+//!     // reads includes the changes it has made regardless of the commit state of that
+//!     // transaction.
+
+//!     // In the code above, "foo" and "bar" were put into the store, then "foo" was
+//!     // deleted so only "bar" will return a result when the database is queried via the
+//!     // writer.
+//!     println!("It should be None! ({:?})", store.get(&writer, "foo").unwrap());
+//!     println!("Get bar ({:?})", store.get(&writer, "bar").unwrap());
+//!
+//!     // But a reader won't see that change until the write transaction is committed.
+//!     {
+//!         let reader = env.read().expect("reader");
+//!         println!("Get foo {:?}", store.get(&reader, "foo").unwrap());
+//!         println!("Get bar {:?}", store.get(&reader, "bar").unwrap());
+//!     }
+//!     writer.commit().unwrap();
+//!     {
+//!         let reader = env.read().expect("reader");
+//!         println!("It should be None! ({:?})", store.get(&reader, "foo").unwrap());
+//!         println!("Get bar {:?}", store.get(&reader, "bar").unwrap());
+//!     }
+//!
+//!     // Committing a transaction consumes the writer, preventing you from reusing it by
+//!     // failing at compile time with an error. This line would report "error[E0382]:
+//!     // borrow of moved value: `writer`".
+//!     // store.put(&mut writer, "baz", &Value::Str("buz")).unwrap();
+//! }
+//!
+//! {
+//!     // Clearing all the entries in the store with a write transaction.
+//!     {
+//!         let mut writer = env.write().unwrap();
+//!         store.put(&mut writer, "foo", &Value::Str("bar")).unwrap();
+//!         store.put(&mut writer, "bar", &Value::Str("baz")).unwrap();
+//!         writer.commit().unwrap();
+//!     }
+//!
+//!     {
+//!         let mut writer = env.write().unwrap();
+//!         store.clear(&mut writer).unwrap();
+//!         writer.commit().unwrap();
+//!     }
+//!
+//!     {
+//!         let reader = env.read().expect("reader");
+//!         println!("It should be None! ({:?})", store.get(&reader, "foo").unwrap());
+//!         println!("It should be None! ({:?})", store.get(&reader, "bar").unwrap());
+//!     }
+//!
+//! }
+//!
+//! ```
+
+mod env;
+mod error;
+mod helpers;
+mod manager;
+mod readwrite;
+
+pub mod backend;
+#[cfg(feature = "lmdb")]
+pub mod migrator;
+pub mod store;
+pub mod value;
+
+pub use backend::{DatabaseFlags, EnvironmentFlags, WriteFlags};
+pub use env::Rkv;
+pub use error::{DataError, MigrateError, StoreError};
+pub use manager::Manager;
+#[cfg(feature = "lmdb")]
+pub use migrator::Migrator;
+pub use readwrite::{Readable, Reader, Writer};
+pub use store::{keys::EncodableKey, single::SingleStore, CloseOptions, Options as StoreOptions};
+pub use value::{OwnedValue, Value};
+
+#[cfg(feature = "db-dup-sort")]
+pub use store::multi::MultiStore;
+
+#[cfg(feature = "db-int-key")]
+pub use store::integer::IntegerStore;
+#[cfg(feature = "db-int-key")]
+pub use store::keys::PrimitiveInt;
+
+#[cfg(all(feature = "db-dup-sort", feature = "db-int-key"))]
+pub use store::integermulti::MultiIntegerStore;
diff --git a/third_party/rust/rkv/src/manager.rs b/third_party/rust/rkv/src/manager.rs
new file mode 100644
index 0000000000..045564497c
--- /dev/null
+++ b/third_party/rust/rkv/src/manager.rs
@@ -0,0 +1,247 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+use std::{
+    collections::{btree_map::Entry, BTreeMap},
+    os::raw::c_uint,
+    path::{Path, PathBuf},
+    result,
+    sync::{Arc, RwLock},
+};
+
+use lazy_static::lazy_static;
+
+#[cfg(feature = "lmdb")]
+use crate::backend::LmdbEnvironment;
+use crate::{
+    backend::{BackendEnvironment, BackendEnvironmentBuilder, SafeModeEnvironment},
+    error::{CloseError, StoreError},
+    helpers::canonicalize_path,
+    store::CloseOptions,
+    Rkv,
+};
+
+type Result<T> = result::Result<T, StoreError>;
+type CloseResult<T> = result::Result<T, CloseError>;
+type SharedRkv<E> = Arc<RwLock<Rkv<E>>>;
+
+#[cfg(feature = "lmdb")]
+lazy_static! {
+    static ref MANAGER_LMDB: RwLock<Manager<LmdbEnvironment>> = RwLock::new(Manager::new());
+}
+
+lazy_static! {
+    static ref MANAGER_SAFE_MODE: RwLock<Manager<SafeModeEnvironment>> =
+        RwLock::new(Manager::new());
+}
+
+/// A process is only permitted to have one open handle to each Rkv environment. This
+/// manager exists to enforce that constraint: don't open environments directly.
+///
+/// By default, path canonicalization is enabled for identifying RKV instances. This
+/// is true by default, because it helps enforce the constraints guaranteed by
+/// this manager. However, path canonicalization might crash in some fringe
+/// circumstances, so the `no-canonicalize-path` feature offers the possibility of
+/// disabling it. See: https://bugzilla.mozilla.org/show_bug.cgi?id=1531887
+///
+/// When path canonicalization is disabled, you *must* ensure an RKV environment is
+/// always created or retrieved with the same path.
+pub struct Manager<E> {
+    environments: BTreeMap<PathBuf, SharedRkv<E>>,
+}
+
+impl<'e, E> Manager<E>
+where
+    E: BackendEnvironment<'e>,
+{
+    fn new() -> Manager<E> {
+        Manager {
+            environments: Default::default(),
+        }
+    }
+
+    /// Return the open env at `path`, returning `None` if it has not already been opened.
+    pub fn get<'p, P>(&self, path: P) -> Result<Option<SharedRkv<E>>>
+    where
+        P: Into<&'p Path>,
+    {
+        let canonical = if cfg!(feature = "no-canonicalize-path") {
+            path.into().to_path_buf()
+        } else {
+            canonicalize_path(path)?
+        };
+        Ok(self.environments.get(&canonical).cloned())
+    }
+
+    /// Return the open env at `path`, or create it by calling `f`.
+    pub fn get_or_create<'p, F, P>(&mut self, path: P, f: F) -> Result<SharedRkv<E>>
+    where
+        F: FnOnce(&Path) -> Result<Rkv<E>>,
+        P: Into<&'p Path>,
+    {
+        let canonical = if cfg!(feature = "no-canonicalize-path") {
+            path.into().to_path_buf()
+        } else {
+            canonicalize_path(path)?
+        };
+        Ok(match self.environments.entry(canonical) {
+            Entry::Occupied(e) => e.get().clone(),
+            Entry::Vacant(e) => {
+                let k = Arc::new(RwLock::new(f(e.key().as_path())?));
+                e.insert(k).clone()
+            }
+        })
+    }
+
+    /// Return the open env at `path` with `capacity`, or create it by calling `f`.
+    pub fn get_or_create_with_capacity<'p, F, P>(
+        &mut self,
+        path: P,
+        capacity: c_uint,
+        f: F,
+    ) -> Result<SharedRkv<E>>
+    where
+        F: FnOnce(&Path, c_uint) -> Result<Rkv<E>>,
+        P: Into<&'p Path>,
+    {
+        let canonical = if cfg!(feature = "no-canonicalize-path") {
+            path.into().to_path_buf()
+        } else {
+            canonicalize_path(path)?
+        };
+        Ok(match self.environments.entry(canonical) {
+            Entry::Occupied(e) => e.get().clone(),
+            Entry::Vacant(e) => {
+                let k = Arc::new(RwLock::new(f(e.key().as_path(), capacity)?));
+                e.insert(k).clone()
+            }
+        })
+    }
+
+    /// Return a new Rkv environment from the builder, or create it by calling `f`.
+    pub fn get_or_create_from_builder<'p, F, P, B>(
+        &mut self,
+        path: P,
+        builder: B,
+        f: F,
+    ) -> Result<SharedRkv<E>>
+    where
+        F: FnOnce(&Path, B) -> Result<Rkv<E>>,
+        P: Into<&'p Path>,
+        B: BackendEnvironmentBuilder<'e, Environment = E>,
+    {
+        let canonical = if cfg!(feature = "no-canonicalize-path") {
+            path.into().to_path_buf()
+        } else {
+            canonicalize_path(path)?
+        };
+        Ok(match self.environments.entry(canonical) {
+            Entry::Occupied(e) => e.get().clone(),
+            Entry::Vacant(e) => {
+                let k = Arc::new(RwLock::new(f(e.key().as_path(), builder)?));
+                e.insert(k).clone()
+            }
+        })
+    }
+
+    /// Tries to close the specified environment.
+    /// Returns an error when other users of this environment still exist.
+    pub fn try_close<'p, P>(&mut self, path: P, options: CloseOptions) -> CloseResult<()>
+    where
+        P: Into<&'p Path>,
+    {
+        let canonical = if cfg!(feature = "no-canonicalize-path") {
+            path.into().to_path_buf()
+        } else {
+            canonicalize_path(path)?
+        };
+        match self.environments.entry(canonical) {
+            Entry::Vacant(_) => Ok(()),
+            Entry::Occupied(e) if Arc::strong_count(e.get()) > 1 => {
+                Err(CloseError::EnvironmentStillOpen)
+            }
+            Entry::Occupied(e) => {
+                let env = Arc::try_unwrap(e.remove())
+                    .map_err(|_| CloseError::UnknownEnvironmentStillOpen)?;
+                env.into_inner()?.close(options)?;
+                Ok(())
+            }
+        }
+    }
+}
+
+#[cfg(feature = "lmdb")]
+impl Manager<LmdbEnvironment> {
+    pub fn singleton() -> &'static RwLock<Manager<LmdbEnvironment>> {
+        &MANAGER_LMDB
+    }
+}
+
+impl Manager<SafeModeEnvironment> {
+    pub fn singleton() -> &'static RwLock<Manager<SafeModeEnvironment>> {
+        &MANAGER_SAFE_MODE
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::*;
+
+    use std::fs;
+
+    use tempfile::Builder;
+
+    #[cfg(feature = "lmdb")]
+    use backend::Lmdb;
+
+    /// Test that one can mutate managed Rkv instances in surprising ways.
+    #[cfg(feature = "lmdb")]
+    #[test]
+    fn test_mutate_managed_rkv() {
+        let mut manager = Manager::<LmdbEnvironment>::new();
+
+        let root1 = Builder::new()
+            .prefix("test_mutate_managed_rkv_1")
+            .tempdir()
+            .expect("tempdir");
+        fs::create_dir_all(root1.path()).expect("dir created");
+        let path1 = root1.path();
+        let arc = manager
+            .get_or_create(path1, Rkv::new::<Lmdb>)
+            .expect("created");
+
+        // Arc<RwLock<>> has interior mutability, so we can replace arc's Rkv instance with a new
+        // instance that has a different path.
+        let root2 = Builder::new()
+            .prefix("test_mutate_managed_rkv_2")
+            .tempdir()
+            .expect("tempdir");
+        fs::create_dir_all(root2.path()).expect("dir created");
+        let path2 = root2.path();
+        {
+            let mut rkv = arc.write().expect("guard");
+            let rkv2 = Rkv::new::<Lmdb>(path2).expect("Rkv");
+            *rkv = rkv2;
+        }
+
+        // Arc now has a different internal Rkv with path2, but it's still mapped to path1 in
+        // manager, so its pointer is equal to a new Arc for path1.
+        let path1_arc = manager.get(path1).expect("success").expect("existed");
+        assert!(Arc::ptr_eq(&path1_arc, &arc));
+
+        // Meanwhile, a new Arc for path2 has a different pointer, even though its Rkv's path is
+        // the same as arc's current path.
+        let path2_arc = manager
+            .get_or_create(path2, Rkv::new::<Lmdb>)
+            .expect("success");
+        assert!(!Arc::ptr_eq(&path2_arc, &arc));
+    }
+}
diff --git a/third_party/rust/rkv/src/migrator.rs b/third_party/rust/rkv/src/migrator.rs
new file mode 100644
index 0000000000..6dc8d60371
--- /dev/null
+++ b/third_party/rust/rkv/src/migrator.rs
@@ -0,0 +1,179 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+//! A simple utility for migrating data from one RVK environment to another. Notably, this
+//! tool can migrate data from an enviroment created with a different backend than the
+//! current RKV consumer (e.g from Lmdb to SafeMode).
+//!
+//! The utility doesn't support migrating between 32-bit and 64-bit LMDB environments yet,
+//! see `arch_migrator` if this is needed. However, this utility is ultimately intended to
+//! handle all possible migrations.
+//!
+//! The destination environment should be empty of data, otherwise an error is returned.
+//!
+//! There are 3 versions of the migration methods:
+//! * `migrate_<src>_to_<dst>`, where `<src>` and `<dst>` are the source and destination
+//!   environment types. You're responsive with opening both these environments, handling
+//!   all errors, and performing any cleanup if necessary.
+//! * `open_and_migrate_<src>_to_<dst>`, which is similar the the above, but automatically
+//!   attempts to open the source environment and delete all of its supporting files if
+//!   there's no other environment open at that path. You're still responsible with
+//!   handling all errors.
+//! * `easy_migrate_<src>_to_<dst>` which is similar to the above, but ignores the
+//!   migration and doesn't delete any files if the source environment is invalid
+//!   (corrupted), unavailable (path not accessible or incompatible with configuration),
+//!   or empty (database has no records).
+//!
+//! The tool currently has these limitations:
+//!
+//! 1. It doesn't support migration from environments created with
+//!    `EnvironmentFlags::NO_SUB_DIR`. To migrate such an environment, create a temporary
+//!    directory, copy the environment's data files in the temporary directory, then
+//!    migrate the temporary directory as the source environment.
+//! 2. It doesn't support migration from databases created with DatabaseFlags::DUP_SORT`
+//!    (with or without `DatabaseFlags::DUP_FIXED`) nor with `DatabaseFlags::INTEGER_KEY`.
+//!    This effectively means that migration is limited to `SingleStore`s.
+//! 3. It doesn't allow for existing data in the destination environment, which means that
+//!    it cannot overwrite nor append data.
+
+use crate::{
+    backend::{LmdbEnvironment, SafeModeEnvironment},
+    error::MigrateError,
+    Rkv, StoreOptions,
+};
+
+pub use crate::backend::{LmdbArchMigrateError, LmdbArchMigrateResult, LmdbArchMigrator};
+
+// FIXME: should parametrize this instead.
+
+macro_rules! fn_migrator {
+    ($name:tt, $src_env:ty, $dst_env:ty) => {
+        /// Migrate all data in all of databases from the source environment to the destination
+        /// environment. This includes all key/value pairs in the main database that aren't
+        /// metadata about subdatabases and all key/value pairs in all subdatabases.
+        ///
+        /// Other backend-specific metadata such as map size or maximum databases left intact on
+        /// the given environments.
+        ///
+        /// The destination environment should be empty of data, otherwise an error is returned.
+        pub fn $name<S, D>(src_env: S, dst_env: D) -> Result<(), MigrateError>
+        where
+            S: std::ops::Deref<Target = Rkv<$src_env>>,
+            D: std::ops::Deref<Target = Rkv<$dst_env>>,
+        {
+            let src_dbs = src_env.get_dbs().unwrap();
+            if src_dbs.is_empty() {
+                return Err(MigrateError::SourceEmpty);
+            }
+            let dst_dbs = dst_env.get_dbs().unwrap();
+            if !dst_dbs.is_empty() {
+                return Err(MigrateError::DestinationNotEmpty);
+            }
+            for name in src_dbs {
+                let src_store = src_env.open_single(name.as_deref(), StoreOptions::default())?;
+                let dst_store = dst_env.open_single(name.as_deref(), StoreOptions::create())?;
+                let reader = src_env.read()?;
+                let mut writer = dst_env.write()?;
+                let mut iter = src_store.iter_start(&reader)?;
+                while let Some(Ok((key, value))) = iter.next() {
+                    dst_store.put(&mut writer, key, &value).expect("wrote");
+                }
+                writer.commit()?;
+            }
+            Ok(())
+        }
+    };
+
+    (open $migrate:tt, $name:tt, $builder:tt, $src_env:ty, $dst_env:ty) => {
+        /// Same as the the `migrate_x_to_y` migration method above, but automatically attempts
+        /// to open the source environment. Finally, deletes all of its supporting files if
+        /// there's no other environment open at that path and the migration succeeded.
+        pub fn $name<F, D>(path: &std::path::Path, build: F, dst_env: D) -> Result<(), MigrateError>
+        where
+            F: FnOnce(crate::backend::$builder) -> crate::backend::$builder,
+            D: std::ops::Deref<Target = Rkv<$dst_env>>,
+        {
+            use crate::{backend::*, CloseOptions};
+
+            let mut manager = crate::Manager::<$src_env>::singleton().write()?;
+            let mut builder = Rkv::<$src_env>::environment_builder::<$builder>();
+            builder.set_max_dbs(crate::env::DEFAULT_MAX_DBS);
+            builder = build(builder);
+
+            let src_env =
+                manager.get_or_create_from_builder(path, builder, Rkv::from_builder::<$builder>)?;
+            Migrator::$migrate(src_env.read()?, dst_env)?;
+
+            drop(src_env);
+            manager.try_close(path, CloseOptions::delete_files_on_disk())?;
+
+            Ok(())
+        }
+    };
+
+    (easy $migrate:tt, $name:tt, $src_env:ty, $dst_env:ty) => {
+        /// Same as the `open_and_migrate_x_to_y` migration method above, but ignores the
+        /// migration and doesn't delete any files if the following conditions apply:
+        /// - Source environment is invalid/corrupted, unavailable, or empty.
+        /// - Destination environment is not empty.
+        /// Use this instead of the other migration methods if:
+        /// - You're not concerned by throwing away old data and starting fresh with a new store.
+        /// - You'll never want to overwrite data in the new store from the old store.
+        pub fn $name<D>(path: &std::path::Path, dst_env: D) -> Result<(), MigrateError>
+        where
+            D: std::ops::Deref<Target = Rkv<$dst_env>>,
+        {
+            match Migrator::$migrate(path, |builder| builder, dst_env) {
+                // Source environment is an invalid file or corrupted database.
+                Err(crate::MigrateError::StoreError(crate::StoreError::FileInvalid)) => Ok(()),
+                Err(crate::MigrateError::StoreError(crate::StoreError::DatabaseCorrupted)) => {
+                    Ok(())
+                }
+                // Path not accessible.
+                Err(crate::MigrateError::StoreError(crate::StoreError::IoError(_))) => Ok(()),
+                // Path accessible but incompatible for configuration.
+                Err(crate::MigrateError::StoreError(
+                    crate::StoreError::UnsuitableEnvironmentPath(_),
+                )) => Ok(()),
+                // Couldn't close source environment and delete files on disk (e.g. other stores still open).
+                Err(crate::MigrateError::CloseError(_)) => Ok(()),
+                // Nothing to migrate.
+                Err(crate::MigrateError::SourceEmpty) => Ok(()),
+                // Migrating would overwrite.
+                Err(crate::MigrateError::DestinationNotEmpty) => Ok(()),
+                result => result,
+            }?;
+
+            Ok(())
+        }
+    };
+}
+
+macro_rules! fns_migrator {
+    ($src:tt, $dst:tt) => {
+        paste::item! {
+            fns_migrator!([<migrate_ $src _to_ $dst>], $src, $dst);
+            fns_migrator!([<migrate_ $dst _to_ $src>], $dst, $src);
+        }
+    };
+    ($name:tt, $src:tt, $dst:tt) => {
+        paste::item! {
+            fn_migrator!($name, [<$src:camel Environment>], [<$dst:camel Environment>]);
+            fn_migrator!(open $name, [<open_and_ $name>], [<$src:camel>], [<$src:camel Environment>], [<$dst:camel Environment>]);
+            fn_migrator!(easy [<open_and_ $name>], [<easy_ $name>], [<$src:camel Environment>], [<$dst:camel Environment>]);
+        }
+    };
+}
+
+pub struct Migrator;
+
+impl Migrator {
+    fns_migrator!(lmdb, safe_mode);
+}
diff --git a/third_party/rust/rkv/src/readwrite.rs b/third_party/rust/rkv/src/readwrite.rs
new file mode 100644
index 0000000000..38b4746210
--- /dev/null
+++ b/third_party/rust/rkv/src/readwrite.rs
@@ -0,0 +1,154 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+use crate::{
+    backend::{
+        BackendDatabase, BackendRoCursor, BackendRoCursorTransaction, BackendRoTransaction,
+        BackendRwCursorTransaction, BackendRwTransaction,
+    },
+    error::StoreError,
+    helpers::read_transform,
+    value::Value,
+};
+
+pub struct Reader<T>(T);
+pub struct Writer<T>(T);
+
+pub trait Readable<'r> {
+    type Database: BackendDatabase;
+    type RoCursor: BackendRoCursor<'r>;
+
+    fn get<K>(&'r self, db: &Self::Database, k: &K) -> Result<Option<Value<'r>>, StoreError>
+    where
+        K: AsRef<[u8]>;
+
+    fn open_ro_cursor(&'r self, db: &Self::Database) -> Result<Self::RoCursor, StoreError>;
+}
+
+impl<'r, T> Readable<'r> for Reader<T>
+where
+    T: BackendRoCursorTransaction<'r>,
+{
+    type Database = T::Database;
+    type RoCursor = T::RoCursor;
+
+    fn get<K>(&'r self, db: &T::Database, k: &K) -> Result<Option<Value<'r>>, StoreError>
+    where
+        K: AsRef<[u8]>,
+    {
+        let bytes = self.0.get(db, k.as_ref()).map_err(|e| e.into());
+        match read_transform(bytes).map(Some) {
+            Err(StoreError::KeyValuePairNotFound) => Ok(None),
+            result => result,
+        }
+    }
+
+    fn open_ro_cursor(&'r self, db: &T::Database) -> Result<T::RoCursor, StoreError> {
+        self.0.open_ro_cursor(db).map_err(|e| e.into())
+    }
+}
+
+impl<T> Reader<T> {
+    pub(crate) fn new(txn: T) -> Reader<T> {
+        Reader(txn)
+    }
+}
+
+impl<T> Reader<T>
+where
+    T: BackendRoTransaction,
+{
+    pub fn abort(self) {
+        self.0.abort();
+    }
+}
+
+impl<'r, T> Readable<'r> for Writer<T>
+where
+    T: BackendRwCursorTransaction<'r>,
+{
+    type Database = T::Database;
+    type RoCursor = T::RoCursor;
+
+    fn get<K>(&'r self, db: &T::Database, k: &K) -> Result<Option<Value<'r>>, StoreError>
+    where
+        K: AsRef<[u8]>,
+    {
+        let bytes = self.0.get(db, k.as_ref()).map_err(|e| e.into());
+        match read_transform(bytes).map(Some) {
+            Err(StoreError::KeyValuePairNotFound) => Ok(None),
+            result => result,
+        }
+    }
+
+    fn open_ro_cursor(&'r self, db: &T::Database) -> Result<T::RoCursor, StoreError> {
+        self.0.open_ro_cursor(db).map_err(|e| e.into())
+    }
+}
+
+impl<T> Writer<T> {
+    pub(crate) fn new(txn: T) -> Writer<T> {
+        Writer(txn)
+    }
+}
+
+impl<T> Writer<T>
+where
+    T: BackendRwTransaction,
+{
+    pub fn commit(self) -> Result<(), StoreError> {
+        self.0.commit().map_err(|e| e.into())
+    }
+
+    pub fn abort(self) {
+        self.0.abort();
+    }
+
+    pub(crate) fn put<K>(
+        &mut self,
+        db: &T::Database,
+        k: &K,
+        v: &Value,
+        flags: T::Flags,
+    ) -> Result<(), StoreError>
+    where
+        K: AsRef<[u8]>,
+    {
+        // TODO: don't allocate twice.
+        self.0
+            .put(db, k.as_ref(), &v.to_bytes()?, flags)
+            .map_err(|e| e.into())
+    }
+
+    #[cfg(not(feature = "db-dup-sort"))]
+    pub(crate) fn delete<K>(&mut self, db: &T::Database, k: &K) -> Result<(), StoreError>
+    where
+        K: AsRef<[u8]>,
+    {
+        self.0.del(db, k.as_ref()).map_err(|e| e.into())
+    }
+
+    #[cfg(feature = "db-dup-sort")]
+    pub(crate) fn delete<K>(
+        &mut self,
+        db: &T::Database,
+        k: &K,
+        v: Option<&[u8]>,
+    ) -> Result<(), StoreError>
+    where
+        K: AsRef<[u8]>,
+    {
+        self.0.del(db, k.as_ref(), v).map_err(|e| e.into())
+    }
+
+    pub(crate) fn clear(&mut self, db: &T::Database) -> Result<(), StoreError> {
+        self.0.clear_db(db).map_err(|e| e.into())
+    }
+}
diff --git a/third_party/rust/rkv/src/store.rs b/third_party/rust/rkv/src/store.rs
new file mode 100644
index 0000000000..c8ac2c583f
--- /dev/null
+++ b/third_party/rust/rkv/src/store.rs
@@ -0,0 +1,52 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+pub mod keys;
+pub mod single;
+
+#[cfg(feature = "db-dup-sort")]
+pub mod multi;
+
+#[cfg(feature = "db-int-key")]
+pub mod integer;
+
+#[cfg(all(feature = "db-dup-sort", feature = "db-int-key"))]
+pub mod integermulti;
+
+use crate::backend::BackendDatabaseFlags;
+
+#[derive(Default, Debug, Copy, Clone)]
+pub struct Options<F> {
+    pub create: bool,
+    pub flags: F,
+}
+
+impl<F> Options<F>
+where
+    F: BackendDatabaseFlags,
+{
+    pub fn create() -> Options<F> {
+        Options {
+            create: true,
+            flags: F::empty(),
+        }
+    }
+}
+
+#[derive(Default, Debug, Copy, Clone)]
+pub struct CloseOptions {
+    pub delete: bool,
+}
+
+impl CloseOptions {
+    pub fn delete_files_on_disk() -> CloseOptions {
+        CloseOptions { delete: true }
+    }
+}
diff --git a/third_party/rust/rkv/src/store/integer.rs b/third_party/rust/rkv/src/store/integer.rs
new file mode 100644
index 0000000000..f150f09c5f
--- /dev/null
+++ b/third_party/rust/rkv/src/store/integer.rs
@@ -0,0 +1,637 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+use std::marker::PhantomData;
+
+use crate::{
+    backend::{BackendDatabase, BackendRwTransaction},
+    error::StoreError,
+    readwrite::{Readable, Writer},
+    store::{
+        keys::{Key, PrimitiveInt},
+        single::SingleStore,
+    },
+    value::Value,
+};
+
+type EmptyResult = Result<(), StoreError>;
+
+#[derive(Debug, Eq, PartialEq, Copy, Clone)]
+pub struct IntegerStore<D, K> {
+    inner: SingleStore<D>,
+    phantom: PhantomData<K>,
+}
+
+impl<D, K> IntegerStore<D, K>
+where
+    D: BackendDatabase,
+    K: PrimitiveInt,
+{
+    pub(crate) fn new(db: D) -> IntegerStore<D, K> {
+        IntegerStore {
+            inner: SingleStore::new(db),
+            phantom: PhantomData,
+        }
+    }
+
+    pub fn get<'r, R>(&self, reader: &'r R, k: K) -> Result<Option<Value<'r>>, StoreError>
+    where
+        R: Readable<'r, Database = D>,
+    {
+        self.inner.get(reader, Key::new(&k)?)
+    }
+
+    pub fn put<T>(&self, writer: &mut Writer<T>, k: K, v: &Value) -> EmptyResult
+    where
+        T: BackendRwTransaction<Database = D>,
+    {
+        self.inner.put(writer, Key::new(&k)?, v)
+    }
+
+    pub fn delete<T>(&self, writer: &mut Writer<T>, k: K) -> EmptyResult
+    where
+        T: BackendRwTransaction<Database = D>,
+    {
+        self.inner.delete(writer, Key::new(&k)?)
+    }
+
+    pub fn clear<T>(&self, writer: &mut Writer<T>) -> EmptyResult
+    where
+        T: BackendRwTransaction<Database = D>,
+    {
+        self.inner.clear(writer)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::*;
+
+    use std::fs;
+
+    use tempfile::Builder;
+
+    #[test]
+    fn test_integer_keys() {
+        let root = Builder::new()
+            .prefix("test_integer_keys")
+            .tempdir()
+            .expect("tempdir");
+        fs::create_dir_all(root.path()).expect("dir created");
+
+        let k = Rkv::new::<backend::SafeMode>(root.path()).expect("new succeeded");
+        let s = k.open_integer("s", StoreOptions::create()).expect("open");
+
+        macro_rules! test_integer_keys {
+            ($type:ty, $key:expr) => {{
+                let mut writer = k.write().expect("writer");
+
+                s.put(&mut writer, $key, &Value::Str("hello!"))
+                    .expect("write");
+                assert_eq!(
+                    s.get(&writer, $key).expect("read"),
+                    Some(Value::Str("hello!"))
+                );
+                writer.commit().expect("committed");
+
+                let reader = k.read().expect("reader");
+                assert_eq!(
+                    s.get(&reader, $key).expect("read"),
+                    Some(Value::Str("hello!"))
+                );
+            }};
+        }
+
+        test_integer_keys!(u32, std::u32::MIN);
+        test_integer_keys!(u32, std::u32::MAX);
+    }
+
+    #[test]
+    fn test_clear() {
+        let root = Builder::new()
+            .prefix("test_integer_clear")
+            .tempdir()
+            .expect("tempdir");
+        fs::create_dir_all(root.path()).expect("dir created");
+
+        let k = Rkv::new::<backend::SafeMode>(root.path()).expect("new succeeded");
+        let s = k.open_integer("s", StoreOptions::create()).expect("open");
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.put(&mut writer, 1, &Value::Str("hello!")).expect("write");
+            s.put(&mut writer, 2, &Value::Str("hello!")).expect("write");
+            s.put(&mut writer, 3, &Value::Str("hello!")).expect("write");
+            assert_eq!(s.get(&writer, 1).expect("read"), Some(Value::Str("hello!")));
+            assert_eq!(s.get(&writer, 2).expect("read"), Some(Value::Str("hello!")));
+            assert_eq!(s.get(&writer, 3).expect("read"), Some(Value::Str("hello!")));
+            writer.commit().expect("committed");
+        }
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.clear(&mut writer).expect("cleared");
+            writer.commit().expect("committed");
+
+            let reader = k.read().expect("reader");
+            assert_eq!(s.get(&reader, 1).expect("read"), None);
+            assert_eq!(s.get(&reader, 2).expect("read"), None);
+            assert_eq!(s.get(&reader, 3).expect("read"), None);
+        }
+    }
+
+    #[test]
+    fn test_dup() {
+        let root = Builder::new()
+            .prefix("test_integer_dup")
+            .tempdir()
+            .expect("tempdir");
+        fs::create_dir_all(root.path()).expect("dir created");
+
+        let k = Rkv::new::<backend::SafeMode>(root.path()).expect("new succeeded");
+        let s = k.open_integer("s", StoreOptions::create()).expect("open");
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.put(&mut writer, 1, &Value::Str("hello!")).expect("write");
+            s.put(&mut writer, 1, &Value::Str("foo!")).expect("write");
+            s.put(&mut writer, 1, &Value::Str("bar!")).expect("write");
+            assert_eq!(s.get(&writer, 1).expect("read"), Some(Value::Str("bar!")));
+            assert_eq!(s.get(&writer, 2).expect("read"), None);
+            assert_eq!(s.get(&writer, 3).expect("read"), None);
+            writer.commit().expect("committed");
+        }
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.clear(&mut writer).expect("cleared");
+            writer.commit().expect("committed");
+
+            let reader = k.read().expect("reader");
+            assert_eq!(s.get(&reader, 1).expect("read"), None);
+            assert_eq!(s.get(&reader, 2).expect("read"), None);
+            assert_eq!(s.get(&reader, 3).expect("read"), None);
+        }
+    }
+
+    #[test]
+    fn test_del() {
+        let root = Builder::new()
+            .prefix("test_integer_del")
+            .tempdir()
+            .expect("tempdir");
+        fs::create_dir_all(root.path()).expect("dir created");
+
+        let k = Rkv::new::<backend::SafeMode>(root.path()).expect("new succeeded");
+        let s = k.open_integer("s", StoreOptions::create()).expect("open");
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.put(&mut writer, 1, &Value::Str("hello!")).expect("write");
+            s.put(&mut writer, 1, &Value::Str("foo!")).expect("write");
+            s.put(&mut writer, 1, &Value::Str("bar!")).expect("write");
+            assert_eq!(s.get(&writer, 1).expect("read"), Some(Value::Str("bar!")));
+            assert_eq!(s.get(&writer, 2).expect("read"), None);
+            assert_eq!(s.get(&writer, 3).expect("read"), None);
+            writer.commit().expect("committed");
+        }
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.delete(&mut writer, 1).expect("deleted");
+            writer.commit().expect("committed");
+
+            let reader = k.read().expect("reader");
+            assert_eq!(s.get(&reader, 1).expect("read"), None);
+        }
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.delete(&mut writer, 2).expect_err("not deleted");
+            writer.commit().expect("committed");
+
+            let reader = k.read().expect("reader");
+            assert_eq!(s.get(&reader, 2).expect("read"), None);
+        }
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.delete(&mut writer, 3).expect_err("not deleted");
+            writer.commit().expect("committed");
+
+            let reader = k.read().expect("reader");
+            assert_eq!(s.get(&reader, 3).expect("read"), None);
+        }
+    }
+
+    #[test]
+    fn test_persist() {
+        let root = Builder::new()
+            .prefix("test_integer_persist")
+            .tempdir()
+            .expect("tempdir");
+        fs::create_dir_all(root.path()).expect("dir created");
+
+        {
+            let k = Rkv::new::<backend::SafeMode>(root.path()).expect("new succeeded");
+            let s = k.open_integer("s", StoreOptions::create()).expect("open");
+
+            let mut writer = k.write().expect("writer");
+            s.put(&mut writer, 1, &Value::Str("hello!")).expect("write");
+            s.put(&mut writer, 2, &Value::Str("hello!")).expect("write");
+            s.put(&mut writer, 3, &Value::Str("hello!")).expect("write");
+            assert_eq!(s.get(&writer, 1).expect("read"), Some(Value::Str("hello!")));
+            assert_eq!(s.get(&writer, 2).expect("read"), Some(Value::Str("hello!")));
+            assert_eq!(s.get(&writer, 3).expect("read"), Some(Value::Str("hello!")));
+            writer.commit().expect("committed");
+        }
+
+        {
+            let k = Rkv::new::<backend::SafeMode>(root.path()).expect("new succeeded");
+            let s = k.open_integer("s", StoreOptions::create()).expect("open");
+
+            let reader = k.read().expect("reader");
+            assert_eq!(s.get(&reader, 1).expect("read"), Some(Value::Str("hello!")));
+            assert_eq!(s.get(&reader, 2).expect("read"), Some(Value::Str("hello!")));
+            assert_eq!(s.get(&reader, 3).expect("read"), Some(Value::Str("hello!")));
+        }
+    }
+
+    #[test]
+    fn test_intertwine_read_write() {
+        let root = Builder::new()
+            .prefix("test_integer_intertwine_read_write")
+            .tempdir()
+            .expect("tempdir");
+        fs::create_dir_all(root.path()).expect("dir created");
+
+        let k = Rkv::new::<backend::SafeMode>(root.path()).expect("new succeeded");
+        let s = k.open_integer("s", StoreOptions::create()).expect("open");
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.put(&mut writer, 1, &Value::Str("hello!")).expect("write");
+            assert_eq!(s.get(&writer, 1).expect("read"), Some(Value::Str("hello!")));
+            assert_eq!(s.get(&writer, 2).expect("read"), None);
+            assert_eq!(s.get(&writer, 3).expect("read"), None);
+            writer.commit().expect("committed");
+        }
+
+        let reader = k.read().expect("reader");
+        let mut writer = k.write().expect("writer");
+
+        {
+            assert_eq!(s.get(&reader, 1).expect("read"), Some(Value::Str("hello!")));
+            assert_eq!(s.get(&reader, 2).expect("read"), None);
+            assert_eq!(s.get(&reader, 3).expect("read"), None);
+        }
+
+        {
+            s.put(&mut writer, 1, &Value::Str("goodbye!"))
+                .expect("write");
+            s.put(&mut writer, 2, &Value::Str("goodbye!"))
+                .expect("write");
+            s.put(&mut writer, 3, &Value::Str("goodbye!"))
+                .expect("write");
+            assert_eq!(
+                s.get(&writer, 1).expect("read"),
+                Some(Value::Str("goodbye!"))
+            );
+            assert_eq!(
+                s.get(&writer, 2).expect("read"),
+                Some(Value::Str("goodbye!"))
+            );
+            assert_eq!(
+                s.get(&writer, 3).expect("read"),
+                Some(Value::Str("goodbye!"))
+            );
+            writer.commit().expect("committed");
+        }
+
+        {
+            assert_eq!(s.get(&reader, 1).expect("read"), Some(Value::Str("hello!")));
+            assert_eq!(s.get(&reader, 2).expect("read"), None);
+            assert_eq!(s.get(&reader, 3).expect("read"), None);
+        }
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.put(&mut writer, 1, &Value::Str("hello!")).expect("write");
+            assert_eq!(s.get(&writer, 1).expect("read"), Some(Value::Str("hello!")));
+            assert_eq!(
+                s.get(&writer, 2).expect("read"),
+                Some(Value::Str("goodbye!"))
+            );
+            assert_eq!(
+                s.get(&writer, 3).expect("read"),
+                Some(Value::Str("goodbye!"))
+            );
+            writer.commit().expect("committed");
+        }
+
+        {
+            let reader = k.write().expect("reader");
+            assert_eq!(s.get(&reader, 1).expect("read"), Some(Value::Str("hello!")));
+            assert_eq!(
+                s.get(&reader, 2).expect("read"),
+                Some(Value::Str("goodbye!"))
+            );
+            assert_eq!(
+                s.get(&reader, 3).expect("read"),
+                Some(Value::Str("goodbye!"))
+            );
+            reader.commit().expect("committed");
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests_safe {
+    use super::*;
+    use crate::*;
+
+    use std::fs;
+
+    use tempfile::Builder;
+
+    #[test]
+    fn test_integer_keys() {
+        let root = Builder::new()
+            .prefix("test_integer_keys")
+            .tempdir()
+            .expect("tempdir");
+        fs::create_dir_all(root.path()).expect("dir created");
+
+        let k = Rkv::new::<backend::SafeMode>(root.path()).expect("new succeeded");
+        let s = k.open_integer("s", StoreOptions::create()).expect("open");
+
+        macro_rules! test_integer_keys {
+            ($type:ty, $key:expr) => {{
+                let mut writer = k.write().expect("writer");
+
+                s.put(&mut writer, $key, &Value::Str("hello!"))
+                    .expect("write");
+                assert_eq!(
+                    s.get(&writer, $key).expect("read"),
+                    Some(Value::Str("hello!"))
+                );
+                writer.commit().expect("committed");
+
+                let reader = k.read().expect("reader");
+                assert_eq!(
+                    s.get(&reader, $key).expect("read"),
+                    Some(Value::Str("hello!"))
+                );
+            }};
+        }
+
+        test_integer_keys!(u32, std::u32::MIN);
+        test_integer_keys!(u32, std::u32::MAX);
+    }
+
+    #[test]
+    fn test_clear() {
+        let root = Builder::new()
+            .prefix("test_integer_clear")
+            .tempdir()
+            .expect("tempdir");
+        fs::create_dir_all(root.path()).expect("dir created");
+
+        let k = Rkv::new::<backend::SafeMode>(root.path()).expect("new succeeded");
+        let s = k.open_integer("s", StoreOptions::create()).expect("open");
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.put(&mut writer, 1, &Value::Str("hello!")).expect("write");
+            s.put(&mut writer, 2, &Value::Str("hello!")).expect("write");
+            s.put(&mut writer, 3, &Value::Str("hello!")).expect("write");
+            assert_eq!(s.get(&writer, 1).expect("read"), Some(Value::Str("hello!")));
+            assert_eq!(s.get(&writer, 2).expect("read"), Some(Value::Str("hello!")));
+            assert_eq!(s.get(&writer, 3).expect("read"), Some(Value::Str("hello!")));
+            writer.commit().expect("committed");
+        }
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.clear(&mut writer).expect("cleared");
+            writer.commit().expect("committed");
+
+            let reader = k.read().expect("reader");
+            assert_eq!(s.get(&reader, 1).expect("read"), None);
+            assert_eq!(s.get(&reader, 2).expect("read"), None);
+            assert_eq!(s.get(&reader, 3).expect("read"), None);
+        }
+    }
+
+    #[test]
+    fn test_dup() {
+        let root = Builder::new()
+            .prefix("test_integer_dup")
+            .tempdir()
+            .expect("tempdir");
+        fs::create_dir_all(root.path()).expect("dir created");
+
+        let k = Rkv::new::<backend::SafeMode>(root.path()).expect("new succeeded");
+        let s = k.open_integer("s", StoreOptions::create()).expect("open");
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.put(&mut writer, 1, &Value::Str("hello!")).expect("write");
+            s.put(&mut writer, 1, &Value::Str("foo!")).expect("write");
+            s.put(&mut writer, 1, &Value::Str("bar!")).expect("write");
+            assert_eq!(s.get(&writer, 1).expect("read"), Some(Value::Str("bar!")));
+            assert_eq!(s.get(&writer, 2).expect("read"), None);
+            assert_eq!(s.get(&writer, 3).expect("read"), None);
+            writer.commit().expect("committed");
+        }
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.clear(&mut writer).expect("cleared");
+            writer.commit().expect("committed");
+
+            let reader = k.read().expect("reader");
+            assert_eq!(s.get(&reader, 1).expect("read"), None);
+            assert_eq!(s.get(&reader, 2).expect("read"), None);
+            assert_eq!(s.get(&reader, 3).expect("read"), None);
+        }
+    }
+
+    #[test]
+    fn test_del() {
+        let root = Builder::new()
+            .prefix("test_integer_del")
+            .tempdir()
+            .expect("tempdir");
+        fs::create_dir_all(root.path()).expect("dir created");
+
+        let k = Rkv::new::<backend::SafeMode>(root.path()).expect("new succeeded");
+        let s = k.open_integer("s", StoreOptions::create()).expect("open");
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.put(&mut writer, 1, &Value::Str("hello!")).expect("write");
+            s.put(&mut writer, 1, &Value::Str("foo!")).expect("write");
+            s.put(&mut writer, 1, &Value::Str("bar!")).expect("write");
+            assert_eq!(s.get(&writer, 1).expect("read"), Some(Value::Str("bar!")));
+            assert_eq!(s.get(&writer, 2).expect("read"), None);
+            assert_eq!(s.get(&writer, 3).expect("read"), None);
+            writer.commit().expect("committed");
+        }
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.delete(&mut writer, 1).expect("deleted");
+            writer.commit().expect("committed");
+
+            let reader = k.read().expect("reader");
+            assert_eq!(s.get(&reader, 1).expect("read"), None);
+        }
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.delete(&mut writer, 2).expect_err("not deleted");
+            writer.commit().expect("committed");
+
+            let reader = k.read().expect("reader");
+            assert_eq!(s.get(&reader, 2).expect("read"), None);
+        }
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.delete(&mut writer, 3).expect_err("not deleted");
+            writer.commit().expect("committed");
+
+            let reader = k.read().expect("reader");
+            assert_eq!(s.get(&reader, 3).expect("read"), None);
+        }
+    }
+
+    #[test]
+    fn test_persist() {
+        let root = Builder::new()
+            .prefix("test_integer_persist")
+            .tempdir()
+            .expect("tempdir");
+        fs::create_dir_all(root.path()).expect("dir created");
+
+        {
+            let k = Rkv::new::<backend::SafeMode>(root.path()).expect("new succeeded");
+            let s = k.open_integer("s", StoreOptions::create()).expect("open");
+
+            let mut writer = k.write().expect("writer");
+            s.put(&mut writer, 1, &Value::Str("hello!")).expect("write");
+            s.put(&mut writer, 2, &Value::Str("hello!")).expect("write");
+            s.put(&mut writer, 3, &Value::Str("hello!")).expect("write");
+            assert_eq!(s.get(&writer, 1).expect("read"), Some(Value::Str("hello!")));
+            assert_eq!(s.get(&writer, 2).expect("read"), Some(Value::Str("hello!")));
+            assert_eq!(s.get(&writer, 3).expect("read"), Some(Value::Str("hello!")));
+            writer.commit().expect("committed");
+        }
+
+        {
+            let k = Rkv::new::<backend::SafeMode>(root.path()).expect("new succeeded");
+            let s = k.open_integer("s", StoreOptions::create()).expect("open");
+
+            let reader = k.read().expect("reader");
+            assert_eq!(s.get(&reader, 1).expect("read"), Some(Value::Str("hello!")));
+            assert_eq!(s.get(&reader, 2).expect("read"), Some(Value::Str("hello!")));
+            assert_eq!(s.get(&reader, 3).expect("read"), Some(Value::Str("hello!")));
+        }
+    }
+
+    #[test]
+    fn test_intertwine_read_write() {
+        let root = Builder::new()
+            .prefix("test_integer_intertwine_read_write")
+            .tempdir()
+            .expect("tempdir");
+        fs::create_dir_all(root.path()).expect("dir created");
+
+        let k = Rkv::new::<backend::SafeMode>(root.path()).expect("new succeeded");
+        let s = k.open_integer("s", StoreOptions::create()).expect("open");
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.put(&mut writer, 1, &Value::Str("hello!")).expect("write");
+            assert_eq!(s.get(&writer, 1).expect("read"), Some(Value::Str("hello!")));
+            assert_eq!(s.get(&writer, 2).expect("read"), None);
+            assert_eq!(s.get(&writer, 3).expect("read"), None);
+            writer.commit().expect("committed");
+        }
+
+        let reader = k.read().expect("reader");
+        let mut writer = k.write().expect("writer");
+
+        {
+            assert_eq!(s.get(&reader, 1).expect("read"), Some(Value::Str("hello!")));
+            assert_eq!(s.get(&reader, 2).expect("read"), None);
+            assert_eq!(s.get(&reader, 3).expect("read"), None);
+        }
+
+        {
+            s.put(&mut writer, 1, &Value::Str("goodbye!"))
+                .expect("write");
+            s.put(&mut writer, 2, &Value::Str("goodbye!"))
+                .expect("write");
+            s.put(&mut writer, 3, &Value::Str("goodbye!"))
+                .expect("write");
+            assert_eq!(
+                s.get(&writer, 1).expect("read"),
+                Some(Value::Str("goodbye!"))
+            );
+            assert_eq!(
+                s.get(&writer, 2).expect("read"),
+                Some(Value::Str("goodbye!"))
+            );
+            assert_eq!(
+                s.get(&writer, 3).expect("read"),
+                Some(Value::Str("goodbye!"))
+            );
+            writer.commit().expect("committed");
+        }
+
+        {
+            assert_eq!(s.get(&reader, 1).expect("read"), Some(Value::Str("hello!")));
+            assert_eq!(s.get(&reader, 2).expect("read"), None);
+            assert_eq!(s.get(&reader, 3).expect("read"), None);
+        }
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.put(&mut writer, 1, &Value::Str("hello!")).expect("write");
+            assert_eq!(s.get(&writer, 1).expect("read"), Some(Value::Str("hello!")));
+            assert_eq!(
+                s.get(&writer, 2).expect("read"),
+                Some(Value::Str("goodbye!"))
+            );
+            assert_eq!(
+                s.get(&writer, 3).expect("read"),
+                Some(Value::Str("goodbye!"))
+            );
+            writer.commit().expect("committed");
+        }
+
+        {
+            let reader = k.write().expect("reader");
+            assert_eq!(s.get(&reader, 1).expect("read"), Some(Value::Str("hello!")));
+            assert_eq!(
+                s.get(&reader, 2).expect("read"),
+                Some(Value::Str("goodbye!"))
+            );
+            assert_eq!(
+                s.get(&reader, 3).expect("read"),
+                Some(Value::Str("goodbye!"))
+            );
+            reader.commit().expect("committed");
+        }
+    }
+}
diff --git a/third_party/rust/rkv/src/store/integermulti.rs b/third_party/rust/rkv/src/store/integermulti.rs
new file mode 100644
index 0000000000..71dd0aae04
--- /dev/null
+++ b/third_party/rust/rkv/src/store/integermulti.rs
@@ -0,0 +1,709 @@
+// Copyright 2018 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+use std::marker::PhantomData;
+
+use crate::{
+    backend::{BackendDatabase, BackendIter, BackendRoCursor, BackendRwTransaction},
+    error::StoreError,
+    readwrite::{Readable, Writer},
+    store::{
+        keys::{Key, PrimitiveInt},
+        multi::{Iter, MultiStore},
+    },
+    value::Value,
+};
+
+type EmptyResult = Result<(), StoreError>;
+
+#[derive(Debug, Eq, PartialEq, Copy, Clone)]
+pub struct MultiIntegerStore<D, K> {
+    inner: MultiStore<D>,
+    phantom: PhantomData<K>,
+}
+
+impl<D, K> MultiIntegerStore<D, K>
+where
+    D: BackendDatabase,
+    K: PrimitiveInt,
+{
+    pub(crate) fn new(db: D) -> MultiIntegerStore<D, K> {
+        MultiIntegerStore {
+            inner: MultiStore::new(db),
+            phantom: PhantomData,
+        }
+    }
+
+    pub fn get<'r, R, I, C>(&self, reader: &'r R, k: K) -> Result<Iter<'r, I>, StoreError>
+    where
+        R: Readable<'r, Database = D, RoCursor = C>,
+        I: BackendIter<'r>,
+        C: BackendRoCursor<'r, Iter = I>,
+        K: 'r,
+    {
+        self.inner.get(reader, Key::new(&k)?)
+    }
+
+    pub fn get_first<'r, R>(&self, reader: &'r R, k: K) -> Result<Option<Value<'r>>, StoreError>
+    where
+        R: Readable<'r, Database = D>,
+    {
+        self.inner.get_first(reader, Key::new(&k)?)
+    }
+
+    pub fn put<T>(&self, writer: &mut Writer<T>, k: K, v: &Value) -> EmptyResult
+    where
+        T: BackendRwTransaction<Database = D>,
+    {
+        self.inner.put(writer, Key::new(&k)?, v)
+    }
+
+    pub fn put_with_flags<T>(
+        &self,
+        writer: &mut Writer<T>,
+        k: K,
+        v: &Value,
+        flags: T::Flags,
+    ) -> EmptyResult
+    where
+        T: BackendRwTransaction<Database = D>,
+    {
+        self.inner.put_with_flags(writer, Key::new(&k)?, v, flags)
+    }
+
+    pub fn delete_all<T>(&self, writer: &mut Writer<T>, k: K) -> EmptyResult
+    where
+        T: BackendRwTransaction<Database = D>,
+    {
+        self.inner.delete_all(writer, Key::new(&k)?)
+    }
+
+    pub fn delete<T>(&self, writer: &mut Writer<T>, k: K, v: &Value) -> EmptyResult
+    where
+        T: BackendRwTransaction<Database = D>,
+    {
+        self.inner.delete(writer, Key::new(&k)?, v)
+    }
+
+    pub fn clear<T>(&self, writer: &mut Writer<T>) -> EmptyResult
+    where
+        T: BackendRwTransaction<Database = D>,
+    {
+        self.inner.clear(writer)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::*;
+
+    use std::fs;
+
+    use tempfile::Builder;
+
+    #[test]
+    fn test_integer_keys() {
+        let root = Builder::new()
+            .prefix("test_integer_keys")
+            .tempdir()
+            .expect("tempdir");
+        fs::create_dir_all(root.path()).expect("dir created");
+
+        let k = Rkv::new::<backend::SafeMode>(root.path()).expect("new succeeded");
+        let s = k
+            .open_multi_integer("s", StoreOptions::create())
+            .expect("open");
+
+        macro_rules! test_integer_keys {
+            ($type:ty, $key:expr) => {{
+                let mut writer = k.write().expect("writer");
+
+                s.put(&mut writer, $key, &Value::Str("hello!"))
+                    .expect("write");
+                assert_eq!(
+                    s.get_first(&writer, $key).expect("read"),
+                    Some(Value::Str("hello!"))
+                );
+                writer.commit().expect("committed");
+
+                let reader = k.read().expect("reader");
+                assert_eq!(
+                    s.get_first(&reader, $key).expect("read"),
+                    Some(Value::Str("hello!"))
+                );
+            }};
+        }
+
+        test_integer_keys!(u32, std::u32::MIN);
+        test_integer_keys!(u32, std::u32::MAX);
+    }
+
+    #[test]
+    fn test_clear() {
+        let root = Builder::new()
+            .prefix("test_multi_integer_clear")
+            .tempdir()
+            .expect("tempdir");
+        fs::create_dir_all(root.path()).expect("dir created");
+
+        let k = Rkv::new::<backend::SafeMode>(root.path()).expect("new succeeded");
+        let s = k
+            .open_multi_integer("s", StoreOptions::create())
+            .expect("open");
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.put(&mut writer, 1, &Value::Str("hello!")).expect("write");
+            s.put(&mut writer, 1, &Value::Str("hello1!"))
+                .expect("write");
+            s.put(&mut writer, 2, &Value::Str("hello!")).expect("write");
+            assert_eq!(
+                s.get_first(&writer, 1).expect("read"),
+                Some(Value::Str("hello!"))
+            );
+            assert_eq!(
+                s.get_first(&writer, 2).expect("read"),
+                Some(Value::Str("hello!"))
+            );
+            assert_eq!(s.get_first(&writer, 3).expect("read"), None);
+            writer.commit().expect("committed");
+        }
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.clear(&mut writer).expect("cleared");
+            writer.commit().expect("committed");
+
+            let reader = k.read().expect("reader");
+            assert_eq!(s.get_first(&reader, 1).expect("read"), None);
+            assert_eq!(s.get_first(&reader, 2).expect("read"), None);
+            assert_eq!(s.get_first(&reader, 3).expect("read"), None);
+        }
+    }
+
+    #[test]
+    fn test_dup() {
+        let root = Builder::new()
+            .prefix("test_multi_integer_dup")
+            .tempdir()
+            .expect("tempdir");
+        fs::create_dir_all(root.path()).expect("dir created");
+
+        let k = Rkv::new::<backend::SafeMode>(root.path()).expect("new succeeded");
+        let s = k
+            .open_multi_integer("s", StoreOptions::create())
+            .expect("open");
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.put(&mut writer, 1, &Value::Str("hello!")).expect("write");
+            s.put(&mut writer, 1, &Value::Str("hello!")).expect("write");
+            s.put(&mut writer, 1, &Value::Str("hello1!"))
+                .expect("write");
+            assert_eq!(
+                s.get_first(&writer, 1).expect("read"),
+                Some(Value::Str("hello!"))
+            );
+            assert_eq!(s.get_first(&writer, 2).expect("read"), None);
+            assert_eq!(s.get_first(&writer, 3).expect("read"), None);
+            writer.commit().expect("committed");
+        }
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.clear(&mut writer).expect("cleared");
+            writer.commit().expect("committed");
+
+            let reader = k.read().expect("reader");
+            assert_eq!(s.get_first(&reader, 1).expect("read"), None);
+            assert_eq!(s.get_first(&reader, 2).expect("read"), None);
+            assert_eq!(s.get_first(&reader, 3).expect("read"), None);
+        }
+    }
+
+    #[test]
+    fn test_dup_2() {
+        let root = Builder::new()
+            .prefix("test_multi_integer_dup")
+            .tempdir()
+            .expect("tempdir");
+        fs::create_dir_all(root.path()).expect("dir created");
+
+        let k = Rkv::new::<backend::SafeMode>(root.path()).expect("new succeeded");
+        let s = k
+            .open_multi_integer("s", StoreOptions::create())
+            .expect("open");
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.put(&mut writer, 1, &Value::Str("hello!")).expect("write");
+            s.put(&mut writer, 1, &Value::Str("hello!")).expect("write");
+            s.put(&mut writer, 1, &Value::Str("hello1!"))
+                .expect("write");
+
+            let mut iter = s.get(&writer, 1).expect("read");
+            assert_eq!(
+                iter.next().expect("first").expect("ok").1,
+                Value::Str("hello!")
+            );
+            assert_eq!(
+                iter.next().expect("second").expect("ok").1,
+                Value::Str("hello1!")
+            );
+            assert!(iter.next().is_none());
+        }
+    }
+
+    #[test]
+    fn test_del() {
+        let root = Builder::new()
+            .prefix("test_multi_integer_dup")
+            .tempdir()
+            .expect("tempdir");
+        fs::create_dir_all(root.path()).expect("dir created");
+
+        let k = Rkv::new::<backend::SafeMode>(root.path()).expect("new succeeded");
+        let s = k
+            .open_multi_integer("s", StoreOptions::create())
+            .expect("open");
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.put(&mut writer, 1, &Value::Str("hello!")).expect("write");
+            s.put(&mut writer, 1, &Value::Str("hello!")).expect("write");
+            s.put(&mut writer, 1, &Value::Str("hello1!"))
+                .expect("write");
+            {
+                let mut iter = s.get(&writer, 1).expect("read");
+                assert_eq!(
+                    iter.next().expect("first").expect("ok").1,
+                    Value::Str("hello!")
+                );
+                assert_eq!(
+                    iter.next().expect("second").expect("ok").1,
+                    Value::Str("hello1!")
+                );
+                assert!(iter.next().is_none());
+            }
+            writer.commit().expect("committed");
+        }
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.delete(&mut writer, 1, &Value::Str("hello!"))
+                .expect("deleted");
+            writer.commit().expect("committed");
+
+            let reader = k.read().expect("reader");
+            let mut iter = s.get(&reader, 1).expect("read");
+            assert_eq!(
+                iter.next().expect("first").expect("ok").1,
+                Value::Str("hello1!")
+            );
+            assert!(iter.next().is_none());
+        }
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.delete(&mut writer, 1, &Value::Str("hello!"))
+                .expect_err("deleted");
+            writer.commit().expect("committed");
+
+            let reader = k.read().expect("reader");
+            let mut iter = s.get(&reader, 1).expect("read");
+            assert_eq!(
+                iter.next().expect("first").expect("ok").1,
+                Value::Str("hello1!")
+            );
+            assert!(iter.next().is_none());
+        }
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.delete(&mut writer, 1, &Value::Str("hello1!"))
+                .expect("deleted");
+            writer.commit().expect("committed");
+
+            let reader = k.read().expect("reader");
+            let mut iter = s.get(&reader, 1).expect("read");
+            assert!(iter.next().is_none());
+        }
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.delete(&mut writer, 1, &Value::Str("hello1!"))
+                .expect_err("deleted");
+            writer.commit().expect("committed");
+
+            let reader = k.read().expect("reader");
+            let mut iter = s.get(&reader, 1).expect("read");
+            assert!(iter.next().is_none());
+        }
+    }
+
+    #[test]
+    fn test_persist() {
+        let root = Builder::new()
+            .prefix("test_multi_integer_persist")
+            .tempdir()
+            .expect("tempdir");
+        fs::create_dir_all(root.path()).expect("dir created");
+
+        {
+            let k = Rkv::new::<backend::SafeMode>(root.path()).expect("new succeeded");
+            let s = k
+                .open_multi_integer("s", StoreOptions::create())
+                .expect("open");
+
+            let mut writer = k.write().expect("writer");
+            s.put(&mut writer, 1, &Value::Str("hello!")).expect("write");
+            s.put(&mut writer, 1, &Value::Str("hello1!"))
+                .expect("write");
+            s.put(&mut writer, 2, &Value::Str("hello!")).expect("write");
+            {
+                let mut iter = s.get(&writer, 1).expect("read");
+                assert_eq!(
+                    iter.next().expect("first").expect("ok").1,
+                    Value::Str("hello!")
+                );
+                assert_eq!(
+                    iter.next().expect("second").expect("ok").1,
+                    Value::Str("hello1!")
+                );
+                assert!(iter.next().is_none());
+            }
+            writer.commit().expect("committed");
+        }
+
+        {
+            let k = Rkv::new::<backend::SafeMode>(root.path()).expect("new succeeded");
+            let s = k
+                .open_multi_integer("s", StoreOptions::create())
+                .expect("open");
+
+            let reader = k.read().expect("reader");
+            let mut iter = s.get(&reader, 1).expect("read");
+            assert_eq!(
+                iter.next().expect("first").expect("ok").1,
+                Value::Str("hello!")
+            );
+            assert_eq!(
+                iter.next().expect("second").expect("ok").1,
+                Value::Str("hello1!")
+            );
+            assert!(iter.next().is_none());
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests_safe {
+    use super::*;
+    use crate::*;
+
+    use std::fs;
+
+    use tempfile::Builder;
+
+    #[test]
+    fn test_integer_keys() {
+        let root = Builder::new()
+            .prefix("test_integer_keys")
+            .tempdir()
+            .expect("tempdir");
+        fs::create_dir_all(root.path()).expect("dir created");
+
+        let k = Rkv::new::<backend::SafeMode>(root.path()).expect("new succeeded");
+        let s = k
+            .open_multi_integer("s", StoreOptions::create())
+            .expect("open");
+
+        macro_rules! test_integer_keys {
+            ($type:ty, $key:expr) => {{
+                let mut writer = k.write().expect("writer");
+
+                s.put(&mut writer, $key, &Value::Str("hello!"))
+                    .expect("write");
+                assert_eq!(
+                    s.get_first(&writer, $key).expect("read"),
+                    Some(Value::Str("hello!"))
+                );
+                writer.commit().expect("committed");
+
+                let reader = k.read().expect("reader");
+                assert_eq!(
+                    s.get_first(&reader, $key).expect("read"),
+                    Some(Value::Str("hello!"))
+                );
+            }};
+        }
+
+        test_integer_keys!(u32, std::u32::MIN);
+        test_integer_keys!(u32, std::u32::MAX);
+    }
+
+    #[test]
+    fn test_clear() {
+        let root = Builder::new()
+            .prefix("test_multi_integer_clear")
+            .tempdir()
+            .expect("tempdir");
+        fs::create_dir_all(root.path()).expect("dir created");
+
+        let k = Rkv::new::<backend::SafeMode>(root.path()).expect("new succeeded");
+        let s = k
+            .open_multi_integer("s", StoreOptions::create())
+            .expect("open");
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.put(&mut writer, 1, &Value::Str("hello!")).expect("write");
+            s.put(&mut writer, 1, &Value::Str("hello1!"))
+                .expect("write");
+            s.put(&mut writer, 2, &Value::Str("hello!")).expect("write");
+            assert_eq!(
+                s.get_first(&writer, 1).expect("read"),
+                Some(Value::Str("hello!"))
+            );
+            assert_eq!(
+                s.get_first(&writer, 2).expect("read"),
+                Some(Value::Str("hello!"))
+            );
+            assert_eq!(s.get_first(&writer, 3).expect("read"), None);
+            writer.commit().expect("committed");
+        }
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.clear(&mut writer).expect("cleared");
+            writer.commit().expect("committed");
+
+            let reader = k.read().expect("reader");
+            assert_eq!(s.get_first(&reader, 1).expect("read"), None);
+            assert_eq!(s.get_first(&reader, 2).expect("read"), None);
+            assert_eq!(s.get_first(&reader, 3).expect("read"), None);
+        }
+    }
+
+    #[test]
+    fn test_dup() {
+        let root = Builder::new()
+            .prefix("test_multi_integer_dup")
+            .tempdir()
+            .expect("tempdir");
+        fs::create_dir_all(root.path()).expect("dir created");
+
+        let k = Rkv::new::<backend::SafeMode>(root.path()).expect("new succeeded");
+        let s = k
+            .open_multi_integer("s", StoreOptions::create())
+            .expect("open");
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.put(&mut writer, 1, &Value::Str("hello!")).expect("write");
+            s.put(&mut writer, 1, &Value::Str("hello!")).expect("write");
+            s.put(&mut writer, 1, &Value::Str("hello1!"))
+                .expect("write");
+            assert_eq!(
+                s.get_first(&writer, 1).expect("read"),
+                Some(Value::Str("hello!"))
+            );
+            assert_eq!(s.get_first(&writer, 2).expect("read"), None);
+            assert_eq!(s.get_first(&writer, 3).expect("read"), None);
+            writer.commit().expect("committed");
+        }
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.clear(&mut writer).expect("cleared");
+            writer.commit().expect("committed");
+
+            let reader = k.read().expect("reader");
+            assert_eq!(s.get_first(&reader, 1).expect("read"), None);
+            assert_eq!(s.get_first(&reader, 2).expect("read"), None);
+            assert_eq!(s.get_first(&reader, 3).expect("read"), None);
+        }
+    }
+
+    #[test]
+    fn test_dup_2() {
+        let root = Builder::new()
+            .prefix("test_multi_integer_dup")
+            .tempdir()
+            .expect("tempdir");
+        fs::create_dir_all(root.path()).expect("dir created");
+
+        let k = Rkv::new::<backend::SafeMode>(root.path()).expect("new succeeded");
+        let s = k
+            .open_multi_integer("s", StoreOptions::create())
+            .expect("open");
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.put(&mut writer, 1, &Value::Str("hello!")).expect("write");
+            s.put(&mut writer, 1, &Value::Str("hello!")).expect("write");
+            s.put(&mut writer, 1, &Value::Str("hello1!"))
+                .expect("write");
+
+            let mut iter = s.get(&writer, 1).expect("read");
+            assert_eq!(
+                iter.next().expect("first").expect("ok").1,
+                Value::Str("hello!")
+            );
+            assert_eq!(
+                iter.next().expect("second").expect("ok").1,
+                Value::Str("hello1!")
+            );
+            assert!(iter.next().is_none());
+        }
+    }
+
+    #[test]
+    fn test_del() {
+        let root = Builder::new()
+            .prefix("test_multi_integer_dup")
+            .tempdir()
+            .expect("tempdir");
+        fs::create_dir_all(root.path()).expect("dir created");
+
+        let k = Rkv::new::<backend::SafeMode>(root.path()).expect("new succeeded");
+        let s = k
+            .open_multi_integer("s", StoreOptions::create())
+            .expect("open");
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.put(&mut writer, 1, &Value::Str("hello!")).expect("write");
+            s.put(&mut writer, 1, &Value::Str("hello!")).expect("write");
+            s.put(&mut writer, 1, &Value::Str("hello1!"))
+                .expect("write");
+            {
+                let mut iter = s.get(&writer, 1).expect("read");
+                assert_eq!(
+                    iter.next().expect("first").expect("ok").1,
+                    Value::Str("hello!")
+                );
+                assert_eq!(
+                    iter.next().expect("second").expect("ok").1,
+                    Value::Str("hello1!")
+                );
+                assert!(iter.next().is_none());
+            }
+            writer.commit().expect("committed");
+        }
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.delete(&mut writer, 1, &Value::Str("hello!"))
+                .expect("deleted");
+            writer.commit().expect("committed");
+
+            let reader = k.read().expect("reader");
+            let mut iter = s.get(&reader, 1).expect("read");
+            assert_eq!(
+                iter.next().expect("first").expect("ok").1,
+                Value::Str("hello1!")
+            );
+            assert!(iter.next().is_none());
+        }
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.delete(&mut writer, 1, &Value::Str("hello!"))
+                .expect_err("deleted");
+            writer.commit().expect("committed");
+
+            let reader = k.read().expect("reader");
+            let mut iter = s.get(&reader, 1).expect("read");
+            assert_eq!(
+                iter.next().expect("first").expect("ok").1,
+                Value::Str("hello1!")
+            );
+            assert!(iter.next().is_none());
+        }
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.delete(&mut writer, 1, &Value::Str("hello1!"))
+                .expect("deleted");
+            writer.commit().expect("committed");
+
+            let reader = k.read().expect("reader");
+            let mut iter = s.get(&reader, 1).expect("read");
+            assert!(iter.next().is_none());
+        }
+
+        {
+            let mut writer = k.write().expect("writer");
+            s.delete(&mut writer, 1, &Value::Str("hello1!"))
+                .expect_err("deleted");
+            writer.commit().expect("committed");
+
+            let reader = k.read().expect("reader");
+            let mut iter = s.get(&reader, 1).expect("read");
+            assert!(iter.next().is_none());
+        }
+    }
+
+    #[test]
+    fn test_persist() {
+        let root = Builder::new()
+            .prefix("test_multi_integer_persist")
+            .tempdir()
+            .expect("tempdir");
+        fs::create_dir_all(root.path()).expect("dir created");
+
+        {
+            let k = Rkv::new::<backend::SafeMode>(root.path()).expect("new succeeded");
+            let s = k
+                .open_multi_integer("s", StoreOptions::create())
+                .expect("open");
+
+            let mut writer = k.write().expect("writer");
+            s.put(&mut writer, 1, &Value::Str("hello!")).expect("write");
+            s.put(&mut writer, 1, &Value::Str("hello1!"))
+                .expect("write");
+            s.put(&mut writer, 2, &Value::Str("hello!")).expect("write");
+            {
+                let mut iter = s.get(&writer, 1).expect("read");
+                assert_eq!(
+                    iter.next().expect("first").expect("ok").1,
+                    Value::Str("hello!")
+                );
+                assert_eq!(
+                    iter.next().expect("second").expect("ok").1,
+                    Value::Str("hello1!")
+                );
+                assert!(iter.next().is_none());
+            }
+            writer.commit().expect("committed");
+        }
+
+        {
+            let k = Rkv::new::<backend::SafeMode>(root.path()).expect("new succeeded");
+            let s = k
+                .open_multi_integer("s", StoreOptions::create())
+                .expect("open");
+
+            let reader = k.read().expect("reader");
+            let mut iter = s.get(&reader, 1).expect("read");
+            assert_eq!(
+                iter.next().expect("first").expect("ok").1,
+                Value::Str("hello!")
+            );
+            assert_eq!(
+                iter.next().expect("second").expect("ok").1,
+                Value::Str("hello1!")
+            );
+            assert!(iter.next().is_none());
+        }
+    }
+}
diff --git a/third_party/rust/rkv/src/store/keys.rs b/third_party/rust/rkv/src/store/keys.rs
new file mode 100644
index 0000000000..26a13db47a
--- /dev/null
+++ b/third_party/rust/rkv/src/store/keys.rs
@@ -0,0 +1,46 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+mod encodables;
+mod primitives;
+
+use std::marker::PhantomData;
+
+use crate::error::DataError;
+
+pub use encodables::*;
+pub use primitives::*;
+
+pub(crate) struct Key<K> {
+    bytes: Vec<u8>,
+    phantom: PhantomData<K>,
+}
+
+impl<K> AsRef<[u8]> for Key<K>
+where
+    K: EncodableKey,
+{
+    fn as_ref(&self) -> &[u8] {
+        self.bytes.as_ref()
+    }
+}
+
+impl<K> Key<K>
+where
+    K: EncodableKey,
+{
+    #[allow(clippy::new_ret_no_self)]
+    pub fn new(k: &K) -> Result<Key<K>, DataError> {
+        Ok(Key {
+            bytes: k.to_bytes()?,
+            phantom: PhantomData,
+        })
+    }
+}
diff --git a/third_party/rust/rkv/src/store/keys/encodables.rs b/third_party/rust/rkv/src/store/keys/encodables.rs
new file mode 100644
index 0000000000..85e3eacdf5
--- /dev/null
+++ b/third_party/rust/rkv/src/store/keys/encodables.rs
@@ -0,0 +1,27 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+use bincode::serialize;
+use serde::Serialize;
+
+use crate::error::DataError;
+
+pub trait EncodableKey {
+    fn to_bytes(&self) -> Result<Vec<u8>, DataError>;
+}
+
+impl<T> EncodableKey for T
+where
+    T: Serialize,
+{
+    fn to_bytes(&self) -> Result<Vec<u8>, DataError> {
+        serialize(self).map_err(|e| e.into())
+    }
+}
diff --git a/third_party/rust/rkv/src/store/keys/primitives.rs b/third_party/rust/rkv/src/store/keys/primitives.rs
new file mode 100644
index 0000000000..26282b7cbb
--- /dev/null
+++ b/third_party/rust/rkv/src/store/keys/primitives.rs
@@ -0,0 +1,15 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+use crate::store::keys::EncodableKey;
+
+pub trait PrimitiveInt: EncodableKey {}
+
+impl PrimitiveInt for u32 {}
diff --git a/third_party/rust/rkv/src/store/multi.rs b/third_party/rust/rkv/src/store/multi.rs
new file mode 100644
index 0000000000..c7f816b1ec
--- /dev/null
+++ b/third_party/rust/rkv/src/store/multi.rs
@@ -0,0 +1,133 @@
+// Copyright 2018 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+use std::marker::PhantomData;
+
+use crate::{
+    backend::{BackendDatabase, BackendFlags, BackendIter, BackendRoCursor, BackendRwTransaction},
+    error::StoreError,
+    helpers::read_transform,
+    readwrite::{Readable, Writer},
+    value::Value,
+};
+
+type EmptyResult = Result<(), StoreError>;
+
+#[derive(Debug, Eq, PartialEq, Copy, Clone)]
+pub struct MultiStore<D> {
+    db: D,
+}
+
+pub struct Iter<'i, I> {
+    iter: I,
+    phantom: PhantomData<&'i ()>,
+}
+
+impl<D> MultiStore<D>
+where
+    D: BackendDatabase,
+{
+    pub(crate) fn new(db: D) -> MultiStore<D> {
+        MultiStore { db }
+    }
+
+    /// Provides a cursor to all of the values for the duplicate entries that match this
+    /// key
+    pub fn get<'r, R, I, C, K>(&self, reader: &'r R, k: K) -> Result<Iter<'r, I>, StoreError>
+    where
+        R: Readable<'r, Database = D, RoCursor = C>,
+        I: BackendIter<'r>,
+        C: BackendRoCursor<'r, Iter = I>,
+        K: AsRef<[u8]> + 'r,
+    {
+        let cursor = reader.open_ro_cursor(&self.db)?;
+        let iter = cursor.into_iter_dup_of(k);
+
+        Ok(Iter {
+            iter,
+            phantom: PhantomData,
+        })
+    }
+
+    /// Provides the first value that matches this key
+    pub fn get_first<'r, R, K>(&self, reader: &'r R, k: K) -> Result<Option<Value<'r>>, StoreError>
+    where
+        R: Readable<'r, Database = D>,
+        K: AsRef<[u8]>,
+    {
+        reader.get(&self.db, &k)
+    }
+
+    /// Insert a value at the specified key.
+    /// This put will allow duplicate entries.  If you wish to have duplicate entries
+    /// rejected, use the `put_with_flags` function and specify NO_DUP_DATA
+    pub fn put<T, K>(&self, writer: &mut Writer<T>, k: K, v: &Value) -> EmptyResult
+    where
+        T: BackendRwTransaction<Database = D>,
+        K: AsRef<[u8]>,
+    {
+        writer.put(&self.db, &k, v, T::Flags::empty())
+    }
+
+    pub fn put_with_flags<T, K>(
+        &self,
+        writer: &mut Writer<T>,
+        k: K,
+        v: &Value,
+        flags: T::Flags,
+    ) -> EmptyResult
+    where
+        T: BackendRwTransaction<Database = D>,
+        K: AsRef<[u8]>,
+    {
+        writer.put(&self.db, &k, v, flags)
+    }
+
+    pub fn delete_all<T, K>(&self, writer: &mut Writer<T>, k: K) -> EmptyResult
+    where
+        T: BackendRwTransaction<Database = D>,
+        K: AsRef<[u8]>,
+    {
+        writer.delete(&self.db, &k, None)
+    }
+
+    pub fn delete<T, K>(&self, writer: &mut Writer<T>, k: K, v: &Value) -> EmptyResult
+    where
+        T: BackendRwTransaction<Database = D>,
+        K: AsRef<[u8]>,
+    {
+        writer.delete(&self.db, &k, Some(&v.to_bytes()?))
+    }
+
+    pub fn clear<T>(&self, writer: &mut Writer<T>) -> EmptyResult
+    where
+        T: BackendRwTransaction<Database = D>,
+    {
+        writer.clear(&self.db)
+    }
+}
+
+impl<'i, I> Iterator for Iter<'i, I>
+where
+    I: BackendIter<'i>,
+{
+    type Item = Result<(&'i [u8], Value<'i>), StoreError>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        match self.iter.next() {
+            None => None,
+            Some(Ok((key, bytes))) => match read_transform(Ok(bytes)) {
+                Ok(val) => Some(Ok((key, val))),
+                Err(err) => Some(Err(err)),
+            },
+            Some(Err(err)) => Some(Err(err.into())),
+        }
+    }
+}
diff --git a/third_party/rust/rkv/src/store/single.rs b/third_party/rust/rkv/src/store/single.rs
new file mode 100644
index 0000000000..c456e6aaa6
--- /dev/null
+++ b/third_party/rust/rkv/src/store/single.rs
@@ -0,0 +1,132 @@
+// Copyright 2018 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+use std::marker::PhantomData;
+
+use crate::{
+    backend::{BackendDatabase, BackendFlags, BackendIter, BackendRoCursor, BackendRwTransaction},
+    error::StoreError,
+    helpers::read_transform,
+    readwrite::{Readable, Writer},
+    value::Value,
+};
+
+type EmptyResult = Result<(), StoreError>;
+
+#[derive(Debug, Eq, PartialEq, Copy, Clone)]
+pub struct SingleStore<D> {
+    db: D,
+}
+
+pub struct Iter<'i, I> {
+    iter: I,
+    phantom: PhantomData<&'i ()>,
+}
+
+impl<D> SingleStore<D>
+where
+    D: BackendDatabase,
+{
+    pub(crate) fn new(db: D) -> SingleStore<D> {
+        SingleStore { db }
+    }
+
+    pub fn get<'r, R, K>(&self, reader: &'r R, k: K) -> Result<Option<Value<'r>>, StoreError>
+    where
+        R: Readable<'r, Database = D>,
+        K: AsRef<[u8]>,
+    {
+        reader.get(&self.db, &k)
+    }
+
+    // TODO: flags
+    pub fn put<T, K>(&self, writer: &mut Writer<T>, k: K, v: &Value) -> EmptyResult
+    where
+        T: BackendRwTransaction<Database = D>,
+        K: AsRef<[u8]>,
+    {
+        writer.put(&self.db, &k, v, T::Flags::empty())
+    }
+
+    #[cfg(not(feature = "db-dup-sort"))]
+    pub fn delete<T, K>(&self, writer: &mut Writer<T>, k: K) -> EmptyResult
+    where
+        T: BackendRwTransaction<Database = D>,
+        K: AsRef<[u8]>,
+    {
+        writer.delete(&self.db, &k)
+    }
+
+    #[cfg(feature = "db-dup-sort")]
+    pub fn delete<T, K>(&self, writer: &mut Writer<T>, k: K) -> EmptyResult
+    where
+        T: BackendRwTransaction<Database = D>,
+        K: AsRef<[u8]>,
+    {
+        writer.delete(&self.db, &k, None)
+    }
+
+    pub fn iter_start<'r, R, I, C>(&self, reader: &'r R) -> Result<Iter<'r, I>, StoreError>
+    where
+        R: Readable<'r, Database = D, RoCursor = C>,
+        I: BackendIter<'r>,
+        C: BackendRoCursor<'r, Iter = I>,
+    {
+        let cursor = reader.open_ro_cursor(&self.db)?;
+        let iter = cursor.into_iter();
+
+        Ok(Iter {
+            iter,
+            phantom: PhantomData,
+        })
+    }
+
+    pub fn iter_from<'r, R, I, C, K>(&self, reader: &'r R, k: K) -> Result<Iter<'r, I>, StoreError>
+    where
+        R: Readable<'r, Database = D, RoCursor = C>,
+        I: BackendIter<'r>,
+        C: BackendRoCursor<'r, Iter = I>,
+        K: AsRef<[u8]> + 'r,
+    {
+        let cursor = reader.open_ro_cursor(&self.db)?;
+        let iter = cursor.into_iter_from(k);
+
+        Ok(Iter {
+            iter,
+            phantom: PhantomData,
+        })
+    }
+
+    pub fn clear<T>(&self, writer: &mut Writer<T>) -> EmptyResult
+    where
+        D: BackendDatabase,
+        T: BackendRwTransaction<Database = D>,
+    {
+        writer.clear(&self.db)
+    }
+}
+
+impl<'i, I> Iterator for Iter<'i, I>
+where
+    I: BackendIter<'i>,
+{
+    type Item = Result<(&'i [u8], Value<'i>), StoreError>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        match self.iter.next() {
+            None => None,
+            Some(Ok((key, bytes))) => match read_transform(Ok(bytes)) {
+                Ok(val) => Some(Ok((key, val))),
+                Err(err) => Some(Err(err)),
+            },
+            Some(Err(err)) => Some(Err(err.into())),
+        }
+    }
+}
diff --git a/third_party/rust/rkv/src/value.rs b/third_party/rust/rkv/src/value.rs
new file mode 100644
index 0000000000..762607acd3
--- /dev/null
+++ b/third_party/rust/rkv/src/value.rs
@@ -0,0 +1,252 @@
+// Copyright 2018-2019 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+use std::fmt;
+
+use arrayref::array_ref;
+use bincode::{deserialize, serialize, serialized_size};
+use ordered_float::OrderedFloat;
+use uuid::{Bytes, Uuid};
+
+use crate::error::DataError;
+
+/// We define a set of types, associated with simple integers, to annotate values stored
+/// in LMDB. This is to avoid an accidental 'cast' from a value of one type to another.
+/// For this reason we don't simply use `deserialize` from the `bincode` crate.
+#[repr(u8)]
+#[derive(Debug, PartialEq, Eq)]
+pub enum Type {
+    Bool = 1,
+    U64 = 2,
+    I64 = 3,
+    F64 = 4,
+    Instant = 5, // Millisecond-precision timestamp.
+    Uuid = 6,
+    Str = 7,
+    Json = 8,
+    Blob = 9,
+}
+
+/// We use manual tagging, because <https://github.com/serde-rs/serde/issues/610>.
+impl Type {
+    pub fn from_tag(tag: u8) -> Result<Type, DataError> {
+        #![allow(clippy::unnecessary_lazy_evaluations)]
+        Type::from_primitive(tag).ok_or_else(|| DataError::UnknownType(tag))
+    }
+
+    #[allow(clippy::wrong_self_convention)]
+    pub fn to_tag(self) -> u8 {
+        self as u8
+    }
+
+    fn from_primitive(p: u8) -> Option<Type> {
+        match p {
+            1 => Some(Type::Bool),
+            2 => Some(Type::U64),
+            3 => Some(Type::I64),
+            4 => Some(Type::F64),
+            5 => Some(Type::Instant),
+            6 => Some(Type::Uuid),
+            7 => Some(Type::Str),
+            8 => Some(Type::Json),
+            9 => Some(Type::Blob),
+            _ => None,
+        }
+    }
+}
+
+impl fmt::Display for Type {
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        f.write_str(match *self {
+            Type::Bool => "bool",
+            Type::U64 => "u64",
+            Type::I64 => "i64",
+            Type::F64 => "f64",
+            Type::Instant => "instant",
+            Type::Uuid => "uuid",
+            Type::Str => "str",
+            Type::Json => "json",
+            Type::Blob => "blob",
+        })
+    }
+}
+
+#[derive(Debug, Eq, PartialEq)]
+pub enum Value<'v> {
+    Bool(bool),
+    U64(u64),
+    I64(i64),
+    F64(OrderedFloat<f64>),
+    Instant(i64), // Millisecond-precision timestamp.
+    Uuid(&'v Bytes),
+    Str(&'v str),
+    Json(&'v str),
+    Blob(&'v [u8]),
+}
+
+#[derive(Clone, Debug, PartialEq)]
+pub enum OwnedValue {
+    Bool(bool),
+    U64(u64),
+    I64(i64),
+    F64(f64),
+    Instant(i64), // Millisecond-precision timestamp.
+    Uuid(Uuid),
+    Str(String),
+    Json(String), // TODO
+    Blob(Vec<u8>),
+}
+
+fn uuid(bytes: &[u8]) -> Result<Value, DataError> {
+    if bytes.len() == 16 {
+        Ok(Value::Uuid(array_ref![bytes, 0, 16]))
+    } else {
+        Err(DataError::InvalidUuid)
+    }
+}
+
+impl<'v> Value<'v> {
+    pub fn from_tagged_slice(slice: &'v [u8]) -> Result<Value<'v>, DataError> {
+        let (tag, data) = slice.split_first().ok_or(DataError::Empty)?;
+        let t = Type::from_tag(*tag)?;
+        Value::from_type_and_data(t, data)
+    }
+
+    fn from_type_and_data(t: Type, data: &'v [u8]) -> Result<Value<'v>, DataError> {
+        if t == Type::Uuid {
+            return deserialize(data)
+                .map_err(|e| DataError::DecodingError {
+                    value_type: t,
+                    err: e,
+                })
+                .map(uuid)?;
+        }
+
+        match t {
+            Type::Bool => deserialize(data).map(Value::Bool),
+            Type::U64 => deserialize(data).map(Value::U64),
+            Type::I64 => deserialize(data).map(Value::I64),
+            Type::F64 => deserialize(data).map(OrderedFloat).map(Value::F64),
+            Type::Instant => deserialize(data).map(Value::Instant),
+            Type::Str => deserialize(data).map(Value::Str),
+            Type::Json => deserialize(data).map(Value::Json),
+            Type::Blob => deserialize(data).map(Value::Blob),
+            Type::Uuid => {
+                // Processed above to avoid verbose duplication of error transforms.
+                unreachable!()
+            }
+        }
+        .map_err(|e| DataError::DecodingError {
+            value_type: t,
+            err: e,
+        })
+    }
+
+    pub fn to_bytes(&self) -> Result<Vec<u8>, DataError> {
+        match self {
+            Value::Bool(v) => serialize(&(Type::Bool.to_tag(), *v)),
+            Value::U64(v) => serialize(&(Type::U64.to_tag(), *v)),
+            Value::I64(v) => serialize(&(Type::I64.to_tag(), *v)),
+            Value::F64(v) => serialize(&(Type::F64.to_tag(), v.0)),
+            Value::Instant(v) => serialize(&(Type::Instant.to_tag(), *v)),
+            Value::Str(v) => serialize(&(Type::Str.to_tag(), v)),
+            Value::Json(v) => serialize(&(Type::Json.to_tag(), v)),
+            Value::Blob(v) => serialize(&(Type::Blob.to_tag(), v)),
+            Value::Uuid(v) => serialize(&(Type::Uuid.to_tag(), v)),
+        }
+        .map_err(DataError::EncodingError)
+    }
+
+    pub fn serialized_size(&self) -> Result<u64, DataError> {
+        match self {
+            Value::Bool(v) => serialized_size(&(Type::Bool.to_tag(), *v)),
+            Value::U64(v) => serialized_size(&(Type::U64.to_tag(), *v)),
+            Value::I64(v) => serialized_size(&(Type::I64.to_tag(), *v)),
+            Value::F64(v) => serialized_size(&(Type::F64.to_tag(), v.0)),
+            Value::Instant(v) => serialized_size(&(Type::Instant.to_tag(), *v)),
+            Value::Str(v) => serialized_size(&(Type::Str.to_tag(), v)),
+            Value::Json(v) => serialized_size(&(Type::Json.to_tag(), v)),
+            Value::Blob(v) => serialized_size(&(Type::Blob.to_tag(), v)),
+            Value::Uuid(v) => serialized_size(&(Type::Uuid.to_tag(), v)),
+        }
+        .map_err(DataError::EncodingError)
+    }
+}
+
+impl<'v> From<&'v Value<'v>> for OwnedValue {
+    fn from(value: &Value) -> OwnedValue {
+        match value {
+            Value::Bool(v) => OwnedValue::Bool(*v),
+            Value::U64(v) => OwnedValue::U64(*v),
+            Value::I64(v) => OwnedValue::I64(*v),
+            Value::F64(v) => OwnedValue::F64(**v),
+            Value::Instant(v) => OwnedValue::Instant(*v),
+            Value::Uuid(v) => OwnedValue::Uuid(Uuid::from_bytes(**v)),
+            Value::Str(v) => OwnedValue::Str((*v).to_string()),
+            Value::Json(v) => OwnedValue::Json((*v).to_string()),
+            Value::Blob(v) => OwnedValue::Blob(v.to_vec()),
+        }
+    }
+}
+
+impl<'v> From<&'v OwnedValue> for Value<'v> {
+    fn from(value: &OwnedValue) -> Value {
+        match value {
+            OwnedValue::Bool(v) => Value::Bool(*v),
+            OwnedValue::U64(v) => Value::U64(*v),
+            OwnedValue::I64(v) => Value::I64(*v),
+            OwnedValue::F64(v) => Value::F64(OrderedFloat::from(*v)),
+            OwnedValue::Instant(v) => Value::Instant(*v),
+            OwnedValue::Uuid(v) => Value::Uuid(v.as_bytes()),
+            OwnedValue::Str(v) => Value::Str(v),
+            OwnedValue::Json(v) => Value::Json(v),
+            OwnedValue::Blob(v) => Value::Blob(v),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_value_serialized_size() {
+        // | Value enum    | tag: 1 byte   |     value_payload        |
+        // |----------------------------------------------------------|
+        // |   I64         |     1         |       8                  |
+        // |   U64         |     1         |       8                  |
+        // |   Bool        |     1         |       1                  |
+        // |   Instant     |     1         |       8                  |
+        // |   F64         |     1         |       8                  |
+        // |   Uuid        |     1         |       16                 |
+        // | Str/Blob/Json |     1         |(8: len + sizeof(payload))|
+        assert_eq!(Value::I64(-1000).serialized_size().unwrap(), 9);
+        assert_eq!(Value::U64(1000u64).serialized_size().unwrap(), 9);
+        assert_eq!(Value::Bool(true).serialized_size().unwrap(), 2);
+        assert_eq!(
+            Value::Instant(1_558_020_865_224).serialized_size().unwrap(),
+            9
+        );
+        assert_eq!(
+            Value::F64(OrderedFloat(10000.1)).serialized_size().unwrap(),
+            9
+        );
+        assert_eq!(Value::Str("hello!").serialized_size().unwrap(), 15);
+        assert_eq!(Value::Str("¡Hola").serialized_size().unwrap(), 15);
+        assert_eq!(Value::Blob(b"hello!").serialized_size().unwrap(), 15);
+        assert_eq!(
+            uuid(b"\x9f\xe2\xc4\xe9\x3f\x65\x4f\xdb\xb2\x4c\x02\xb1\x52\x59\x71\x6c")
+                .unwrap()
+                .serialized_size()
+                .unwrap(),
+            17
+        );
+    }
+}