Adding upstream version 115.8.0esr.upstream/115.8.0esr

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

11 files changed, 2336 insertions, 0 deletions

diff --git a/third_party/rust/tabs/src/error.rs b/third_party/rust/tabs/src/error.rs
new file mode 100644
index 0000000000..748c21f656
--- /dev/null
+++ b/third_party/rust/tabs/src/error.rs
@@ -0,0 +1,90 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use error_support::{ErrorHandling, GetErrorHandling};
+
+/// Result enum for the public interface
+pub type ApiResult<T> = std::result::Result<T, TabsApiError>;
+/// Result enum for internal functions
+pub type Result<T> = std::result::Result<T, Error>;
+
+// Errors we return via the public interface.
+#[derive(Debug, thiserror::Error)]
+pub enum TabsApiError {
+    #[error("SyncError: {reason}")]
+    SyncError { reason: String },
+
+    #[error("SqlError: {reason}")]
+    SqlError { reason: String },
+
+    #[error("Unexpected tabs error: {reason}")]
+    UnexpectedTabsError { reason: String },
+}
+
+// Error we use internally
+#[derive(Debug, thiserror::Error)]
+pub enum Error {
+    #[cfg(feature = "full-sync")]
+    #[error("Error synchronizing: {0}")]
+    SyncAdapterError(#[from] sync15::Error),
+
+    // Note we are abusing this as a kind of "mis-matched feature" error.
+    // This works because when `full-sync` isn't enabled we don't actually
+    // handle any sync15 errors as the bridged-engine never returns them.
+    #[cfg(not(feature = "full-sync"))]
+    #[error("Sync feature is disabled: {0}")]
+    SyncAdapterError(String),
+
+    #[error("Error parsing JSON data: {0}")]
+    JsonError(#[from] serde_json::Error),
+
+    #[error("Missing SyncUnlockInfo Local ID")]
+    MissingLocalIdError,
+
+    #[error("Error parsing URL: {0}")]
+    UrlParseError(#[from] url::ParseError),
+
+    #[error("Error executing SQL: {0}")]
+    SqlError(#[from] rusqlite::Error),
+
+    #[error("Error opening database: {0}")]
+    OpenDatabaseError(#[from] sql_support::open_database::Error),
+}
+
+// Define how our internal errors are handled and converted to external errors
+// See `support/error/README.md` for how this works, especially the warning about PII.
+impl GetErrorHandling for Error {
+    type ExternalError = TabsApiError;
+
+    fn get_error_handling(&self) -> ErrorHandling<Self::ExternalError> {
+        match self {
+            Self::SyncAdapterError(e) => ErrorHandling::convert(TabsApiError::SyncError {
+                reason: e.to_string(),
+            })
+            .report_error("tabs-sync-error"),
+            Self::JsonError(e) => ErrorHandling::convert(TabsApiError::UnexpectedTabsError {
+                reason: e.to_string(),
+            })
+            .report_error("tabs-json-error"),
+            Self::MissingLocalIdError => {
+                ErrorHandling::convert(TabsApiError::UnexpectedTabsError {
+                    reason: "MissingLocalId".to_string(),
+                })
+                .report_error("tabs-missing-local-id-error")
+            }
+            Self::UrlParseError(e) => ErrorHandling::convert(TabsApiError::UnexpectedTabsError {
+                reason: e.to_string(),
+            })
+            .report_error("tabs-url-parse-error"),
+            Self::SqlError(e) => ErrorHandling::convert(TabsApiError::SqlError {
+                reason: e.to_string(),
+            })
+            .report_error("tabs-sql-error"),
+            Self::OpenDatabaseError(e) => ErrorHandling::convert(TabsApiError::SqlError {
+                reason: e.to_string(),
+            })
+            .report_error("tabs-open-database-error"),
+        }
+    }
+}
diff --git a/third_party/rust/tabs/src/lib.rs b/third_party/rust/tabs/src/lib.rs
new file mode 100644
index 0000000000..58f32ea5e0
--- /dev/null
+++ b/third_party/rust/tabs/src/lib.rs
@@ -0,0 +1,39 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#![allow(unknown_lints)]
+#![warn(rust_2018_idioms)]
+
+#[macro_use]
+pub mod error;
+mod schema;
+mod storage;
+mod store;
+mod sync;
+
+uniffi::include_scaffolding!("tabs");
+
+// Our UDL uses a `Guid` type.
+use sync_guid::Guid as TabsGuid;
+impl UniffiCustomTypeConverter for TabsGuid {
+    type Builtin = String;
+
+    fn into_custom(val: Self::Builtin) -> uniffi::Result<TabsGuid> {
+        Ok(TabsGuid::new(val.as_str()))
+    }
+
+    fn from_custom(obj: Self) -> Self::Builtin {
+        obj.into()
+    }
+}
+
+pub use crate::storage::{ClientRemoteTabs, RemoteTabRecord, TabsDeviceType};
+pub use crate::store::TabsStore;
+pub use error::{ApiResult, Error, Result, TabsApiError};
+use sync15::DeviceType;
+
+pub use crate::sync::engine::get_registered_sync_engine;
+
+pub use crate::sync::bridge::TabsBridgedEngine;
+pub use crate::sync::engine::TabsEngine;
diff --git a/third_party/rust/tabs/src/schema.rs b/third_party/rust/tabs/src/schema.rs
new file mode 100644
index 0000000000..ad8713b487
--- /dev/null
+++ b/third_party/rust/tabs/src/schema.rs
@@ -0,0 +1,150 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// Tabs is a bit special - it's a trivial SQL schema and is only used as a persistent
+// cache, and the semantics of the "tabs" collection means there's no need for
+// syncChangeCounter/syncStatus nor a mirror etc.
+
+use rusqlite::{Connection, Transaction};
+use sql_support::{
+    open_database::{
+        ConnectionInitializer as MigrationLogic, Error as MigrationError, Result as MigrationResult,
+    },
+    ConnExt,
+};
+
+// The record is the TabsRecord struct in json and this module doesn't need to deserialize, so we just
+// store each client as its own row.
+const CREATE_SCHEMA_SQL: &str = "
+    CREATE TABLE IF NOT EXISTS tabs (
+        guid            TEXT NOT NULL PRIMARY KEY,
+        record          TEXT NOT NULL,
+        last_modified   INTEGER NOT NULL
+    );
+";
+
+const CREATE_META_TABLE_SQL: &str = "
+    CREATE TABLE IF NOT EXISTS moz_meta (
+        key    TEXT PRIMARY KEY,
+        value  NOT NULL
+    )
+";
+
+pub(crate) static LAST_SYNC_META_KEY: &str = "last_sync_time";
+pub(crate) static GLOBAL_SYNCID_META_KEY: &str = "global_sync_id";
+pub(crate) static COLLECTION_SYNCID_META_KEY: &str = "tabs_sync_id";
+// Tabs stores this in the meta table due to a unique requirement that we only know the list
+// of connected clients when syncing, however getting the list of tabs could be called at anytime
+// so we store it so we can translate from the tabs sync record ID to the FxA device id for the client
+pub(crate) static REMOTE_CLIENTS_KEY: &str = "remote_clients";
+
+pub struct TabsMigrationLogic;
+
+impl MigrationLogic for TabsMigrationLogic {
+    const NAME: &'static str = "tabs storage db";
+    const END_VERSION: u32 = 2;
+
+    fn prepare(&self, conn: &Connection, _db_empty: bool) -> MigrationResult<()> {
+        let initial_pragmas = "
+            -- We don't care about temp tables being persisted to disk.
+            PRAGMA temp_store = 2;
+            -- we unconditionally want write-ahead-logging mode.
+            PRAGMA journal_mode=WAL;
+            -- foreign keys seem worth enforcing (and again, we don't care in practice)
+            PRAGMA foreign_keys = ON;
+        ";
+        conn.execute_batch(initial_pragmas)?;
+        // This is where we'd define our sql functions if we had any!
+        conn.set_prepared_statement_cache_capacity(128);
+        Ok(())
+    }
+
+    fn init(&self, db: &Transaction<'_>) -> MigrationResult<()> {
+        log::debug!("Creating schemas");
+        db.execute_all(&[CREATE_SCHEMA_SQL, CREATE_META_TABLE_SQL])?;
+        Ok(())
+    }
+
+    fn upgrade_from(&self, db: &Transaction<'_>, version: u32) -> MigrationResult<()> {
+        match version {
+            1 => upgrade_from_v1(db),
+            _ => Err(MigrationError::IncompatibleVersion(version)),
+        }
+    }
+}
+
+fn upgrade_from_v1(db: &Connection) -> MigrationResult<()> {
+    // The previous version stored the entire payload in one row
+    // and cleared on each sync -- it's fine to just drop it
+    db.execute_batch("DROP TABLE tabs;")?;
+    // Recreate the world
+    db.execute_all(&[CREATE_SCHEMA_SQL, CREATE_META_TABLE_SQL])?;
+    Ok(())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::storage::TabsStorage;
+    use rusqlite::OptionalExtension;
+    use serde_json::json;
+    use sql_support::open_database::test_utils::MigratedDatabaseFile;
+
+    const CREATE_V1_SCHEMA_SQL: &str = "
+        CREATE TABLE IF NOT EXISTS tabs (
+            payload TEXT NOT NULL
+        );
+        PRAGMA user_version=1;
+    ";
+
+    #[test]
+    fn test_create_schema_twice() {
+        let mut db = TabsStorage::new_with_mem_path("test");
+        let conn = db.open_or_create().unwrap();
+        conn.execute_batch(CREATE_SCHEMA_SQL)
+            .expect("should allow running twice");
+    }
+
+    #[test]
+    fn test_tabs_db_upgrade_from_v1() {
+        let db_file = MigratedDatabaseFile::new(TabsMigrationLogic, CREATE_V1_SCHEMA_SQL);
+        db_file.run_all_upgrades();
+        // Verify we can open the DB just fine, since migration is essentially a drop
+        // we don't need to check any data integrity
+        let mut storage = TabsStorage::new(db_file.path);
+        storage.open_or_create().unwrap();
+        assert!(storage.open_if_exists().unwrap().is_some());
+
+        let test_payload = json!({
+            "id": "device-with-a-tab",
+            "clientName": "device with a tab",
+            "tabs": [{
+                "title": "the title",
+                "urlHistory": [
+                    "https://mozilla.org/"
+                ],
+                "icon": "https://mozilla.org/icon",
+                "lastUsed": 1643764207,
+            }]
+        });
+        let db = storage.open_if_exists().unwrap().unwrap();
+        // We should be able to insert without a SQL error after upgrade
+        db.execute(
+            "INSERT INTO tabs (guid, record, last_modified) VALUES (:guid, :record, :last_modified);",
+            rusqlite::named_params! {
+                ":guid": "my-device",
+                ":record": serde_json::to_string(&test_payload).unwrap(),
+                ":last_modified": "1643764207"
+            },
+        )
+        .unwrap();
+
+        let row: Option<String> = db
+            .query_row("SELECT guid FROM tabs;", [], |row| row.get(0))
+            .optional()
+            .unwrap();
+        // Verify we can query for a valid guid now
+        assert_eq!(row.unwrap(), "my-device");
+    }
+}
diff --git a/third_party/rust/tabs/src/storage.rs b/third_party/rust/tabs/src/storage.rs
new file mode 100644
index 0000000000..eaccbc3446
--- /dev/null
+++ b/third_party/rust/tabs/src/storage.rs
@@ -0,0 +1,727 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// From https://searchfox.org/mozilla-central/rev/ea63a0888d406fae720cf24f4727d87569a8cab5/services/sync/modules/constants.js#75
+const URI_LENGTH_MAX: usize = 65536;
+// https://searchfox.org/mozilla-central/rev/ea63a0888d406fae720cf24f4727d87569a8cab5/services/sync/modules/engines/tabs.js#8
+const TAB_ENTRIES_LIMIT: usize = 5;
+
+use crate::error::*;
+use crate::schema;
+use crate::sync::record::TabsRecord;
+use crate::DeviceType;
+use rusqlite::{
+    types::{FromSql, ToSql},
+    Connection, OpenFlags,
+};
+use serde_derive::{Deserialize, Serialize};
+use sql_support::open_database::{self, open_database_with_flags};
+use sql_support::ConnExt;
+use std::cell::RefCell;
+use std::collections::HashMap;
+use std::path::{Path, PathBuf};
+use sync15::{RemoteClient, ServerTimestamp};
+pub type TabsDeviceType = crate::DeviceType;
+pub type RemoteTabRecord = RemoteTab;
+
+pub(crate) const TABS_CLIENT_TTL: u32 = 15_552_000; // 180 days, same as CLIENTS_TTL
+const FAR_FUTURE: i64 = 4_102_405_200_000; // 2100/01/01
+const MAX_PAYLOAD_SIZE: usize = 512 * 1024; // Twice as big as desktop, still smaller than server max (2MB)
+const MAX_TITLE_CHAR_LENGTH: usize = 512; // We put an upper limit on title sizes for tabs to reduce memory
+
+#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
+pub struct RemoteTab {
+    pub title: String,
+    pub url_history: Vec<String>,
+    pub icon: Option<String>,
+    pub last_used: i64, // In ms.
+}
+
+#[derive(Clone, Debug, Serialize, Deserialize)]
+pub struct ClientRemoteTabs {
+    // The fxa_device_id of the client. *Should not* come from the id in the `clients` collection,
+    // because that may or may not be the fxa_device_id (currently, it will not be for desktop
+    // records.)
+    pub client_id: String,
+    pub client_name: String,
+    #[serde(
+        default = "devicetype_default_deser",
+        skip_serializing_if = "devicetype_is_unknown"
+    )]
+    pub device_type: DeviceType,
+    // serde default so we can read old rows that didn't persist this.
+    #[serde(default)]
+    pub last_modified: i64,
+    pub remote_tabs: Vec<RemoteTab>,
+}
+
+fn devicetype_default_deser() -> DeviceType {
+    // replace with `DeviceType::default_deser` once #4861 lands.
+    DeviceType::Unknown
+}
+
+// Unlike most other uses-cases, here we do allow serializing ::Unknown, but skip it.
+fn devicetype_is_unknown(val: &DeviceType) -> bool {
+    matches!(val, DeviceType::Unknown)
+}
+
+// Tabs has unique requirements for storage:
+// * The "local_tabs" exist only so we can sync them out. There's no facility to
+//   query "local tabs", so there's no need to store these persistently - ie, they
+//   are write-only.
+// * The "remote_tabs" exist purely for incoming items via sync - there's no facility
+//   to set them locally - they are read-only.
+// Note that this means a database is only actually needed after Sync fetches remote tabs,
+// and because sync users are in the minority, the use of a database here is purely
+// optional and created on demand. The implication here is that asking for the "remote tabs"
+// when no database exists is considered a normal situation and just implies no remote tabs exist.
+// (Note however we don't attempt to remove the database when no remote tabs exist, so having
+// no remote tabs in an existing DB is also a normal situation)
+pub struct TabsStorage {
+    local_tabs: RefCell<Option<Vec<RemoteTab>>>,
+    db_path: PathBuf,
+    db_connection: Option<Connection>,
+}
+
+impl TabsStorage {
+    pub fn new(db_path: impl AsRef<Path>) -> Self {
+        Self {
+            local_tabs: RefCell::default(),
+            db_path: db_path.as_ref().to_path_buf(),
+            db_connection: None,
+        }
+    }
+
+    /// Arrange for a new memory-based TabsStorage. As per other DB semantics, creating
+    /// this isn't enough to actually create the db!
+    pub fn new_with_mem_path(db_path: &str) -> Self {
+        let name = PathBuf::from(format!("file:{}?mode=memory&cache=shared", db_path));
+        Self::new(name)
+    }
+
+    /// If a DB file exists, open and return it.
+    pub fn open_if_exists(&mut self) -> Result<Option<&Connection>> {
+        if let Some(ref existing) = self.db_connection {
+            return Ok(Some(existing));
+        }
+        let flags = OpenFlags::SQLITE_OPEN_NO_MUTEX
+            | OpenFlags::SQLITE_OPEN_URI
+            | OpenFlags::SQLITE_OPEN_READ_WRITE;
+        match open_database_with_flags(
+            self.db_path.clone(),
+            flags,
+            &crate::schema::TabsMigrationLogic,
+        ) {
+            Ok(conn) => {
+                self.db_connection = Some(conn);
+                Ok(self.db_connection.as_ref())
+            }
+            Err(open_database::Error::SqlError(rusqlite::Error::SqliteFailure(code, _)))
+                if code.code == rusqlite::ErrorCode::CannotOpen =>
+            {
+                Ok(None)
+            }
+            Err(e) => Err(e.into()),
+        }
+    }
+
+    /// Open and return the DB, creating it if necessary.
+    pub fn open_or_create(&mut self) -> Result<&Connection> {
+        if let Some(ref existing) = self.db_connection {
+            return Ok(existing);
+        }
+        let flags = OpenFlags::SQLITE_OPEN_NO_MUTEX
+            | OpenFlags::SQLITE_OPEN_URI
+            | OpenFlags::SQLITE_OPEN_READ_WRITE
+            | OpenFlags::SQLITE_OPEN_CREATE;
+        let conn = open_database_with_flags(
+            self.db_path.clone(),
+            flags,
+            &crate::schema::TabsMigrationLogic,
+        )?;
+        self.db_connection = Some(conn);
+        Ok(self.db_connection.as_ref().unwrap())
+    }
+
+    pub fn update_local_state(&mut self, local_state: Vec<RemoteTab>) {
+        self.local_tabs.borrow_mut().replace(local_state);
+    }
+
+    // We try our best to fit as many tabs in a payload as possible, this includes
+    // limiting the url history entries, title character count and finally drop enough tabs
+    // until we have small enough payload that the server will accept
+    pub fn prepare_local_tabs_for_upload(&self) -> Option<Vec<RemoteTab>> {
+        if let Some(local_tabs) = self.local_tabs.borrow().as_ref() {
+            let mut sanitized_tabs: Vec<RemoteTab> = local_tabs
+                .iter()
+                .cloned()
+                .filter_map(|mut tab| {
+                    if tab.url_history.is_empty() || !is_url_syncable(&tab.url_history[0]) {
+                        return None;
+                    }
+                    let mut sanitized_history = Vec::with_capacity(TAB_ENTRIES_LIMIT);
+                    for url in tab.url_history {
+                        if sanitized_history.len() == TAB_ENTRIES_LIMIT {
+                            break;
+                        }
+                        if is_url_syncable(&url) {
+                            sanitized_history.push(url);
+                        }
+                    }
+
+                    tab.url_history = sanitized_history;
+                    // Potentially truncate the title to some limit
+                    tab.title = slice_up_to(tab.title, MAX_TITLE_CHAR_LENGTH);
+                    Some(tab)
+                })
+                .collect();
+            // Sort the tabs so when we trim tabs it's the oldest tabs
+            sanitized_tabs.sort_by(|a, b| b.last_used.cmp(&a.last_used));
+            // If trimming the tab length failed for some reason, just return the untrimmed tabs
+            trim_tabs_length(&mut sanitized_tabs, MAX_PAYLOAD_SIZE);
+            return Some(sanitized_tabs);
+        }
+        None
+    }
+
+    pub fn get_remote_tabs(&mut self) -> Option<Vec<ClientRemoteTabs>> {
+        let conn = match self.open_if_exists() {
+            Err(e) => {
+                error_support::report_error!(
+                    "tabs-read-remote",
+                    "Failed to read remote tabs: {}",
+                    e
+                );
+                return None;
+            }
+            Ok(None) => return None,
+            Ok(Some(conn)) => conn,
+        };
+
+        let records: Vec<(TabsRecord, ServerTimestamp)> = match conn.query_rows_and_then_cached(
+            "SELECT record, last_modified FROM tabs",
+            [],
+            |row| -> Result<_> {
+                Ok((
+                    serde_json::from_str(&row.get::<_, String>(0)?)?,
+                    ServerTimestamp(row.get::<_, i64>(1)?),
+                ))
+            },
+        ) {
+            Ok(records) => records,
+            Err(e) => {
+                error_support::report_error!("tabs-read-remote", "Failed to read database: {}", e);
+                return None;
+            }
+        };
+        let mut crts: Vec<ClientRemoteTabs> = Vec::new();
+        let remote_clients: HashMap<String, RemoteClient> =
+            match self.get_meta::<String>(schema::REMOTE_CLIENTS_KEY) {
+                Err(e) => {
+                    error_support::report_error!(
+                        "tabs-read-remote",
+                        "Failed to get remote clients: {}",
+                        e
+                    );
+                    return None;
+                }
+                // We don't return early here since we still store tabs even if we don't
+                // "know" about the client it's associated with (incase it becomes available later)
+                Ok(None) => HashMap::default(),
+                Ok(Some(json)) => serde_json::from_str(&json).unwrap(),
+            };
+        for (record, last_modified) in records {
+            let id = record.id.clone();
+            let crt = if let Some(remote_client) = remote_clients.get(&id) {
+                ClientRemoteTabs::from_record_with_remote_client(
+                    remote_client
+                        .fxa_device_id
+                        .as_ref()
+                        .unwrap_or(&id)
+                        .to_owned(),
+                    last_modified,
+                    remote_client,
+                    record,
+                )
+            } else {
+                // A record with a device that's not in our remote clients seems unlikely, but
+                // could happen - in most cases though, it will be due to a disconnected client -
+                // so we really should consider just dropping it? (Sadly though, it does seem
+                // possible it's actually a very recently connected client, so we keep it)
+                // We should get rid of this eventually - https://github.com/mozilla/application-services/issues/5199
+                log::info!(
+                    "Storing tabs from a client that doesn't appear in the devices list: {}",
+                    id,
+                );
+                ClientRemoteTabs::from_record(id, last_modified, record)
+            };
+            crts.push(crt);
+        }
+        Some(crts)
+    }
+
+    // Keep DB from growing infinitely since we only ask for records since our last sync
+    // and may or may not know about the client it's associated with -- but we could at some point
+    // and should start returning those tabs immediately. If that client hasn't been seen in 3 weeks,
+    // we remove it until it reconnects
+    pub fn remove_stale_clients(&mut self) -> Result<()> {
+        let last_sync = self.get_meta::<i64>(schema::LAST_SYNC_META_KEY)?;
+        if let Some(conn) = self.open_if_exists()? {
+            if let Some(last_sync) = last_sync {
+                let client_ttl_ms = (TABS_CLIENT_TTL as i64) * 1000;
+                // On desktop, a quick write temporarily sets the last_sync to FAR_FUTURE
+                // but if it doesn't set it back to the original (crash, etc) it
+                // means we'll most likely trash all our records (as it's more than any TTL we'd ever do)
+                // so we need to detect this for now until we have native quick write support
+                if last_sync - client_ttl_ms >= 0 && last_sync != (FAR_FUTURE * 1000) {
+                    let tx = conn.unchecked_transaction()?;
+                    let num_removed = tx.execute_cached(
+                        "DELETE FROM tabs WHERE last_modified <= :last_sync - :ttl",
+                        rusqlite::named_params! {
+                            ":last_sync": last_sync,
+                            ":ttl": client_ttl_ms,
+                        },
+                    )?;
+                    log::info!(
+                        "removed {} stale clients (threshold was {})",
+                        num_removed,
+                        last_sync - client_ttl_ms
+                    );
+                    tx.commit()?;
+                }
+            }
+        }
+        Ok(())
+    }
+}
+
+impl TabsStorage {
+    pub(crate) fn replace_remote_tabs(
+        &mut self,
+        // This is a tuple because we need to know what the server reports
+        // as the last time a record was modified
+        new_remote_tabs: Vec<(TabsRecord, ServerTimestamp)>,
+    ) -> Result<()> {
+        let connection = self.open_or_create()?;
+        let tx = connection.unchecked_transaction()?;
+
+        // For tabs it's fine if we override the existing tabs for a remote
+        // there can only ever be one record for each client
+        for remote_tab in new_remote_tabs {
+            let record = remote_tab.0;
+            let last_modified = remote_tab.1;
+            log::info!(
+                "inserting tab for device {}, last modified at {}",
+                record.id,
+                last_modified.as_millis()
+            );
+            tx.execute_cached(
+                "INSERT OR REPLACE INTO tabs (guid, record, last_modified) VALUES (:guid, :record, :last_modified);",
+                rusqlite::named_params! {
+                    ":guid": &record.id,
+                    ":record": serde_json::to_string(&record).expect("tabs don't fail to serialize"),
+                    ":last_modified": last_modified.as_millis()
+                },
+            )?;
+        }
+        tx.commit()?;
+        Ok(())
+    }
+
+    pub(crate) fn wipe_remote_tabs(&mut self) -> Result<()> {
+        if let Some(db) = self.open_if_exists()? {
+            db.execute_batch("DELETE FROM tabs")?;
+        }
+        Ok(())
+    }
+
+    pub(crate) fn wipe_local_tabs(&self) {
+        self.local_tabs.replace(None);
+    }
+
+    pub(crate) fn put_meta(&mut self, key: &str, value: &dyn ToSql) -> Result<()> {
+        let db = self.open_or_create()?;
+        db.execute_cached(
+            "REPLACE INTO moz_meta (key, value) VALUES (:key, :value)",
+            &[(":key", &key as &dyn ToSql), (":value", value)],
+        )?;
+        Ok(())
+    }
+
+    pub(crate) fn get_meta<T: FromSql>(&mut self, key: &str) -> Result<Option<T>> {
+        match self.open_if_exists() {
+            Ok(Some(db)) => {
+                let res = db.try_query_one(
+                    "SELECT value FROM moz_meta WHERE key = :key",
+                    &[(":key", &key)],
+                    true,
+                )?;
+                Ok(res)
+            }
+            Err(e) => Err(e),
+            Ok(None) => Ok(None),
+        }
+    }
+
+    pub(crate) fn delete_meta(&mut self, key: &str) -> Result<()> {
+        if let Some(db) = self.open_if_exists()? {
+            db.execute_cached("DELETE FROM moz_meta WHERE key = :key", &[(":key", &key)])?;
+        }
+        Ok(())
+    }
+}
+
+// Trim the amount of tabs in a list to fit the specified memory size
+fn trim_tabs_length(tabs: &mut Vec<RemoteTab>, payload_size_max_bytes: usize) {
+    // Ported from https://searchfox.org/mozilla-central/rev/84fb1c4511312a0b9187f647d90059e3a6dd27f8/services/sync/modules/util.sys.mjs#422
+    // See bug 535326 comment 8 for an explanation of the estimation
+    let max_serialized_size = (payload_size_max_bytes / 4) * 3 - 1500;
+    let size = compute_serialized_size(tabs);
+    if size > max_serialized_size {
+        // Estimate a little more than the direct fraction to maximize packing
+        let cutoff = (tabs.len() * max_serialized_size) / size;
+        tabs.truncate(cutoff);
+
+        // Keep dropping off the last entry until the data fits.
+        while compute_serialized_size(tabs) > max_serialized_size {
+            tabs.pop();
+        }
+    }
+}
+
+fn compute_serialized_size(v: &Vec<RemoteTab>) -> usize {
+    serde_json::to_string(v).unwrap_or_default().len()
+}
+
+// Similar to places/utils.js
+// This method ensures we safely truncate a string up to a certain max_len while
+// respecting char bounds to prevent rust panics. If we do end up truncating, we
+// append an ellipsis to the string
+pub fn slice_up_to(s: String, max_len: usize) -> String {
+    if max_len >= s.len() {
+        return s;
+    }
+
+    let ellipsis = '\u{2026}';
+    // Ensure we leave space for the ellipsis while still being under the max
+    let mut idx = max_len - ellipsis.len_utf8();
+    while !s.is_char_boundary(idx) {
+        idx -= 1;
+    }
+    let mut new_str = s[..idx].to_string();
+    new_str.push(ellipsis);
+    new_str
+}
+
+// Try to keep in sync with https://searchfox.org/mozilla-central/rev/2ad13433da20a0749e1e9a10ec0ab49b987c2c8e/modules/libpref/init/all.js#3927
+fn is_url_syncable(url: &str) -> bool {
+    url.len() <= URI_LENGTH_MAX
+        && !(url.starts_with("about:")
+            || url.starts_with("resource:")
+            || url.starts_with("chrome:")
+            || url.starts_with("wyciwyg:")
+            || url.starts_with("blob:")
+            || url.starts_with("file:")
+            || url.starts_with("moz-extension:")
+            || url.starts_with("data:"))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::sync::record::TabsRecordTab;
+
+    #[test]
+    fn test_is_url_syncable() {
+        assert!(is_url_syncable("https://bobo.com"));
+        assert!(is_url_syncable("ftp://bobo.com"));
+        assert!(!is_url_syncable("about:blank"));
+        // XXX - this smells wrong - we should insist on a valid complete URL?
+        assert!(is_url_syncable("aboutbobo.com"));
+        assert!(!is_url_syncable("file:///Users/eoger/bobo"));
+    }
+
+    #[test]
+    fn test_open_if_exists_no_file() {
+        let dir = tempfile::tempdir().unwrap();
+        let db_name = dir.path().join("test_open_for_read_no_file.db");
+        let mut storage = TabsStorage::new(db_name.clone());
+        assert!(storage.open_if_exists().unwrap().is_none());
+        storage.open_or_create().unwrap(); // will have created it.
+                                           // make a new storage, but leave the file alone.
+        let mut storage = TabsStorage::new(db_name);
+        // db file exists, so opening for read should open it.
+        assert!(storage.open_if_exists().unwrap().is_some());
+    }
+
+    #[test]
+    fn test_tabs_meta() {
+        let dir = tempfile::tempdir().unwrap();
+        let db_name = dir.path().join("test_tabs_meta.db");
+        let mut db = TabsStorage::new(db_name);
+        let test_key = "TEST KEY A";
+        let test_value = "TEST VALUE A";
+        let test_key2 = "TEST KEY B";
+        let test_value2 = "TEST VALUE B";
+
+        // should automatically make the DB if one doesn't exist
+        db.put_meta(test_key, &test_value).unwrap();
+        db.put_meta(test_key2, &test_value2).unwrap();
+
+        let retrieved_value: String = db.get_meta(test_key).unwrap().expect("test value");
+        let retrieved_value2: String = db.get_meta(test_key2).unwrap().expect("test value 2");
+
+        assert_eq!(retrieved_value, test_value);
+        assert_eq!(retrieved_value2, test_value2);
+
+        // check that the value of an existing key can be updated
+        let test_value3 = "TEST VALUE C";
+        db.put_meta(test_key, &test_value3).unwrap();
+
+        let retrieved_value3: String = db.get_meta(test_key).unwrap().expect("test value 3");
+
+        assert_eq!(retrieved_value3, test_value3);
+
+        // check that a deleted key is not retrieved
+        db.delete_meta(test_key).unwrap();
+        let retrieved_value4: Option<String> = db.get_meta(test_key).unwrap();
+        assert!(retrieved_value4.is_none());
+    }
+
+    #[test]
+    fn test_prepare_local_tabs_for_upload() {
+        let mut storage = TabsStorage::new_with_mem_path("test_prepare_local_tabs_for_upload");
+        assert_eq!(storage.prepare_local_tabs_for_upload(), None);
+        storage.update_local_state(vec![
+            RemoteTab {
+                title: "".to_owned(),
+                url_history: vec!["about:blank".to_owned(), "https://foo.bar".to_owned()],
+                icon: None,
+                last_used: 0,
+            },
+            RemoteTab {
+                title: "".to_owned(),
+                url_history: vec![
+                    "https://foo.bar".to_owned(),
+                    "about:blank".to_owned(),
+                    "about:blank".to_owned(),
+                    "about:blank".to_owned(),
+                    "about:blank".to_owned(),
+                    "about:blank".to_owned(),
+                    "about:blank".to_owned(),
+                    "about:blank".to_owned(),
+                ],
+                icon: None,
+                last_used: 0,
+            },
+            RemoteTab {
+                title: "".to_owned(),
+                url_history: vec![
+                    "https://foo.bar".to_owned(),
+                    "about:blank".to_owned(),
+                    "https://foo2.bar".to_owned(),
+                    "https://foo3.bar".to_owned(),
+                    "https://foo4.bar".to_owned(),
+                    "https://foo5.bar".to_owned(),
+                    "https://foo6.bar".to_owned(),
+                ],
+                icon: None,
+                last_used: 0,
+            },
+            RemoteTab {
+                title: "".to_owned(),
+                url_history: vec![],
+                icon: None,
+                last_used: 0,
+            },
+        ]);
+        assert_eq!(
+            storage.prepare_local_tabs_for_upload(),
+            Some(vec![
+                RemoteTab {
+                    title: "".to_owned(),
+                    url_history: vec!["https://foo.bar".to_owned()],
+                    icon: None,
+                    last_used: 0,
+                },
+                RemoteTab {
+                    title: "".to_owned(),
+                    url_history: vec![
+                        "https://foo.bar".to_owned(),
+                        "https://foo2.bar".to_owned(),
+                        "https://foo3.bar".to_owned(),
+                        "https://foo4.bar".to_owned(),
+                        "https://foo5.bar".to_owned()
+                    ],
+                    icon: None,
+                    last_used: 0,
+                },
+            ])
+        );
+    }
+    #[test]
+    fn test_trimming_tab_title() {
+        let mut storage = TabsStorage::new_with_mem_path("test_prepare_local_tabs_for_upload");
+        assert_eq!(storage.prepare_local_tabs_for_upload(), None);
+        storage.update_local_state(vec![RemoteTab {
+            title: "a".repeat(MAX_TITLE_CHAR_LENGTH + 10), // Fill a string more than max
+            url_history: vec!["https://foo.bar".to_owned()],
+            icon: None,
+            last_used: 0,
+        }]);
+        let ellipsis_char = '\u{2026}';
+        let mut truncated_title = "a".repeat(MAX_TITLE_CHAR_LENGTH - ellipsis_char.len_utf8());
+        truncated_title.push(ellipsis_char);
+        assert_eq!(
+            storage.prepare_local_tabs_for_upload(),
+            Some(vec![
+                // title trimmed to 50 characters
+                RemoteTab {
+                    title: truncated_title, // title was trimmed to only max char length
+                    url_history: vec!["https://foo.bar".to_owned()],
+                    icon: None,
+                    last_used: 0,
+                },
+            ])
+        );
+    }
+    #[test]
+    fn test_utf8_safe_title_trim() {
+        let mut storage = TabsStorage::new_with_mem_path("test_prepare_local_tabs_for_upload");
+        assert_eq!(storage.prepare_local_tabs_for_upload(), None);
+        storage.update_local_state(vec![
+            RemoteTab {
+                title: "😍".repeat(MAX_TITLE_CHAR_LENGTH + 10), // Fill a string more than max
+                url_history: vec!["https://foo.bar".to_owned()],
+                icon: None,
+                last_used: 0,
+            },
+            RemoteTab {
+                title: "を".repeat(MAX_TITLE_CHAR_LENGTH + 5), // Fill a string more than max
+                url_history: vec!["https://foo_jp.bar".to_owned()],
+                icon: None,
+                last_used: 0,
+            },
+        ]);
+        let ellipsis_char = '\u{2026}';
+        // (MAX_TITLE_CHAR_LENGTH - ellipsis / "😍" bytes)
+        let mut truncated_title = "😍".repeat(127);
+        // (MAX_TITLE_CHAR_LENGTH - ellipsis / "を" bytes)
+        let mut truncated_jp_title = "を".repeat(169);
+        truncated_title.push(ellipsis_char);
+        truncated_jp_title.push(ellipsis_char);
+        let remote_tabs = storage.prepare_local_tabs_for_upload().unwrap();
+        assert_eq!(
+            remote_tabs,
+            vec![
+                RemoteTab {
+                    title: truncated_title, // title was trimmed to only max char length
+                    url_history: vec!["https://foo.bar".to_owned()],
+                    icon: None,
+                    last_used: 0,
+                },
+                RemoteTab {
+                    title: truncated_jp_title, // title was trimmed to only max char length
+                    url_history: vec!["https://foo_jp.bar".to_owned()],
+                    icon: None,
+                    last_used: 0,
+                },
+            ]
+        );
+        // We should be less than max
+        assert!(remote_tabs[0].title.chars().count() <= MAX_TITLE_CHAR_LENGTH);
+        assert!(remote_tabs[1].title.chars().count() <= MAX_TITLE_CHAR_LENGTH);
+    }
+    #[test]
+    fn test_trim_tabs_length() {
+        let mut storage = TabsStorage::new_with_mem_path("test_prepare_local_tabs_for_upload");
+        assert_eq!(storage.prepare_local_tabs_for_upload(), None);
+        let mut too_many_tabs: Vec<RemoteTab> = Vec::new();
+        for n in 1..5000 {
+            too_many_tabs.push(RemoteTab {
+                title: "aaaa aaaa aaaa aaaa aaaa aaaa aaaa aaaa aaaa aaaa" //50 characters
+                    .to_owned(),
+                url_history: vec![format!("https://foo{}.bar", n)],
+                icon: None,
+                last_used: 0,
+            });
+        }
+        let tabs_mem_size = compute_serialized_size(&too_many_tabs);
+        // ensure we are definitely over the payload limit
+        assert!(tabs_mem_size > MAX_PAYLOAD_SIZE);
+        // Add our over-the-limit tabs to the local state
+        storage.update_local_state(too_many_tabs.clone());
+        // prepare_local_tabs_for_upload did the trimming we needed to get under payload size
+        let tabs_to_upload = &storage.prepare_local_tabs_for_upload().unwrap();
+        assert!(compute_serialized_size(tabs_to_upload) <= MAX_PAYLOAD_SIZE);
+    }
+    // Helper struct to model what's stored in the DB
+    struct TabsSQLRecord {
+        guid: String,
+        record: TabsRecord,
+        last_modified: i64,
+    }
+    #[test]
+    fn test_remove_stale_clients() {
+        let dir = tempfile::tempdir().unwrap();
+        let db_name = dir.path().join("test_remove_stale_clients.db");
+        let mut storage = TabsStorage::new(db_name);
+        storage.open_or_create().unwrap();
+        assert!(storage.open_if_exists().unwrap().is_some());
+
+        let records = vec![
+            TabsSQLRecord {
+                guid: "device-1".to_string(),
+                record: TabsRecord {
+                    id: "device-1".to_string(),
+                    client_name: "Device #1".to_string(),
+                    tabs: vec![TabsRecordTab {
+                        title: "the title".to_string(),
+                        url_history: vec!["https://mozilla.org/".to_string()],
+                        icon: Some("https://mozilla.org/icon".to_string()),
+                        last_used: 1643764207000,
+                    }],
+                },
+                last_modified: 1643764207000,
+            },
+            TabsSQLRecord {
+                guid: "device-outdated".to_string(),
+                record: TabsRecord {
+                    id: "device-outdated".to_string(),
+                    client_name: "Device outdated".to_string(),
+                    tabs: vec![TabsRecordTab {
+                        title: "the title".to_string(),
+                        url_history: vec!["https://mozilla.org/".to_string()],
+                        icon: Some("https://mozilla.org/icon".to_string()),
+                        last_used: 1643764207000,
+                    }],
+                },
+                last_modified: 1443764207000, // old
+            },
+        ];
+        let db = storage.open_if_exists().unwrap().unwrap();
+        for record in records {
+            db.execute(
+                "INSERT INTO tabs (guid, record, last_modified) VALUES (:guid, :record, :last_modified);",
+                rusqlite::named_params! {
+                    ":guid": &record.guid,
+                    ":record": serde_json::to_string(&record.record).unwrap(),
+                    ":last_modified": &record.last_modified,
+                },
+            ).unwrap();
+        }
+        // pretend we just synced
+        let last_synced = 1643764207000_i64;
+        storage
+            .put_meta(schema::LAST_SYNC_META_KEY, &last_synced)
+            .unwrap();
+        storage.remove_stale_clients().unwrap();
+
+        let remote_tabs = storage.get_remote_tabs().unwrap();
+        // We should've removed the outdated device
+        assert_eq!(remote_tabs.len(), 1);
+        // Assert the correct record is still being returned
+        assert_eq!(remote_tabs[0].client_id, "device-1");
+    }
+}
diff --git a/third_party/rust/tabs/src/store.rs b/third_party/rust/tabs/src/store.rs
new file mode 100644
index 0000000000..67bae498f7
--- /dev/null
+++ b/third_party/rust/tabs/src/store.rs
@@ -0,0 +1,41 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use crate::storage::{ClientRemoteTabs, RemoteTab, TabsStorage};
+use std::path::Path;
+use std::sync::Mutex;
+
+pub struct TabsStore {
+    pub storage: Mutex<TabsStorage>,
+}
+
+impl TabsStore {
+    pub fn new(db_path: impl AsRef<Path>) -> Self {
+        Self {
+            storage: Mutex::new(TabsStorage::new(db_path)),
+        }
+    }
+
+    pub fn new_with_mem_path(db_path: &str) -> Self {
+        Self {
+            storage: Mutex::new(TabsStorage::new_with_mem_path(db_path)),
+        }
+    }
+
+    pub fn set_local_tabs(&self, local_state: Vec<RemoteTab>) {
+        self.storage.lock().unwrap().update_local_state(local_state);
+    }
+
+    // like remote_tabs, but serves the uniffi layer
+    pub fn get_all(&self) -> Vec<ClientRemoteTabs> {
+        match self.remote_tabs() {
+            Some(list) => list,
+            None => vec![],
+        }
+    }
+
+    pub fn remote_tabs(&self) -> Option<Vec<ClientRemoteTabs>> {
+        self.storage.lock().unwrap().get_remote_tabs()
+    }
+}
diff --git a/third_party/rust/tabs/src/sync/bridge.rs b/third_party/rust/tabs/src/sync/bridge.rs
new file mode 100644
index 0000000000..6e37a0cf5c
--- /dev/null
+++ b/third_party/rust/tabs/src/sync/bridge.rs
@@ -0,0 +1,436 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use crate::error::TabsApiError;
+use crate::sync::engine::TabsSyncImpl;
+use crate::TabsStore;
+use anyhow::Result;
+use std::sync::{Arc, Mutex};
+use sync15::bso::{IncomingBso, OutgoingBso};
+use sync15::engine::{ApplyResults, BridgedEngine, CollSyncIds, EngineSyncAssociation};
+use sync15::{ClientData, ServerTimestamp};
+use sync_guid::Guid as SyncGuid;
+
+impl TabsStore {
+    // Returns a bridged sync engine for Desktop for this store.
+    pub fn bridged_engine(self: Arc<Self>) -> Arc<TabsBridgedEngine> {
+        let bridge_impl = crate::sync::bridge::BridgedEngineImpl::new(&self);
+        // This is a concrete struct exposed via uniffi.
+        let concrete = TabsBridgedEngine::new(bridge_impl);
+        Arc::new(concrete)
+    }
+}
+
+/// A bridged engine implements all the methods needed to make the
+/// `storage.sync` store work with Desktop's Sync implementation.
+/// Conceptually it's very similar to our SyncEngine, and once we have SyncEngine
+/// and BridgedEngine using the same types we can probably combine them (or at least
+/// implement this bridged engine purely in terms of SyncEngine)
+/// See also #2841 and #5139
+pub struct BridgedEngineImpl {
+    sync_impl: Mutex<TabsSyncImpl>,
+    incoming: Mutex<Vec<IncomingBso>>,
+}
+
+impl BridgedEngineImpl {
+    /// Creates a bridged engine for syncing.
+    pub fn new(store: &Arc<TabsStore>) -> Self {
+        Self {
+            sync_impl: Mutex::new(TabsSyncImpl::new(store.clone())),
+            incoming: Mutex::default(),
+        }
+    }
+}
+
+impl BridgedEngine for BridgedEngineImpl {
+    fn last_sync(&self) -> Result<i64> {
+        Ok(self
+            .sync_impl
+            .lock()
+            .unwrap()
+            .get_last_sync()?
+            .unwrap_or_default()
+            .as_millis())
+    }
+
+    fn set_last_sync(&self, last_sync_millis: i64) -> Result<()> {
+        self.sync_impl
+            .lock()
+            .unwrap()
+            .set_last_sync(ServerTimestamp::from_millis(last_sync_millis))?;
+        Ok(())
+    }
+
+    fn sync_id(&self) -> Result<Option<String>> {
+        Ok(match self.sync_impl.lock().unwrap().get_sync_assoc()? {
+            EngineSyncAssociation::Connected(id) => Some(id.coll.to_string()),
+            EngineSyncAssociation::Disconnected => None,
+        })
+    }
+
+    fn reset_sync_id(&self) -> Result<String> {
+        let new_id = SyncGuid::random().to_string();
+        let new_coll_ids = CollSyncIds {
+            global: SyncGuid::empty(),
+            coll: new_id.clone().into(),
+        };
+        self.sync_impl
+            .lock()
+            .unwrap()
+            .reset(&EngineSyncAssociation::Connected(new_coll_ids))?;
+        Ok(new_id)
+    }
+
+    fn ensure_current_sync_id(&self, sync_id: &str) -> Result<String> {
+        let mut sync_impl = self.sync_impl.lock().unwrap();
+        let assoc = sync_impl.get_sync_assoc()?;
+        if matches!(assoc, EngineSyncAssociation::Connected(c) if c.coll == sync_id) {
+            log::debug!("ensure_current_sync_id is current");
+        } else {
+            let new_coll_ids = CollSyncIds {
+                global: SyncGuid::empty(),
+                coll: sync_id.into(),
+            };
+            sync_impl.reset(&EngineSyncAssociation::Connected(new_coll_ids))?;
+        }
+        Ok(sync_id.to_string())
+    }
+
+    fn prepare_for_sync(&self, client_data: &str) -> Result<()> {
+        let data: ClientData = serde_json::from_str(client_data)?;
+        Ok(self.sync_impl.lock().unwrap().prepare_for_sync(data)?)
+    }
+
+    fn sync_started(&self) -> Result<()> {
+        // This is a no-op for the Tabs Engine
+        Ok(())
+    }
+
+    fn store_incoming(&self, incoming: Vec<IncomingBso>) -> Result<()> {
+        // Store the incoming payload in memory so we can use it in apply
+        *(self.incoming.lock().unwrap()) = incoming;
+        Ok(())
+    }
+
+    fn apply(&self) -> Result<ApplyResults> {
+        let mut incoming = self.incoming.lock().unwrap();
+        // We've a reference to a Vec<> but it's owned by the mutex - swap the mutex owned
+        // value for an empty vec so we can consume the original.
+        let mut records = Vec::new();
+        std::mem::swap(&mut records, &mut *incoming);
+        let mut telem = sync15::telemetry::Engine::new("tabs");
+
+        let mut sync_impl = self.sync_impl.lock().unwrap();
+        let outgoing = sync_impl.apply_incoming(records, &mut telem)?;
+
+        Ok(ApplyResults {
+            records: outgoing,
+            num_reconciled: Some(0),
+        })
+    }
+
+    fn set_uploaded(&self, server_modified_millis: i64, ids: &[SyncGuid]) -> Result<()> {
+        Ok(self
+            .sync_impl
+            .lock()
+            .unwrap()
+            .sync_finished(ServerTimestamp::from_millis(server_modified_millis), ids)?)
+    }
+
+    fn sync_finished(&self) -> Result<()> {
+        *(self.incoming.lock().unwrap()) = Vec::default();
+        Ok(())
+    }
+
+    fn reset(&self) -> Result<()> {
+        self.sync_impl
+            .lock()
+            .unwrap()
+            .reset(&EngineSyncAssociation::Disconnected)?;
+        Ok(())
+    }
+
+    fn wipe(&self) -> Result<()> {
+        self.sync_impl.lock().unwrap().wipe()?;
+        Ok(())
+    }
+}
+
+// This is for uniffi to expose, and does nothing than delegate back to the trait.
+pub struct TabsBridgedEngine {
+    bridge_impl: BridgedEngineImpl,
+}
+
+impl TabsBridgedEngine {
+    pub fn new(bridge_impl: BridgedEngineImpl) -> Self {
+        Self { bridge_impl }
+    }
+
+    pub fn last_sync(&self) -> Result<i64> {
+        self.bridge_impl.last_sync()
+    }
+
+    pub fn set_last_sync(&self, last_sync: i64) -> Result<()> {
+        self.bridge_impl.set_last_sync(last_sync)
+    }
+
+    pub fn sync_id(&self) -> Result<Option<String>> {
+        self.bridge_impl.sync_id()
+    }
+
+    pub fn reset_sync_id(&self) -> Result<String> {
+        self.bridge_impl.reset_sync_id()
+    }
+
+    pub fn ensure_current_sync_id(&self, sync_id: &str) -> Result<String> {
+        self.bridge_impl.ensure_current_sync_id(sync_id)
+    }
+
+    pub fn prepare_for_sync(&self, client_data: &str) -> Result<()> {
+        self.bridge_impl.prepare_for_sync(client_data)
+    }
+
+    pub fn sync_started(&self) -> Result<()> {
+        self.bridge_impl.sync_started()
+    }
+
+    // Decode the JSON-encoded IncomingBso's that UniFFI passes to us
+    fn convert_incoming_bsos(&self, incoming: Vec<String>) -> Result<Vec<IncomingBso>> {
+        let mut bsos = Vec::with_capacity(incoming.len());
+        for inc in incoming {
+            bsos.push(serde_json::from_str::<IncomingBso>(&inc)?);
+        }
+        Ok(bsos)
+    }
+
+    // Encode OutgoingBso's into JSON for UniFFI
+    fn convert_outgoing_bsos(&self, outgoing: Vec<OutgoingBso>) -> Result<Vec<String>> {
+        let mut bsos = Vec::with_capacity(outgoing.len());
+        for e in outgoing {
+            bsos.push(serde_json::to_string(&e)?);
+        }
+        Ok(bsos)
+    }
+
+    pub fn store_incoming(&self, incoming: Vec<String>) -> Result<()> {
+        self.bridge_impl
+            .store_incoming(self.convert_incoming_bsos(incoming)?)
+    }
+
+    pub fn apply(&self) -> Result<Vec<String>> {
+        let apply_results = self.bridge_impl.apply()?;
+        self.convert_outgoing_bsos(apply_results.records)
+    }
+
+    pub fn set_uploaded(&self, server_modified_millis: i64, guids: Vec<SyncGuid>) -> Result<()> {
+        self.bridge_impl
+            .set_uploaded(server_modified_millis, &guids)
+    }
+
+    pub fn sync_finished(&self) -> Result<()> {
+        self.bridge_impl.sync_finished()
+    }
+
+    pub fn reset(&self) -> Result<()> {
+        self.bridge_impl.reset()
+    }
+
+    pub fn wipe(&self) -> Result<()> {
+        self.bridge_impl.wipe()
+    }
+}
+
+impl From<anyhow::Error> for TabsApiError {
+    fn from(value: anyhow::Error) -> Self {
+        TabsApiError::UnexpectedTabsError {
+            reason: value.to_string(),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::storage::{RemoteTab, TABS_CLIENT_TTL};
+    use crate::sync::record::TabsRecordTab;
+    use serde_json::json;
+    use std::collections::HashMap;
+    use sync15::{ClientData, DeviceType, RemoteClient};
+
+    // A copy of the normal "engine" tests but which go via the bridge
+    #[test]
+    fn test_sync_via_bridge() {
+        env_logger::try_init().ok();
+
+        let store = Arc::new(TabsStore::new_with_mem_path("test-bridge_incoming"));
+
+        // Set some local tabs for our device.
+        let my_tabs = vec![
+            RemoteTab {
+                title: "my first tab".to_string(),
+                url_history: vec!["http://1.com".to_string()],
+                icon: None,
+                last_used: 2,
+            },
+            RemoteTab {
+                title: "my second tab".to_string(),
+                url_history: vec!["http://2.com".to_string()],
+                icon: None,
+                last_used: 1,
+            },
+        ];
+        store.set_local_tabs(my_tabs.clone());
+
+        let bridge = store.bridged_engine();
+
+        let client_data = ClientData {
+            local_client_id: "my-device".to_string(),
+            recent_clients: HashMap::from([
+                (
+                    "my-device".to_string(),
+                    RemoteClient {
+                        fxa_device_id: None,
+                        device_name: "my device".to_string(),
+                        device_type: sync15::DeviceType::Unknown,
+                    },
+                ),
+                (
+                    "device-no-tabs".to_string(),
+                    RemoteClient {
+                        fxa_device_id: None,
+                        device_name: "device with no tabs".to_string(),
+                        device_type: DeviceType::Unknown,
+                    },
+                ),
+                (
+                    "device-with-a-tab".to_string(),
+                    RemoteClient {
+                        fxa_device_id: None,
+                        device_name: "device with a tab".to_string(),
+                        device_type: DeviceType::Unknown,
+                    },
+                ),
+            ]),
+        };
+        bridge
+            .prepare_for_sync(&serde_json::to_string(&client_data).unwrap())
+            .expect("should work");
+
+        let records = vec![
+            // my-device should be ignored by sync - here it is acting as what our engine last
+            // wrote, but the actual tabs in our store we set above are what should be used.
+            json!({
+                "id": "my-device",
+                "clientName": "my device",
+                "tabs": [{
+                    "title": "the title",
+                    "urlHistory": [
+                        "https://mozilla.org/"
+                    ],
+                    "icon": "https://mozilla.org/icon",
+                    "lastUsed": 1643764207
+                }]
+            }),
+            json!({
+                "id": "device-no-tabs",
+                "clientName": "device with no tabs",
+                "tabs": [],
+            }),
+            json!({
+                "id": "device-with-a-tab",
+                "clientName": "device with a tab",
+                "tabs": [{
+                    "title": "the title",
+                    "urlHistory": [
+                        "https://mozilla.org/"
+                    ],
+                    "icon": "https://mozilla.org/icon",
+                    "lastUsed": 1643764207
+                }]
+            }),
+            // This has the main payload as OK but the tabs part invalid.
+            json!({
+                "id": "device-with-invalid-tab",
+                "clientName": "device with a tab",
+                "tabs": [{
+                    "foo": "bar",
+                }]
+            }),
+            // We want this to be a valid payload but an invalid tab - so it needs an ID.
+            json!({
+                "id": "invalid-tab",
+                "foo": "bar"
+            }),
+        ];
+
+        let mut incoming = Vec::new();
+        for record in records {
+            // Annoyingly we can't use `IncomingEnvelope` directly as it intentionally doesn't
+            // support Serialize - so need to use explicit json.
+            let envelope = json!({
+                "id": record.get("id"),
+                "modified": 0,
+                "payload": serde_json::to_string(&record).unwrap(),
+            });
+            incoming.push(serde_json::to_string(&envelope).unwrap());
+        }
+
+        bridge.store_incoming(incoming).expect("should store");
+
+        let out = bridge.apply().expect("should apply");
+
+        assert_eq!(out.len(), 1);
+        let ours = serde_json::from_str::<serde_json::Value>(&out[0]).unwrap();
+        // As above, can't use `OutgoingEnvelope` as it doesn't Deserialize.
+        // First, convert my_tabs from the local `RemoteTab` to the Sync specific `TabsRecord`
+        let expected_tabs: Vec<TabsRecordTab> =
+            my_tabs.into_iter().map(|t| t.to_record_tab()).collect();
+        let expected = json!({
+            "id": "my-device".to_string(),
+            "payload": json!({
+                "id": "my-device".to_string(),
+                "clientName": "my device",
+                "tabs": serde_json::to_value(expected_tabs).unwrap(),
+            }).to_string(),
+            "ttl": TABS_CLIENT_TTL,
+        });
+
+        assert_eq!(ours, expected);
+        bridge.set_uploaded(1234, vec![]).unwrap();
+        assert_eq!(bridge.last_sync().unwrap(), 1234);
+    }
+
+    #[test]
+    fn test_sync_meta() {
+        env_logger::try_init().ok();
+
+        let store = Arc::new(TabsStore::new_with_mem_path("test-meta"));
+        let bridge = store.bridged_engine();
+
+        // Should not error or panic
+        assert_eq!(bridge.last_sync().unwrap(), 0);
+        bridge.set_last_sync(3).unwrap();
+        assert_eq!(bridge.last_sync().unwrap(), 3);
+
+        assert!(bridge.sync_id().unwrap().is_none());
+
+        bridge.ensure_current_sync_id("some_guid").unwrap();
+        assert_eq!(bridge.sync_id().unwrap(), Some("some_guid".to_string()));
+        // changing the sync ID should reset the timestamp
+        assert_eq!(bridge.last_sync().unwrap(), 0);
+        bridge.set_last_sync(3).unwrap();
+
+        bridge.reset_sync_id().unwrap();
+        // should now be a random guid.
+        assert_ne!(bridge.sync_id().unwrap(), Some("some_guid".to_string()));
+        // should have reset the last sync timestamp.
+        assert_eq!(bridge.last_sync().unwrap(), 0);
+        bridge.set_last_sync(3).unwrap();
+
+        // `reset` clears the guid and the timestamp
+        bridge.reset().unwrap();
+        assert_eq!(bridge.last_sync().unwrap(), 0);
+        assert!(bridge.sync_id().unwrap().is_none());
+    }
+}
diff --git a/third_party/rust/tabs/src/sync/engine.rs b/third_party/rust/tabs/src/sync/engine.rs
new file mode 100644
index 0000000000..bc0978cbf2
--- /dev/null
+++ b/third_party/rust/tabs/src/sync/engine.rs
@@ -0,0 +1,543 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use crate::schema;
+use crate::storage::{ClientRemoteTabs, RemoteTab, TABS_CLIENT_TTL};
+use crate::store::TabsStore;
+use crate::sync::record::{TabsRecord, TabsRecordTab};
+use crate::Result;
+use std::collections::HashMap;
+use std::sync::{Arc, Mutex, Weak};
+use sync15::bso::{IncomingBso, OutgoingBso, OutgoingEnvelope};
+use sync15::engine::{
+    CollSyncIds, CollectionRequest, EngineSyncAssociation, IncomingChangeset, OutgoingChangeset,
+    SyncEngine, SyncEngineId,
+};
+use sync15::{telemetry, ClientData, DeviceType, RemoteClient, ServerTimestamp};
+use sync_guid::Guid;
+
+// Our "sync manager" will use whatever is stashed here.
+lazy_static::lazy_static! {
+    // Mutex: just taken long enough to update the inner stuff
+    static ref STORE_FOR_MANAGER: Mutex<Weak<TabsStore>> = Mutex::new(Weak::new());
+}
+
+/// Called by the sync manager to get a sync engine via the store previously
+/// registered with the sync manager.
+pub fn get_registered_sync_engine(engine_id: &SyncEngineId) -> Option<Box<dyn SyncEngine>> {
+    let weak = STORE_FOR_MANAGER.lock().unwrap();
+    match weak.upgrade() {
+        None => None,
+        Some(store) => match engine_id {
+            SyncEngineId::Tabs => Some(Box::new(TabsEngine::new(Arc::clone(&store)))),
+            // panicing here seems reasonable - it's a static error if this
+            // it hit, not something that runtime conditions can influence.
+            _ => unreachable!("can't provide unknown engine: {}", engine_id),
+        },
+    }
+}
+
+impl ClientRemoteTabs {
+    pub(crate) fn from_record_with_remote_client(
+        client_id: String,
+        last_modified: ServerTimestamp,
+        remote_client: &RemoteClient,
+        record: TabsRecord,
+    ) -> Self {
+        Self {
+            client_id,
+            client_name: remote_client.device_name.clone(),
+            device_type: remote_client.device_type,
+            last_modified: last_modified.as_millis(),
+            remote_tabs: record.tabs.iter().map(RemoteTab::from_record_tab).collect(),
+        }
+    }
+
+    // Note that this should die as part of https://github.com/mozilla/application-services/issues/5199
+    // If we don't have a `RemoteClient` record, then we don't know whether the ID passed here is
+    // the fxa_device_id (which is must be) or the client_id (which it will be if this ends up being
+    // called for desktop records, where client_id != fxa_device_id)
+    pub(crate) fn from_record(
+        client_id: String,
+        last_modified: ServerTimestamp,
+        record: TabsRecord,
+    ) -> Self {
+        Self {
+            client_id,
+            client_name: record.client_name,
+            device_type: DeviceType::Unknown,
+            last_modified: last_modified.as_millis(),
+            remote_tabs: record.tabs.iter().map(RemoteTab::from_record_tab).collect(),
+        }
+    }
+    fn to_record(&self) -> TabsRecord {
+        TabsRecord {
+            id: self.client_id.clone(),
+            client_name: self.client_name.clone(),
+            tabs: self
+                .remote_tabs
+                .iter()
+                .map(RemoteTab::to_record_tab)
+                .collect(),
+        }
+    }
+}
+
+impl RemoteTab {
+    pub(crate) fn from_record_tab(tab: &TabsRecordTab) -> Self {
+        Self {
+            title: tab.title.clone(),
+            url_history: tab.url_history.clone(),
+            icon: tab.icon.clone(),
+            last_used: tab.last_used.checked_mul(1000).unwrap_or_default(),
+        }
+    }
+    pub(super) fn to_record_tab(&self) -> TabsRecordTab {
+        TabsRecordTab {
+            title: self.title.clone(),
+            url_history: self.url_history.clone(),
+            icon: self.icon.clone(),
+            last_used: self.last_used.checked_div(1000).unwrap_or_default(),
+        }
+    }
+}
+
+// This is the implementation of syncing, which is used by the 2 different "sync engines"
+// (We hope to get these 2 engines even closer in the future, but for now, we suck this up)
+pub struct TabsSyncImpl {
+    pub(super) store: Arc<TabsStore>,
+    pub(super) local_id: String,
+}
+
+impl TabsSyncImpl {
+    pub fn new(store: Arc<TabsStore>) -> Self {
+        Self {
+            store,
+            local_id: Default::default(),
+        }
+    }
+
+    pub fn prepare_for_sync(&mut self, client_data: ClientData) -> Result<()> {
+        let mut storage = self.store.storage.lock().unwrap();
+        // We only know the client list at sync time, but need to return tabs potentially
+        // at any time -- so we store the clients in the meta table to be able to properly
+        // return a ClientRemoteTab struct
+        storage.put_meta(
+            schema::REMOTE_CLIENTS_KEY,
+            &serde_json::to_string(&client_data.recent_clients)?,
+        )?;
+        self.local_id = client_data.local_client_id;
+        Ok(())
+    }
+
+    pub fn apply_incoming(
+        &mut self,
+        inbound: Vec<IncomingBso>,
+        telem: &mut telemetry::Engine,
+    ) -> Result<Vec<OutgoingBso>> {
+        let local_id = self.local_id.clone();
+        let mut remote_tabs = Vec::with_capacity(inbound.len());
+        let mut incoming_telemetry = telemetry::EngineIncoming::new();
+
+        let remote_clients: HashMap<String, RemoteClient> = {
+            let mut storage = self.store.storage.lock().unwrap();
+            match storage.get_meta::<String>(schema::REMOTE_CLIENTS_KEY)? {
+                None => HashMap::default(),
+                Some(json) => serde_json::from_str(&json).unwrap(),
+            }
+        };
+        for incoming in inbound {
+            if incoming.envelope.id == local_id {
+                // That's our own record, ignore it.
+                continue;
+            }
+            let modified = incoming.envelope.modified;
+            let record = match incoming.into_content::<TabsRecord>().content() {
+                Some(record) => record,
+                None => {
+                    // Invalid record or a "tombstone" which tabs don't have.
+                    log::warn!("Ignoring incoming invalid tab");
+                    incoming_telemetry.failed(1);
+                    continue;
+                }
+            };
+            remote_tabs.push((record, modified));
+        }
+
+        // We want to keep the mutex for as short as possible
+        let local_tabs = {
+            let mut storage = self.store.storage.lock().unwrap();
+            // In desktop we might end up here with zero records when doing a quick-write, in
+            // which case we don't want to wipe the DB.
+            if !remote_tabs.is_empty() {
+                storage.replace_remote_tabs(remote_tabs)?;
+            }
+            storage.remove_stale_clients()?;
+            storage.prepare_local_tabs_for_upload()
+        };
+        let outgoing = if let Some(local_tabs) = local_tabs {
+            let (client_name, device_type) = remote_clients
+                .get(&local_id)
+                .map(|client| (client.device_name.clone(), client.device_type))
+                .unwrap_or_else(|| (String::new(), DeviceType::Unknown));
+            let local_record = ClientRemoteTabs {
+                client_id: local_id.clone(),
+                client_name,
+                device_type,
+                last_modified: 0, // ignored for outgoing records.
+                remote_tabs: local_tabs.to_vec(),
+            };
+            log::trace!("outgoing {:?}", local_record);
+            let envelope = OutgoingEnvelope {
+                id: local_id.into(),
+                ttl: Some(TABS_CLIENT_TTL),
+                ..Default::default()
+            };
+            vec![OutgoingBso::from_content(
+                envelope,
+                local_record.to_record(),
+            )?]
+        } else {
+            vec![]
+        };
+        telem.incoming(incoming_telemetry);
+        Ok(outgoing)
+    }
+
+    pub fn sync_finished(
+        &mut self,
+        new_timestamp: ServerTimestamp,
+        records_synced: &[Guid],
+    ) -> Result<()> {
+        log::info!(
+            "sync completed after uploading {} records",
+            records_synced.len()
+        );
+        self.set_last_sync(new_timestamp)?;
+        Ok(())
+    }
+
+    pub fn reset(&mut self, assoc: &EngineSyncAssociation) -> Result<()> {
+        self.set_last_sync(ServerTimestamp(0))?;
+        let mut storage = self.store.storage.lock().unwrap();
+        storage.delete_meta(schema::REMOTE_CLIENTS_KEY)?;
+        storage.wipe_remote_tabs()?;
+        match assoc {
+            EngineSyncAssociation::Disconnected => {
+                storage.delete_meta(schema::GLOBAL_SYNCID_META_KEY)?;
+                storage.delete_meta(schema::COLLECTION_SYNCID_META_KEY)?;
+            }
+            EngineSyncAssociation::Connected(ids) => {
+                storage.put_meta(schema::GLOBAL_SYNCID_META_KEY, &ids.global.to_string())?;
+                storage.put_meta(schema::COLLECTION_SYNCID_META_KEY, &ids.coll.to_string())?;
+            }
+        };
+        Ok(())
+    }
+
+    pub fn wipe(&mut self) -> Result<()> {
+        self.reset(&EngineSyncAssociation::Disconnected)?;
+        // not clear why we need to wipe the local tabs - the app is just going
+        // to re-add them?
+        self.store.storage.lock().unwrap().wipe_local_tabs();
+        Ok(())
+    }
+
+    pub fn get_sync_assoc(&self) -> Result<EngineSyncAssociation> {
+        let mut storage = self.store.storage.lock().unwrap();
+        let global = storage.get_meta::<String>(schema::GLOBAL_SYNCID_META_KEY)?;
+        let coll = storage.get_meta::<String>(schema::COLLECTION_SYNCID_META_KEY)?;
+        Ok(if let (Some(global), Some(coll)) = (global, coll) {
+            EngineSyncAssociation::Connected(CollSyncIds {
+                global: Guid::from_string(global),
+                coll: Guid::from_string(coll),
+            })
+        } else {
+            EngineSyncAssociation::Disconnected
+        })
+    }
+
+    pub fn set_last_sync(&self, last_sync: ServerTimestamp) -> Result<()> {
+        let mut storage = self.store.storage.lock().unwrap();
+        log::debug!("Updating last sync to {}", last_sync);
+        let last_sync_millis = last_sync.as_millis();
+        storage.put_meta(schema::LAST_SYNC_META_KEY, &last_sync_millis)
+    }
+
+    pub fn get_last_sync(&self) -> Result<Option<ServerTimestamp>> {
+        let mut storage = self.store.storage.lock().unwrap();
+        let millis = storage.get_meta::<i64>(schema::LAST_SYNC_META_KEY)?;
+        Ok(millis.map(ServerTimestamp))
+    }
+}
+
+// This is the "SyncEngine" used when syncing via the Sync Manager.
+pub struct TabsEngine {
+    pub sync_impl: Mutex<TabsSyncImpl>,
+}
+
+impl TabsEngine {
+    pub fn new(store: Arc<TabsStore>) -> Self {
+        Self {
+            sync_impl: Mutex::new(TabsSyncImpl::new(store)),
+        }
+    }
+}
+
+impl SyncEngine for TabsEngine {
+    fn collection_name(&self) -> std::borrow::Cow<'static, str> {
+        "tabs".into()
+    }
+
+    fn prepare_for_sync(&self, get_client_data: &dyn Fn() -> ClientData) -> anyhow::Result<()> {
+        Ok(self
+            .sync_impl
+            .lock()
+            .unwrap()
+            .prepare_for_sync(get_client_data())?)
+    }
+
+    fn apply_incoming(
+        &self,
+        inbound: Vec<IncomingChangeset>,
+        telem: &mut telemetry::Engine,
+    ) -> anyhow::Result<OutgoingChangeset> {
+        assert_eq!(inbound.len(), 1, "only requested one set of records");
+        let inbound = inbound.into_iter().next().unwrap();
+        let outgoing_records = self
+            .sync_impl
+            .lock()
+            .unwrap()
+            .apply_incoming(inbound.changes, telem)?;
+
+        Ok(OutgoingChangeset::new("tabs".into(), outgoing_records))
+    }
+
+    fn sync_finished(
+        &self,
+        new_timestamp: ServerTimestamp,
+        records_synced: Vec<Guid>,
+    ) -> anyhow::Result<()> {
+        Ok(self
+            .sync_impl
+            .lock()
+            .unwrap()
+            .sync_finished(new_timestamp, &records_synced)?)
+    }
+
+    fn get_collection_requests(
+        &self,
+        server_timestamp: ServerTimestamp,
+    ) -> anyhow::Result<Vec<CollectionRequest>> {
+        let since = self
+            .sync_impl
+            .lock()
+            .unwrap()
+            .get_last_sync()?
+            .unwrap_or_default();
+        Ok(if since == server_timestamp {
+            vec![]
+        } else {
+            vec![CollectionRequest::new("tabs".into())
+                .full()
+                .newer_than(since)]
+        })
+    }
+
+    fn get_sync_assoc(&self) -> anyhow::Result<EngineSyncAssociation> {
+        Ok(self.sync_impl.lock().unwrap().get_sync_assoc()?)
+    }
+
+    fn reset(&self, assoc: &EngineSyncAssociation) -> anyhow::Result<()> {
+        Ok(self.sync_impl.lock().unwrap().reset(assoc)?)
+    }
+
+    fn wipe(&self) -> anyhow::Result<()> {
+        Ok(self.sync_impl.lock().unwrap().wipe()?)
+    }
+}
+
+impl crate::TabsStore {
+    // This allows the embedding app to say "make this instance available to
+    // the sync manager". The implementation is more like "offer to sync mgr"
+    // (thereby avoiding us needing to link with the sync manager) but
+    // `register_with_sync_manager()` is logically what's happening so that's
+    // the name it gets.
+    pub fn register_with_sync_manager(self: Arc<Self>) {
+        let mut state = STORE_FOR_MANAGER.lock().unwrap();
+        *state = Arc::downgrade(&self);
+    }
+}
+
+#[cfg(test)]
+pub mod test {
+    use super::*;
+    use serde_json::json;
+    use sync15::bso::IncomingBso;
+
+    #[test]
+    fn test_incoming_tabs() {
+        env_logger::try_init().ok();
+
+        let engine = TabsEngine::new(Arc::new(TabsStore::new_with_mem_path("test-incoming")));
+
+        let records = vec![
+            json!({
+                "id": "device-no-tabs",
+                "clientName": "device with no tabs",
+                "tabs": [],
+            }),
+            json!({
+                "id": "device-with-a-tab",
+                "clientName": "device with a tab",
+                "tabs": [{
+                    "title": "the title",
+                    "urlHistory": [
+                        "https://mozilla.org/"
+                    ],
+                    "icon": "https://mozilla.org/icon",
+                    "lastUsed": 1643764207
+                }]
+            }),
+            // test an updated payload will replace the previous record
+            json!({
+                "id": "device-with-a-tab",
+                "clientName": "device with an updated tab",
+                "tabs": [{
+                    "title": "the new title",
+                    "urlHistory": [
+                        "https://mozilla.org/"
+                    ],
+                    "icon": "https://mozilla.org/icon",
+                    "lastUsed": 1643764208
+                }]
+            }),
+            // This has the main payload as OK but the tabs part invalid.
+            json!({
+                "id": "device-with-invalid-tab",
+                "clientName": "device with a tab",
+                "tabs": [{
+                    "foo": "bar",
+                }]
+            }),
+            // We want this to be a valid payload but an invalid tab - so it needs an ID.
+            json!({
+                "id": "invalid-tab",
+                "foo": "bar"
+            }),
+        ];
+
+        let incoming = IncomingChangeset::new_with_changes(
+            engine.collection_name(),
+            ServerTimestamp(0),
+            records
+                .into_iter()
+                .map(IncomingBso::from_test_content)
+                .collect(),
+        );
+        let outgoing = engine
+            .apply_incoming(vec![incoming], &mut telemetry::Engine::new("tabs"))
+            .expect("Should apply incoming and stage outgoing records");
+
+        assert!(outgoing.changes.is_empty());
+
+        // now check the store has what we think it has.
+        let sync_impl = engine.sync_impl.lock().unwrap();
+        let mut storage = sync_impl.store.storage.lock().unwrap();
+        let mut crts = storage.get_remote_tabs().expect("should work");
+        crts.sort_by(|a, b| a.client_name.partial_cmp(&b.client_name).unwrap());
+        assert_eq!(crts.len(), 2, "we currently include devices with no tabs");
+        let crt = &crts[0];
+        assert_eq!(crt.client_name, "device with an updated tab");
+        assert_eq!(crt.device_type, DeviceType::Unknown);
+        assert_eq!(crt.remote_tabs.len(), 1);
+        assert_eq!(crt.remote_tabs[0].title, "the new title");
+
+        let crt = &crts[1];
+        assert_eq!(crt.client_name, "device with no tabs");
+        assert_eq!(crt.device_type, DeviceType::Unknown);
+        assert_eq!(crt.remote_tabs.len(), 0);
+    }
+
+    #[test]
+    fn test_no_incoming_doesnt_write() {
+        env_logger::try_init().ok();
+
+        let engine = TabsEngine::new(Arc::new(TabsStore::new_with_mem_path(
+            "test_no_incoming_doesnt_write",
+        )));
+
+        let records = vec![json!({
+            "id": "device-with-a-tab",
+            "clientName": "device with a tab",
+            "tabs": [{
+                "title": "the title",
+                "urlHistory": [
+                    "https://mozilla.org/"
+                ],
+                "icon": "https://mozilla.org/icon",
+                "lastUsed": 1643764207
+            }]
+        })];
+
+        let incoming = IncomingChangeset::new_with_changes(
+            engine.collection_name(),
+            ServerTimestamp(0),
+            records
+                .into_iter()
+                .map(IncomingBso::from_test_content)
+                .collect(),
+        );
+        engine
+            .apply_incoming(vec![incoming], &mut telemetry::Engine::new("tabs"))
+            .expect("Should apply incoming and stage outgoing records");
+
+        // now check the store has what we think it has.
+        {
+            let sync_impl = engine.sync_impl.lock().unwrap();
+            let mut storage = sync_impl.store.storage.lock().unwrap();
+            assert_eq!(storage.get_remote_tabs().expect("should work").len(), 1);
+        }
+
+        // Now another sync with zero incoming records, should still be able to get back
+        // our one client.
+        let incoming = IncomingChangeset::new_with_changes(
+            engine.collection_name(),
+            ServerTimestamp(0),
+            vec![],
+        );
+        engine
+            .apply_incoming(vec![incoming], &mut telemetry::Engine::new("tabs"))
+            .expect("Should succeed applying zero records");
+
+        {
+            let sync_impl = engine.sync_impl.lock().unwrap();
+            let mut storage = sync_impl.store.storage.lock().unwrap();
+            assert_eq!(storage.get_remote_tabs().expect("should work").len(), 1);
+        }
+    }
+
+    #[test]
+    fn test_sync_manager_registration() {
+        let store = Arc::new(TabsStore::new_with_mem_path("test-registration"));
+        assert_eq!(Arc::strong_count(&store), 1);
+        assert_eq!(Arc::weak_count(&store), 0);
+        Arc::clone(&store).register_with_sync_manager();
+        assert_eq!(Arc::strong_count(&store), 1);
+        assert_eq!(Arc::weak_count(&store), 1);
+        let registered = STORE_FOR_MANAGER
+            .lock()
+            .unwrap()
+            .upgrade()
+            .expect("should upgrade");
+        assert!(Arc::ptr_eq(&store, &registered));
+        drop(registered);
+        // should be no new references
+        assert_eq!(Arc::strong_count(&store), 1);
+        assert_eq!(Arc::weak_count(&store), 1);
+        // dropping the registered object should drop the registration.
+        drop(store);
+        assert!(STORE_FOR_MANAGER.lock().unwrap().upgrade().is_none());
+    }
+}
diff --git a/third_party/rust/tabs/src/sync/full_sync.rs b/third_party/rust/tabs/src/sync/full_sync.rs
new file mode 100644
index 0000000000..768aecce0f
--- /dev/null
+++ b/third_party/rust/tabs/src/sync/full_sync.rs
@@ -0,0 +1,70 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use crate::{sync::engine::TabsSyncImpl, ApiResult, TabsEngine, TabsStore};
+use error_support::handle_error;
+use interrupt_support::NeverInterrupts;
+use std::sync::Arc;
+use sync15::client::{sync_multiple, MemoryCachedState, Sync15StorageClientInit};
+use sync15::engine::EngineSyncAssociation;
+use sync15::KeyBundle;
+
+impl TabsStore {
+    #[handle_error(crate::Error)]
+    pub fn reset(self: Arc<Self>) -> ApiResult<()> {
+        let mut sync_impl = TabsSyncImpl::new(Arc::clone(&self));
+        sync_impl.reset(&EngineSyncAssociation::Disconnected)?;
+        Ok(())
+    }
+
+    /// A convenience wrapper around sync_multiple.
+    #[handle_error(crate::Error)]
+    pub fn sync(
+        self: Arc<Self>,
+        key_id: String,
+        access_token: String,
+        sync_key: String,
+        tokenserver_url: String,
+        local_id: String,
+    ) -> ApiResult<String> {
+        let mut mem_cached_state = MemoryCachedState::default();
+        let engine = TabsEngine::new(Arc::clone(&self));
+
+        // Since we are syncing without the sync manager, there's no
+        // command processor, therefore no clients engine, and in
+        // consequence `TabsStore::prepare_for_sync` is never called
+        // which means our `local_id` will never be set.
+        // Do it here.
+        engine.sync_impl.lock().unwrap().local_id = local_id;
+
+        let storage_init = &Sync15StorageClientInit {
+            key_id,
+            access_token,
+            tokenserver_url: url::Url::parse(tokenserver_url.as_str())?,
+        };
+        let root_sync_key = &KeyBundle::from_ksync_base64(sync_key.as_str())?;
+
+        let mut result = sync_multiple(
+            &[&engine],
+            &mut None,
+            &mut mem_cached_state,
+            storage_init,
+            root_sync_key,
+            &NeverInterrupts,
+            None,
+        );
+
+        // for b/w compat reasons, we do some dances with the result.
+        // XXX - note that this means telemetry isn't going to be reported back
+        // to the app - we need to check with lockwise about whether they really
+        // need these failures to be reported or whether we can loosen this.
+        if let Err(e) = result.result {
+            return Err(e.into());
+        }
+        match result.engine_results.remove("tabs") {
+            None | Some(Ok(())) => Ok(serde_json::to_string(&result.telemetry)?),
+            Some(Err(e)) => Err(e.into()),
+        }
+    }
+}
diff --git a/third_party/rust/tabs/src/sync/mod.rs b/third_party/rust/tabs/src/sync/mod.rs
new file mode 100644
index 0000000000..7d668ff843
--- /dev/null
+++ b/third_party/rust/tabs/src/sync/mod.rs
@@ -0,0 +1,35 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+pub(crate) mod bridge;
+pub(crate) mod engine;
+pub(crate) mod record;
+
+#[cfg(feature = "full-sync")]
+pub mod full_sync;
+
+// When full-sync isn't enabled we need stub versions for these UDL exposed functions.
+#[cfg(not(feature = "full-sync"))]
+impl crate::TabsStore {
+    pub fn reset(self: std::sync::Arc<Self>) -> crate::error::ApiResult<()> {
+        log::warn!("reset: feature not enabled");
+        Err(crate::error::TabsApiError::SyncError {
+            reason: "reset".to_string(),
+        })
+    }
+
+    pub fn sync(
+        self: std::sync::Arc<Self>,
+        _key_id: String,
+        _access_token: String,
+        _sync_key: String,
+        _tokenserver_url: String,
+        _local_id: String,
+    ) -> crate::error::ApiResult<String> {
+        log::warn!("sync: feature not enabled");
+        Err(crate::error::TabsApiError::SyncError {
+            reason: "sync".to_string(),
+        })
+    }
+}
diff --git a/third_party/rust/tabs/src/sync/record.rs b/third_party/rust/tabs/src/sync/record.rs
new file mode 100644
index 0000000000..a1da1a33e8
--- /dev/null
+++ b/third_party/rust/tabs/src/sync/record.rs
@@ -0,0 +1,97 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use serde_derive::{Deserialize, Serialize};
+
+#[derive(Debug, Clone, Hash, PartialEq, Eq, Serialize, Deserialize, Default)]
+#[serde(rename_all = "camelCase")]
+pub struct TabsRecordTab {
+    pub title: String,
+    pub url_history: Vec<String>,
+    pub icon: Option<String>,
+    pub last_used: i64, // Seconds since epoch!
+}
+
+#[derive(Debug, Clone, Hash, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+// This struct mirrors what is stored on the server
+pub struct TabsRecord {
+    // `String` instead of `SyncGuid` because some IDs are FxA device ID (XXX - that doesn't
+    // matter though - this could easily be a Guid!)
+    pub id: String,
+    pub client_name: String,
+    pub tabs: Vec<TabsRecordTab>,
+}
+
+#[cfg(test)]
+pub mod test {
+    use super::*;
+    use serde_json::json;
+
+    #[test]
+    fn test_payload() {
+        let payload = json!({
+            "id": "JkeBPC50ZI0m",
+            "clientName": "client name",
+            "tabs": [{
+                "title": "the title",
+                "urlHistory": [
+                    "https://mozilla.org/"
+                ],
+                "icon": "https://mozilla.org/icon",
+                "lastUsed": 1643764207
+            }]
+        });
+        let record: TabsRecord = serde_json::from_value(payload).expect("should work");
+        assert_eq!(record.id, "JkeBPC50ZI0m");
+        assert_eq!(record.client_name, "client name");
+        assert_eq!(record.tabs.len(), 1);
+        let tab = &record.tabs[0];
+        assert_eq!(tab.title, "the title");
+        assert_eq!(tab.icon, Some("https://mozilla.org/icon".to_string()));
+        assert_eq!(tab.last_used, 1643764207);
+    }
+
+    #[test]
+    fn test_roundtrip() {
+        let tab = TabsRecord {
+            id: "JkeBPC50ZI0m".into(),
+            client_name: "client name".into(),
+            tabs: vec![TabsRecordTab {
+                title: "the title".into(),
+                url_history: vec!["https://mozilla.org/".into()],
+                icon: Some("https://mozilla.org/icon".into()),
+                last_used: 1643764207,
+            }],
+        };
+        let round_tripped =
+            serde_json::from_value(serde_json::to_value(tab.clone()).unwrap()).unwrap();
+        assert_eq!(tab, round_tripped);
+    }
+
+    #[test]
+    fn test_extra_fields() {
+        let payload = json!({
+            "id": "JkeBPC50ZI0m",
+            // Let's say we agree on new tabs to record, we want old versions to
+            // ignore them!
+            "ignoredField": "??",
+            "clientName": "client name",
+            "tabs": [{
+                "title": "the title",
+                "urlHistory": [
+                    "https://mozilla.org/"
+                ],
+                "icon": "https://mozilla.org/icon",
+                "lastUsed": 1643764207,
+                // Ditto - make sure we ignore unexpected fields in each tab.
+                "ignoredField": "??",
+            }]
+        });
+        let record: TabsRecord = serde_json::from_value(payload).unwrap();
+        // The point of this test is really just to ensure the deser worked, so
+        // just check the ID.
+        assert_eq!(record.id, "JkeBPC50ZI0m");
+    }
+}
diff --git a/third_party/rust/tabs/src/tabs.udl b/third_party/rust/tabs/src/tabs.udl
new file mode 100644
index 0000000000..d1964ea0d2
--- /dev/null
+++ b/third_party/rust/tabs/src/tabs.udl
@@ -0,0 +1,108 @@
+[Custom]
+typedef string TabsGuid;
+
+namespace tabs {
+
+};
+
+[Error]
+interface TabsApiError {
+    SyncError(string reason);
+    SqlError(string reason);
+    UnexpectedTabsError(string reason);
+};
+
+
+interface TabsStore {
+    constructor(string path);
+
+    sequence<ClientRemoteTabs> get_all();
+
+    void set_local_tabs(sequence<RemoteTabRecord> remote_tabs);
+
+    [Self=ByArc]
+    void register_with_sync_manager();
+
+    [Throws=TabsApiError, Self=ByArc]
+    void reset();
+
+    [Throws=TabsApiError, Self=ByArc]
+    string sync(string key_id, string access_token, string sync_key, string tokenserver_url, string local_id);
+
+    [Self=ByArc]
+    TabsBridgedEngine bridged_engine();
+
+};
+
+// Note that this enum is duplicated in fxa-client.udl (although the underlying type *is*
+// shared). This duplication exists because there's no direct dependency between that crate and
+// this one. We can probably remove the duplication when sync15 gets a .udl file, then we could
+// reference it via an `[Extern=...]typedef`
+enum TabsDeviceType { "Desktop", "Mobile", "Tablet", "VR", "TV", "Unknown" };
+
+dictionary RemoteTabRecord {
+    string title;
+    sequence<string> url_history;
+    string? icon;
+    // Number of ms since the unix epoch (as reported by the client's clock)
+    i64 last_used;
+};
+
+dictionary ClientRemoteTabs {
+    string client_id;
+    string client_name;
+    TabsDeviceType device_type;
+    // Number of ms since the unix epoch (as reported by the server's clock)
+    i64 last_modified;
+    sequence<RemoteTabRecord> remote_tabs;
+};
+
+// Note the canonical docs for this are in https://searchfox.org/mozilla-central/source/services/interfaces/mozIBridgedSyncEngine.idl
+// It's only actually used in desktop, but it's fine to expose this everywhere.
+// NOTE: all timestamps here are milliseconds.
+interface TabsBridgedEngine {
+    //readonly attribute long storageVersion;
+    // readonly attribute boolean allowSkippedRecord;
+
+    // XXX - better logging story than this?
+    // attribute mozIServicesLogSink logger;
+
+    [Throws=TabsApiError]
+    i64 last_sync();
+
+    [Throws=TabsApiError]
+    void set_last_sync(i64 last_sync);
+
+    [Throws=TabsApiError]
+    string? sync_id();
+
+    [Throws=TabsApiError]
+    string reset_sync_id();
+
+    [Throws=TabsApiError]
+    string ensure_current_sync_id([ByRef]string new_sync_id);
+
+    [Throws=TabsApiError]
+    void prepare_for_sync([ByRef]string client_data);
+
+    [Throws=TabsApiError]
+    void sync_started();
+
+    [Throws=TabsApiError]
+    void store_incoming(sequence<string> incoming_envelopes_as_json);
+
+    [Throws=TabsApiError]
+    sequence<string> apply();
+
+    [Throws=TabsApiError]
+    void set_uploaded(i64 new_timestamp, sequence<TabsGuid> uploaded_ids);
+
+    [Throws=TabsApiError]
+    void sync_finished();
+
+    [Throws=TabsApiError]
+    void reset();
+
+    [Throws=TabsApiError]
+    void wipe();
+};