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diff --git a/third_party/rust/sync15/src/engine/bridged_engine.rs b/third_party/rust/sync15/src/engine/bridged_engine.rs
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+/* 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 anyhow::Result;
+
+use crate::bso::{IncomingBso, OutgoingBso};
+use crate::Guid;
+
+/// A BridgedEngine acts as a bridge between application-services, rust
+/// implemented sync engines and sync engines as defined by Desktop Firefox.
+///
+/// [Desktop Firefox has an abstract implementation of a Sync
+/// Engine](https://searchfox.org/mozilla-central/source/services/sync/modules/engines.js)
+/// with a number of functions each engine is expected to override. Engines
+/// implemented in Rust use a different shape (specifically, the
+/// [SyncEngine](crate::SyncEngine) trait), so this BridgedEngine trait adapts
+/// between the 2.
+pub trait BridgedEngine: Send + Sync {
+    /// Returns the last sync time, in milliseconds, for this engine's
+    /// collection. This is called before each sync, to determine the lower
+    /// bound for new records to fetch from the server.
+    fn last_sync(&self) -> Result<i64>;
+
+    /// Sets the last sync time, in milliseconds. This is called throughout
+    /// the sync, to fast-forward the stored last sync time to match the
+    /// timestamp on the uploaded records.
+    fn set_last_sync(&self, last_sync_millis: i64) -> Result<()>;
+
+    /// Returns the sync ID for this engine's collection. This is only used in
+    /// tests.
+    fn sync_id(&self) -> Result<Option<String>>;
+
+    /// Resets the sync ID for this engine's collection, returning the new ID.
+    /// As a side effect, implementations should reset all local Sync state,
+    /// as in `reset`.
+    fn reset_sync_id(&self) -> Result<String>;
+
+    /// Ensures that the locally stored sync ID for this engine's collection
+    /// matches the `new_sync_id` from the server. If the two don't match,
+    /// implementations should reset all local Sync state, as in `reset`.
+    /// This method returns the assigned sync ID, which can be either the
+    /// `new_sync_id`, or a different one if the engine wants to force other
+    /// devices to reset their Sync state for this collection the next time they
+    /// sync.
+    fn ensure_current_sync_id(&self, new_sync_id: &str) -> Result<String>;
+
+    /// Tells the tabs engine about recent FxA devices. A bit of a leaky abstration as it only
+    /// makes sense for tabs.
+    /// The arg is a json serialized `ClientData` struct.
+    fn prepare_for_sync(&self, _client_data: &str) -> Result<()> {
+        Ok(())
+    }
+
+    /// Indicates that the engine is about to start syncing. This is called
+    /// once per sync, and always before `store_incoming`.
+    fn sync_started(&self) -> Result<()>;
+
+    /// Stages a batch of incoming Sync records. This is called multiple
+    /// times per sync, once for each batch. Implementations can use the
+    /// signal to check if the operation was aborted, and cancel any
+    /// pending work.
+    fn store_incoming(&self, incoming_records: Vec<IncomingBso>) -> Result<()>;
+
+    /// Applies all staged records, reconciling changes on both sides and
+    /// resolving conflicts. Returns a list of records to upload.
+    fn apply(&self) -> Result<ApplyResults>;
+
+    /// Indicates that the given record IDs were uploaded successfully to the
+    /// server. This is called multiple times per sync, once for each batch
+    /// upload.
+    fn set_uploaded(&self, server_modified_millis: i64, ids: &[Guid]) -> Result<()>;
+
+    /// Indicates that all records have been uploaded. At this point, any record
+    /// IDs marked for upload that haven't been passed to `set_uploaded`, can be
+    /// assumed to have failed: for example, because the server rejected a record
+    /// with an invalid TTL or sort index.
+    fn sync_finished(&self) -> Result<()>;
+
+    /// Resets all local Sync state, including any change flags, mirrors, and
+    /// the last sync time, such that the next sync is treated as a first sync
+    /// with all new local data. Does not erase any local user data.
+    fn reset(&self) -> Result<()>;
+
+    /// Erases all local user data for this collection, and any Sync metadata.
+    /// This method is destructive, and unused for most collections.
+    fn wipe(&self) -> Result<()>;
+}
+
+// TODO: We should replace this with OutgoingChangeset to reduce the number
+// of types engines need to deal with.
+#[derive(Debug, Default)]
+pub struct ApplyResults {
+    /// List of records
+    pub records: Vec<OutgoingBso>,
+    /// The number of incoming records whose contents were merged because they
+    /// changed on both sides. None indicates we aren't reporting this
+    /// information.
+    pub num_reconciled: Option<usize>,
+}
+
+impl ApplyResults {
+    pub fn new(records: Vec<OutgoingBso>, num_reconciled: impl Into<Option<usize>>) -> Self {
+        Self {
+            records,
+            num_reconciled: num_reconciled.into(),
+        }
+    }
+}
+
+// Shorthand for engines that don't care.
+impl From<Vec<OutgoingBso>> for ApplyResults {
+    fn from(records: Vec<OutgoingBso>) -> Self {
+        Self {
+            records,
+            num_reconciled: None,
+        }
+    }
+}