Clear the resync bit after resyncing device lists (#9867)

Fixes #9866.

changelog.d/9867.bugfix

@@ -0,0 +1 @@
+Fix a bug which could cause Synapse to get stuck in a loop of resyncing device lists.

synapse/handlers/device.py

@@ -925,6 +925,10 @@ async def user_device_resync(
         else:
             cached_devices = await self.store.get_cached_devices_for_user(user_id)
             if cached_devices == {d["device_id"]: d for d in devices}:
+                logging.info(
+                    "Skipping device list resync for %s, as our cache matches already",
+                    user_id,
+                )
                 devices = []
                 ignore_devices = True
 
@@ -940,6 +944,9 @@ async def user_device_resync(
             await self.store.update_remote_device_list_cache(
                 user_id, devices, stream_id
             )
+        # mark the cache as valid, whether or not we actually processed any device
+        # list updates.
+        await self.store.mark_remote_user_device_cache_as_valid(user_id)
         device_ids = [device["device_id"] for device in devices]
 
         # Handle cross-signing keys.

synapse/storage/databases/main/devices.py

@@ -717,7 +717,15 @@ async def mark_remote_user_device_cache_as_stale(self, user_id: str) -> None:
             keyvalues={"user_id": user_id},
             values={},
             insertion_values={"added_ts": self._clock.time_msec()},
-            desc="make_remote_user_device_cache_as_stale",
+            desc="mark_remote_user_device_cache_as_stale",
+        )
+
+    async def mark_remote_user_device_cache_as_valid(self, user_id: str) -> None:
+        # Remove the database entry that says we need to resync devices, after a resync
+        await self.db_pool.simple_delete(
+            table="device_lists_remote_resync",
+            keyvalues={"user_id": user_id},
+            desc="mark_remote_user_device_cache_as_valid",
         )
 
     async def mark_remote_user_device_list_as_unsubscribed(self, user_id: str) -> None:
@@ -1289,15 +1297,6 @@ def _update_remote_device_list_cache_txn(
             lock=False,
         )
 
-        # If we're replacing the remote user's device list cache presumably
-        # we've done a full resync, so we remove the entry that says we need
-        # to resync
-        self.db_pool.simple_delete_txn(
-            txn,
-            table="device_lists_remote_resync",
-            keyvalues={"user_id": user_id},
-        )
-
     async def add_device_change_to_streams(
         self, user_id: str, device_ids: Collection[str], hosts: List[str]
     ):