Only delete N devices at a time

synapse/handlers/device.py

@@ -52,6 +52,7 @@
 from synapse.util.async_helpers import Linearizer
 from synapse.util.caches.expiringcache import ExpiringCache
 from synapse.util.cancellation import cancellable
+from synapse.util.iterutils import batch_iter
 from synapse.util.metrics import measure_func
 from synapse.util.retryutils import NotRetryingDestination
 
@@ -461,7 +462,24 @@ async def _prune_too_many_devices(self, user_id: str) -> None:
         if not device_ids:
             return
 
-        await self.delete_devices(user_id, device_ids)
+        # We don't want to block and try and delete tonnes of devices at once,
+        # so we cap the number of devices we delete synchronously.
+        first_batch, remaining_device_ids = device_ids[:10], device_ids[10:]
+        await self.delete_devices(user_id, first_batch)
+
+        if not remaining_device_ids:
+            return
+
+        # Now spawn a background loop that deletes the rest.
+        async def _prune_too_many_devices_loop() -> None:
+            for batch in batch_iter(remaining_device_ids, 10):
+                await self.delete_devices(user_id, batch)
+
+                await self.clock.sleep(1)
+
+        run_as_background_process(
+            "_prune_too_many_devices_loop", _prune_too_many_devices_loop
+        )
 
     async def _delete_stale_devices(self) -> None:
         """Background task that deletes devices which haven't been accessed for more than

synapse/storage/databases/main/devices.py

@@ -1533,7 +1533,7 @@ def _txn(txn: LoggingTransaction) -> int:
 
         return rows
 
-    async def check_too_many_devices_for_user(self, user_id: str) -> Collection[str]:
+    async def check_too_many_devices_for_user(self, user_id: str) -> List[str]:
         """Check if the user has a lot of devices, and if so return the set of
         devices we can prune.
 
@@ -1585,13 +1585,14 @@ async def check_too_many_devices_for_user(self, user_id: str) -> Collection[str]
                 AND NOT hidden
                 AND last_seen < ?
                 AND key_json IS NULL
+            ORDER BY last_seen
         """
 
         def check_too_many_devices_for_user_txn(
             txn: LoggingTransaction,
         ) -> Collection[str]:
             txn.execute(sql, (user_id, max_last_seen))
-            return {device_id for device_id, in txn}
+            return [device_id for device_id, in txn]
 
         return await self.db_pool.runInteraction(
             "check_too_many_devices_for_user",