Retry to sync out of sync device lists (#7453)

When a call to `user_device_resync` fails, we don't currently mark the remote user's device list as out of sync, nor do we retry to sync it.

https://github.com/matrix-org/synapse/pull/6776 introduced some code infrastructure to mark device lists as stale/out of sync.

This commit uses that code infrastructure to mark device lists as out of sync if processing an incoming device list update makes the device handler realise that the device list is out of sync, but we can't resync right now.

It also adds a looping call to retry all failed resync every 30s. This shouldn't cause too much spam in the logs as this commit also removes the "Failed to handle device list update for..." warning logs when catching `NotRetryingDestination`.

Fixes #7418 

1 files changed, 73 insertions, 7 deletions

diff --git a/synapse/handlers/device.py b/synapse/handlers/device.py
index 9bd941b5a0..29a19b4572 100644
--- a/synapse/handlers/device.py
+++ b/synapse/handlers/device.py
@@ -29,6 +29,7 @@ from synapse.api.errors import (
     SynapseError,
 )
 from synapse.logging.opentracing import log_kv, set_tag, trace
+from synapse.metrics.background_process_metrics import run_as_background_process
 from synapse.types import RoomStreamToken, get_domain_from_id
 from synapse.util import stringutils
 from synapse.util.async_helpers import Linearizer
@@ -535,6 +536,15 @@ class DeviceListUpdater(object):
             iterable=True,
         )
 
+        # Attempt to resync out of sync device lists every 30s.
+        self._resync_retry_in_progress = False
+        self.clock.looping_call(
+            run_as_background_process,
+            30 * 1000,
+            func=self._maybe_retry_device_resync,
+            desc="_maybe_retry_device_resync",
+        )
+
     @trace
     @defer.inlineCallbacks
     def incoming_device_list_update(self, origin, edu_content):
@@ -679,11 +689,50 @@ class DeviceListUpdater(object):
         return False
 
     @defer.inlineCallbacks
-    def user_device_resync(self, user_id):
+    def _maybe_retry_device_resync(self):
+        """Retry to resync device lists that are out of sync, except if another retry is
+        in progress.
+        """
+        if self._resync_retry_in_progress:
+            return
+
+        try:
+            # Prevent another call of this function to retry resyncing device lists so
+            # we don't send too many requests.
+            self._resync_retry_in_progress = True
+            # Get all of the users that need resyncing.
+            need_resync = yield self.store.get_user_ids_requiring_device_list_resync()
+            # Iterate over the set of user IDs.
+            for user_id in need_resync:
+                # Try to resync the current user's devices list. Exception handling
+                # isn't necessary here, since user_device_resync catches all instances
+                # of "Exception" that might be raised from the federation request. This
+                # means that if an exception is raised by this function, it must be
+                # because of a database issue, which means _maybe_retry_device_resync
+                # probably won't be able to go much further anyway.
+                result = yield self.user_device_resync(
+                    user_id=user_id, mark_failed_as_stale=False,
+                )
+                # user_device_resync only returns a result if it managed to successfully
+                # resync and update the database. Updating the table of users requiring
+                # resync isn't necessary here as user_device_resync already does it
+                # (through self.store.update_remote_device_list_cache).
+                if result:
+                    logger.debug(
+                        "Successfully resynced the device list for %s" % user_id,
+                    )
+        finally:
+            # Allow future calls to retry resyncinc out of sync device lists.
+            self._resync_retry_in_progress = False
+
+    @defer.inlineCallbacks
+    def user_device_resync(self, user_id, mark_failed_as_stale=True):
         """Fetches all devices for a user and updates the device cache with them.
 
         Args:
             user_id (str): The user's id whose device_list will be updated.
+            mark_failed_as_stale (bool): Whether to mark the user's device list as stale
+                if the attempt to resync failed.
         Returns:
             Deferred[dict]: a dict with device info as under the "devices" in the result of this
             request:
@@ -694,10 +743,23 @@ class DeviceListUpdater(object):
         origin = get_domain_from_id(user_id)
         try:
             result = yield self.federation.query_user_devices(origin, user_id)
-        except (NotRetryingDestination, RequestSendFailed, HttpResponseException):
-            # TODO: Remember that we are now out of sync and try again
-            # later
-            logger.warning("Failed to handle device list update for %s", user_id)
+        except NotRetryingDestination:
+            if mark_failed_as_stale:
+                # Mark the remote user's device list as stale so we know we need to retry
+                # it later.
+                yield self.store.mark_remote_user_device_cache_as_stale(user_id)
+
+            return
+        except (RequestSendFailed, HttpResponseException) as e:
+            logger.warning(
+                "Failed to handle device list update for %s: %s", user_id, e,
+            )
+
+            if mark_failed_as_stale:
+                # Mark the remote user's device list as stale so we know we need to retry
+                # it later.
+                yield self.store.mark_remote_user_device_cache_as_stale(user_id)
+
             # We abort on exceptions rather than accepting the update
             # as otherwise synapse will 'forget' that its device list
             # is out of date. If we bail then we will retry the resync
@@ -711,13 +773,17 @@ class DeviceListUpdater(object):
             logger.info(e)
             return
         except Exception as e:
-            # TODO: Remember that we are now out of sync and try again
-            # later
             set_tag("error", True)
             log_kv(
                 {"message": "Exception raised by federation request", "exception": e}
             )
             logger.exception("Failed to handle device list update for %s", user_id)
+
+            if mark_failed_as_stale:
+                # Mark the remote user's device list as stale so we know we need to retry
+                # it later.
+                yield self.store.mark_remote_user_device_cache_as_stale(user_id)
+
             return
         log_kv({"result": result})
         stream_id = result["stream_id"]