From ec5fcfceff6bf37b20d55963f3bb5fbaa0dde796 Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Tue, 28 Jan 2020 17:10:57 +0000
Subject: [PATCH 1/2] When a client asks for remote keys check if should
 resync.

If we detect that the remote users' keys may have changed then we should
attempt to resync against the remote server rather than using the
(potentially) stale local cache.
---
 synapse/storage/data_stores/main/devices.py | 32 +++++++++++++++++++--
 1 file changed, 29 insertions(+), 3 deletions(-)

diff --git a/synapse/storage/data_stores/main/devices.py b/synapse/storage/data_stores/main/devices.py
index 30bf66b2b69f..a34415ff14ca 100644
--- a/synapse/storage/data_stores/main/devices.py
+++ b/synapse/storage/data_stores/main/devices.py
@@ -32,7 +32,7 @@
 from synapse.metrics.background_process_metrics import run_as_background_process
 from synapse.storage._base import SQLBaseStore, db_to_json, make_in_list_sql_clause
 from synapse.storage.database import Database
-from synapse.types import get_verify_key_from_cross_signing_key
+from synapse.types import Collection, get_verify_key_from_cross_signing_key
 from synapse.util.caches.descriptors import (
     Cache,
     cached,
@@ -443,8 +443,15 @@ def get_user_devices_from_cache(self, query_list):
         """
         user_ids = set(user_id for user_id, _ in query_list)
         user_map = yield self.get_device_list_last_stream_id_for_remotes(list(user_ids))
-        user_ids_in_cache = set(
-            user_id for user_id, stream_id in user_map.items() if stream_id
+
+        # We go and check if any of the users need to have their device lists
+        # resynced. If they do then we remove them from the cached list.
+        users_needing_resync = yield self.get_user_ids_requiring_device_list_resync(
+            user_ids
+        )
+        user_ids_in_cache = (
+            set(user_id for user_id, stream_id in user_map.items() if stream_id)
+            - users_needing_resync
         )
         user_ids_not_in_cache = user_ids - user_ids_in_cache
 
@@ -641,6 +648,25 @@ def get_device_list_last_stream_id_for_remotes(self, user_ids):
 
         return results
 
+    @defer.inlineCallbacks
+    def get_user_ids_requiring_device_list_resync(self, user_ids: Collection[str]):
+        """Given a list of remote users return the list of users that we
+        should resync the device lists for.
+
+        Returns:
+            Deferred[Set[str]]
+        """
+
+        rows = yield self.db.simple_select_many_batch(
+            table="device_lists_remote_resync",
+            column="user_id",
+            iterable=user_ids,
+            retcols=("user_id",),
+            desc="get_user_ids_requiring_device_list_resync",
+        )
+
+        return {row["user_id"] for row in rows}
+
     def mark_remote_user_device_cache_as_stale(self, user_id: str):
         """Records that the server has reason to believe the cache of the devices
         for the remote users is out of date.

From 0798c1447b8447bccd4d5887e0f8e867adf544c9 Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Tue, 28 Jan 2020 17:45:31 +0000
Subject: [PATCH 2/2] Newsfile

---
 changelog.d/6797.misc | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 changelog.d/6797.misc

diff --git a/changelog.d/6797.misc b/changelog.d/6797.misc
new file mode 100644
index 000000000000..e9127bac517c
--- /dev/null
+++ b/changelog.d/6797.misc
@@ -0,0 +1 @@
+When a client asks for a remote user's device keys check if the local cache for that user has been marked as potentially stale.