From 69018acbd2d1f331d6a52335b4938c3753b16de6 Mon Sep 17 00:00:00 2001
From: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>
Date: Thu, 22 Apr 2021 16:53:24 +0100
Subject: [PATCH] Clear the resync bit after resyncing device lists (#9867)

Fixes #9866.
---
 changelog.d/9867.bugfix                   |  1 +
 synapse/handlers/device.py                |  7 +++++++
 synapse/storage/databases/main/devices.py | 19 +++++++++----------
 3 files changed, 17 insertions(+), 10 deletions(-)
 create mode 100644 changelog.d/9867.bugfix

diff --git a/changelog.d/9867.bugfix b/changelog.d/9867.bugfix
new file mode 100644
index 0000000000..f236de247d
--- /dev/null
+++ b/changelog.d/9867.bugfix
@@ -0,0 +1 @@
+Fix a bug which could cause Synapse to get stuck in a loop of resyncing device lists.
diff --git a/synapse/handlers/device.py b/synapse/handlers/device.py
index 34d39e3b44..95bdc5902a 100644
--- a/synapse/handlers/device.py
+++ b/synapse/handlers/device.py
@@ -925,6 +925,10 @@ class DeviceListUpdater:
         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 @@ class DeviceListUpdater:
             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.
diff --git a/synapse/storage/databases/main/devices.py b/synapse/storage/databases/main/devices.py
index 9be713399f..c9346de316 100644
--- a/synapse/storage/databases/main/devices.py
+++ b/synapse/storage/databases/main/devices.py
@@ -717,7 +717,15 @@ class DeviceWorkerStore(SQLBaseStore):
             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 @@ class DeviceStore(DeviceWorkerStore, DeviceBackgroundUpdateStore):
             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]
     ):