From 57383d4e25e0153c8a4498492e342b00824da6f1 Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Fri, 21 Jul 2023 12:09:26 +0100
Subject: [PATCH 1/2] Resync stale devices in background

This is so we don't block responding to federation transaction while we
try and fetch the device lists.
---
 synapse/handlers/device.py | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/synapse/handlers/device.py b/synapse/handlers/device.py
index 5d12a39e26a4..d73d9dca083d 100644
--- a/synapse/handlers/device.py
+++ b/synapse/handlers/device.py
@@ -1124,7 +1124,14 @@ async def _handle_device_updates(self, user_id: str) -> None:
                 )
 
             if resync:
-                await self.multi_user_device_resync([user_id])
+                # We mark as stale up front in case we get restarted.
+                await self.store.mark_remote_users_device_caches_as_stale([user_id])
+                run_as_background_process(
+                    "_maybe_retry_device_resync",
+                    self.multi_user_device_resync,
+                    [user_id],
+                    False,
+                )
             else:
                 # Simply update the single device, since we know that is the only
                 # change (because of the single prev_id matching the current cache)

From 3a4772392f60e5f9e011ebbbdf7aa24f6f3bdeee Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Fri, 21 Jul 2023 12:11:34 +0100
Subject: [PATCH 2/2] Newsfile

---
 changelog.d/15975.bugfix | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 changelog.d/15975.bugfix

diff --git a/changelog.d/15975.bugfix b/changelog.d/15975.bugfix
new file mode 100644
index 000000000000..59738cca0ae7
--- /dev/null
+++ b/changelog.d/15975.bugfix
@@ -0,0 +1 @@
+Fix bug where resyncing stale device lists could block responding to federation transactions, and thus delay receiving new data from the remote server.