Fix logging context warnings when losing replication connection (#10984)

Instead of triggering `__exit__` manually on the replication handler's
logging context, use it as a context manager so that there is an
`__enter__` call to balance the `__exit__`.

1 files changed, 13 insertions, 5 deletions

diff --git a/synapse/replication/tcp/redis.py b/synapse/replication/tcp/redis.py
index 062fe2f33e..8d28bd3f3f 100644
--- a/synapse/replication/tcp/redis.py
+++ b/synapse/replication/tcp/redis.py
@@ -100,9 +100,13 @@ class RedisSubscriber(txredisapi.SubscriberProtocol):
 
         # a logcontext which we use for processing incoming commands. We declare it as a
         # background process so that the CPU stats get reported to prometheus.
-        self._logging_context = BackgroundProcessLoggingContext(
-            "replication_command_handler"
-        )
+        with PreserveLoggingContext():
+            # thanks to `PreserveLoggingContext()`, the new logcontext is guaranteed to
+            # capture the sentinel context as its containing context and won't prevent
+            # GC of / unintentionally reactivate what would be the current context.
+            self._logging_context = BackgroundProcessLoggingContext(
+                "replication_command_handler"
+            )
 
     def connectionMade(self):
         logger.info("Connected to redis")
@@ -182,8 +186,12 @@ class RedisSubscriber(txredisapi.SubscriberProtocol):
         super().connectionLost(reason)
         self.synapse_handler.lost_connection(self)
 
-        # mark the logging context as finished
-        self._logging_context.__exit__(None, None, None)
+        # mark the logging context as finished by triggering `__exit__()`
+        with PreserveLoggingContext():
+            with self._logging_context:
+                pass
+            # the sentinel context is now active, which may not be correct.
+            # PreserveLoggingContext() will restore the correct logging context.
 
     def send_command(self, cmd: Command):
         """Send a command if connection has been established.