Fix dropped logcontexts during high outbound traffic. (#5277)

Fixes #5271.

changelog.d/5277.bugfix

@@ -0,0 +1 @@
+Fix dropped logcontexts during high outbound traffic.

synapse/app/_base.py

@@ -344,14 +344,20 @@ class _LimitedHostnameResolver(object):
 
     def resolveHostName(self, resolutionReceiver, hostName, portNumber=0,
                         addressTypes=None, transportSemantics='TCP'):
-        # Note this is happening deep within the reactor, so we don't need to
-        # worry about log contexts.
-
         # We need this function to return `resolutionReceiver` so we do all the
         # actual logic involving deferreds in a separate function.
+
+        # even though this is happening within the depths of twisted, we need to drop
+        # our logcontext before starting _resolve, otherwise: (a) _resolve will drop
+        # the logcontext if it returns an incomplete deferred; (b) _resolve will
+        # call the resolutionReceiver *with* a logcontext, which it won't be expecting.
+        with PreserveLoggingContext():
             self._resolve(
-            resolutionReceiver, hostName, portNumber,
-            addressTypes, transportSemantics,
+                resolutionReceiver,
+                hostName,
+                portNumber,
+                addressTypes,
+                transportSemantics,
             )
 
         return resolutionReceiver