Prevent logging context going missing on federation request timeout

changelog.d/10810.bugfix

@@ -0,0 +1 @@
+Fix a case where logging contexts would go missing when federation requests time out.

synapse/http/matrixfederationclient.py

@@ -66,7 +66,7 @@
 )
 from synapse.http.federation.matrix_federation_agent import MatrixFederationAgent
 from synapse.logging import opentracing
-from synapse.logging.context import make_deferred_yieldable
+from synapse.logging.context import make_deferred_yieldable, run_in_background
 from synapse.logging.opentracing import set_tag, start_active_span, tags
 from synapse.types import JsonDict
 from synapse.util import json_decoder
@@ -553,20 +553,29 @@ async def _send_request(
                         with Measure(self.clock, "outbound_request"):
                             # we don't want all the fancy cookie and redirect handling
                             # that treq.request gives: just use the raw Agent.
-                            request_deferred = self.agent.request(
+
+                            # To preserve the logging context, the timeout is treated
+                            # in a similar way to `defer.gatherResults`:
+                            # * Each logging context-preserving fork is wrapped in
+                            #   `run_in_background`. In this case there is only one,
+                            #   since the timeout fork is not logging-context aware.
+                            # * The `Deferred` that joins the forks back together is
+                            #   wrapped in `make_deferred_yieldable` to restore the
+                            #   logging context regardless of the path taken.
+                            request_deferred = run_in_background(
+                                self.agent.request,
                                 method_bytes,
                                 url_bytes,
                                 headers=Headers(headers_dict),
                                 bodyProducer=producer,
                             )
-
                             request_deferred = timeout_deferred(
                                 request_deferred,
                                 timeout=_sec_timeout,
                                 reactor=self.reactor,
                             )
 
-                            response = await request_deferred
+                            response = await make_deferred_yieldable(request_deferred)
                     except DNSLookupError as e:
                         raise RequestSendFailed(e, can_retry=retry_on_dns_fail) from e
                     except Exception as e: