From 49a683d871add82fb1a8125c6803ac15ec7d341b Mon Sep 17 00:00:00 2001
From: Sean Quah <8349537+squahtx@users.noreply.github.com>
Date: Fri, 8 Oct 2021 12:27:16 +0100
Subject: Fix long-standing bug where `ReadWriteLock` could drop logging
 contexts (#10993)

Use `PreserveLoggingContext()` to ensure that logging contexts are not
lost when exiting a read/write lock.

When exiting a read/write lock, callbacks on a `Deferred` are triggered
as a signal to any waiting coroutines. Any waiting coroutine that
becomes runnable is likely to follow the Synapse logging context rules
and will restore its own logging context, then either run to completion
or await another `Deferred`, resetting the logging context in the
process.
---
 synapse/util/async_helpers.py | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

(limited to 'synapse/util')

diff --git a/synapse/util/async_helpers.py b/synapse/util/async_helpers.py
index 82d918a05f..5df80ea8e7 100644
--- a/synapse/util/async_helpers.py
+++ b/synapse/util/async_helpers.py
@@ -438,7 +438,8 @@ class ReadWriteLock:
             try:
                 yield
             finally:
-                new_defer.callback(None)
+                with PreserveLoggingContext():
+                    new_defer.callback(None)
                 self.key_to_current_readers.get(key, set()).discard(new_defer)
 
         return _ctx_manager()
@@ -466,7 +467,8 @@ class ReadWriteLock:
             try:
                 yield
             finally:
-                new_defer.callback(None)
+                with PreserveLoggingContext():
+                    new_defer.callback(None)
                 if self.key_to_current_writer[key] == new_defer:
                     self.key_to_current_writer.pop(key)
 
-- 
cgit 1.4.1