Change the way we do logging contexts so that they survive divergences

synapse/util/logcontext.py

@@ -12,6 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from twisted.internet import defer
+
 import threading
 import logging
 
@@ -129,3 +131,32 @@ class PreserveLoggingContext(object):
     def __exit__(self, type, value, traceback):
         """Restores the current logging context"""
         LoggingContext.thread_local.current_context = self.current_context
+
+
+def preserve_context_over_fn(fn, *args, **kwargs):
+    with PreserveLoggingContext():
+        deferred = fn(*args, **kwargs)
+
+    return preserve_context_over_deferred(deferred)
+
+
+def preserve_context_over_deferred(deferred):
+    d = defer.Deferred()
+
+    current_context = LoggingContext.current_context()
+
+    def cb(res):
+        with PreserveLoggingContext():
+            LoggingContext.thread_local.current_context = current_context
+            res = d.callback(res)
+        return res
+
+    def eb(failure):
+        with PreserveLoggingContext():
+            LoggingContext.thread_local.current_context = current_context
+            res = d.errback(failure)
+        return res
+
+    deferred.addCallbacks(cb, eb)
+
+    return d