Fix stacktraces when using ObservableDeferred and async/await (#6836)

2 files changed, 5 insertions, 0 deletions

diff --git a/changelog.d/6836.misc b/changelog.d/6836.misc
new file mode 100644

index 0000000000..232488e1e5
--- /dev/null
+++ b/changelog.d/6836.misc
@@ -0,0 +1 @@
+Fix stacktraces when using `ObservableDeferred` and async/await.
diff --git a/synapse/util/async_helpers.py b/synapse/util/async_helpers.py

index 04b6abdc24..581dffd8a0 100644
--- a/synapse/util/async_helpers.py
+++ b/synapse/util/async_helpers.py
@@ -73,6 +73,10 @@ class ObservableDeferred(object):
         def errback(f):
             object.__setattr__(self, "_result", (False, f))
             while self._observers:
+                # This is a little bit of magic to correctly propagate stack
+                # traces when we `await` on one of the observer deferreds.
+                f.value.__failure__ = f
+
                 try:
                     # TODO: Handle errors here.
                     self._observers.pop().errback(f)