From 9898470e7d7b1b2f3c0dfa7d07832ec4662221da Mon Sep 17 00:00:00 2001
From: Jonathan de Jong <jonathan@automatia.nl>
Date: Tue, 9 Mar 2021 12:09:31 +0100
Subject: [PATCH] Add logging to ObservableDeferred callbacks (#9523)

---
 changelog.d/9523.misc         |  1 +
 synapse/util/async_helpers.py | 26 ++++++++++++++++++--------
 2 files changed, 19 insertions(+), 8 deletions(-)
 create mode 100644 changelog.d/9523.misc

diff --git a/changelog.d/9523.misc b/changelog.d/9523.misc
new file mode 100644
index 0000000000..f03e939efb
--- /dev/null
+++ b/changelog.d/9523.misc
@@ -0,0 +1 @@
+Add extra logging to ObservableDeferred when callbacks throw exceptions.
\ No newline at end of file
diff --git a/synapse/util/async_helpers.py b/synapse/util/async_helpers.py
index 719e35b78d..f33c115844 100644
--- a/synapse/util/async_helpers.py
+++ b/synapse/util/async_helpers.py
@@ -76,11 +76,16 @@ class ObservableDeferred:
         def callback(r):
             object.__setattr__(self, "_result", (True, r))
             while self._observers:
+                observer = self._observers.pop()
                 try:
-                    # TODO: Handle errors here.
-                    self._observers.pop().callback(r)
-                except Exception:
-                    pass
+                    observer.callback(r)
+                except Exception as e:
+                    logger.exception(
+                        "%r threw an exception on .callback(%r), ignoring...",
+                        observer,
+                        r,
+                        exc_info=e,
+                    )
             return r
 
         def errback(f):
@@ -90,11 +95,16 @@ class ObservableDeferred:
                 # traces when we `await` on one of the observer deferreds.
                 f.value.__failure__ = f
 
+                observer = self._observers.pop()
                 try:
-                    # TODO: Handle errors here.
-                    self._observers.pop().errback(f)
-                except Exception:
-                    pass
+                    observer.errback(f)
+                except Exception as e:
+                    logger.exception(
+                        "%r threw an exception on .errback(%r), ignoring...",
+                        observer,
+                        f,
+                        exc_info=e,
+                    )
 
             if consumeErrors:
                 return None