From 6525fd65ee52e36929b9c35253c772da16aa2b99 Mon Sep 17 00:00:00 2001
From: David Robertson <davidr@element.io>
Date: Fri, 1 Sep 2023 12:41:56 +0100
Subject: [PATCH] Log the details of background update failures (#16212)

---
 changelog.d/16212.misc                  |  1 +
 synapse/storage/background_updates.py   |  4 ++--
 tests/storage/test_background_update.py | 24 +++++++++++++++++++++++-
 3 files changed, 26 insertions(+), 3 deletions(-)
 create mode 100644 changelog.d/16212.misc

diff --git a/changelog.d/16212.misc b/changelog.d/16212.misc
new file mode 100644
index 000000000..19cf9b102
--- /dev/null
+++ b/changelog.d/16212.misc
@@ -0,0 +1 @@
+Log the details of background update failures.
diff --git a/synapse/storage/background_updates.py b/synapse/storage/background_updates.py
index ddca0af1d..7619f405f 100644
--- a/synapse/storage/background_updates.py
+++ b/synapse/storage/background_updates.py
@@ -405,14 +405,14 @@ class BackgroundUpdater:
                 try:
                     result = await self.do_next_background_update(sleep)
                     back_to_back_failures = 0
-                except Exception:
+                except Exception as e:
+                    logger.exception("Error doing update: %s", e)
                     back_to_back_failures += 1
                     if back_to_back_failures >= 5:
                         self._aborted = True
                         raise RuntimeError(
                             "5 back-to-back background update failures; aborting."
                         )
-                    logger.exception("Error doing update")
                 else:
                     if result:
                         logger.info(
diff --git a/tests/storage/test_background_update.py b/tests/storage/test_background_update.py
index 52beb4e89..abf7d0564 100644
--- a/tests/storage/test_background_update.py
+++ b/tests/storage/test_background_update.py
@@ -11,7 +11,7 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-
+import logging
 from unittest.mock import AsyncMock, Mock
 
 import yaml
@@ -330,6 +330,28 @@ class BackgroundUpdateTestCase(unittest.HomeserverTestCase):
         self.update_handler.side_effect = update_short
         self.get_success(self.updates.do_next_background_update(False))
 
+    def test_failed_update_logs_exception_details(self) -> None:
+        needle = "RUH ROH RAGGY"
+
+        def failing_update(progress: JsonDict, count: int) -> int:
+            raise Exception(needle)
+
+        self.update_handler.side_effect = failing_update
+        self.update_handler.reset_mock()
+
+        self.get_success(
+            self.store.db_pool.simple_insert(
+                "background_updates",
+                values={"update_name": "test_update", "progress_json": "{}"},
+            )
+        )
+
+        with self.assertLogs(level=logging.ERROR) as logs:
+            # Expect a back-to-back RuntimeError to be raised
+            self.get_failure(self.updates.run_background_updates(False), RuntimeError)
+
+        self.assertTrue(any(needle in log for log in logs.output), logs.output)
+
 
 class BackgroundUpdateControllerTestCase(unittest.HomeserverTestCase):
     def prepare(self, reactor: MemoryReactor, clock: Clock, hs: HomeServer) -> None: