From 8eec25a1d9d656905db18a2c62a5552e63db2667 Mon Sep 17 00:00:00 2001
From: Sean Quah <8349537+squahtx@users.noreply.github.com>
Date: Thu, 4 Nov 2021 10:33:53 +0000
Subject: [PATCH] Track ongoing event fetches correctly in the presence of
 failure (#11240)

When an event fetcher aborts due to an exception, `_event_fetch_ongoing`
must be decremented, otherwise the event fetcher would never be
replaced. If enough event fetchers were to fail, no more events would be
fetched and requests would get stuck waiting for events.
---
 changelog.d/11240.bugfix                      |  1 +
 .../storage/databases/main/events_worker.py   | 52 ++++++++++++-------
 2 files changed, 33 insertions(+), 20 deletions(-)
 create mode 100644 changelog.d/11240.bugfix

diff --git a/changelog.d/11240.bugfix b/changelog.d/11240.bugfix
new file mode 100644
index 000000000..94d73f67e
--- /dev/null
+++ b/changelog.d/11240.bugfix
@@ -0,0 +1 @@
+Fix a long-standing bug where all requests that read events from the database could get stuck as a result of losing the database connection.
diff --git a/synapse/storage/databases/main/events_worker.py b/synapse/storage/databases/main/events_worker.py
index ae37901be..c6bf316d5 100644
--- a/synapse/storage/databases/main/events_worker.py
+++ b/synapse/storage/databases/main/events_worker.py
@@ -28,6 +28,7 @@ from typing import (
 
 import attr
 from constantly import NamedConstant, Names
+from prometheus_client import Gauge
 from typing_extensions import Literal
 
 from twisted.internet import defer
@@ -81,6 +82,12 @@ EVENT_QUEUE_ITERATIONS = 3  # No. times we block waiting for requests for events
 EVENT_QUEUE_TIMEOUT_S = 0.1  # Timeout when waiting for requests for events
 
 
+event_fetch_ongoing_gauge = Gauge(
+    "synapse_event_fetch_ongoing",
+    "The number of event fetchers that are running",
+)
+
+
 @attr.s(slots=True, auto_attribs=True)
 class _EventCacheEntry:
     event: EventBase
@@ -222,6 +229,7 @@ class EventsWorkerStore(SQLBaseStore):
         self._event_fetch_lock = threading.Condition()
         self._event_fetch_list = []
         self._event_fetch_ongoing = 0
+        event_fetch_ongoing_gauge.set(self._event_fetch_ongoing)
 
         # We define this sequence here so that it can be referenced from both
         # the DataStore and PersistEventStore.
@@ -732,28 +740,31 @@ class EventsWorkerStore(SQLBaseStore):
         """Takes a database connection and waits for requests for events from
         the _event_fetch_list queue.
         """
-        i = 0
-        while True:
-            with self._event_fetch_lock:
-                event_list = self._event_fetch_list
-                self._event_fetch_list = []
+        try:
+            i = 0
+            while True:
+                with self._event_fetch_lock:
+                    event_list = self._event_fetch_list
+                    self._event_fetch_list = []
 
-                if not event_list:
-                    single_threaded = self.database_engine.single_threaded
-                    if (
-                        not self.USE_DEDICATED_DB_THREADS_FOR_EVENT_FETCHING
-                        or single_threaded
-                        or i > EVENT_QUEUE_ITERATIONS
-                    ):
-                        self._event_fetch_ongoing -= 1
-                        return
-                    else:
-                        self._event_fetch_lock.wait(EVENT_QUEUE_TIMEOUT_S)
-                        i += 1
-                        continue
-                i = 0
+                    if not event_list:
+                        single_threaded = self.database_engine.single_threaded
+                        if (
+                            not self.USE_DEDICATED_DB_THREADS_FOR_EVENT_FETCHING
+                            or single_threaded
+                            or i > EVENT_QUEUE_ITERATIONS
+                        ):
+                            break
+                        else:
+                            self._event_fetch_lock.wait(EVENT_QUEUE_TIMEOUT_S)
+                            i += 1
+                            continue
+                    i = 0
 
-            self._fetch_event_list(conn, event_list)
+                self._fetch_event_list(conn, event_list)
+        finally:
+            self._event_fetch_ongoing -= 1
+            event_fetch_ongoing_gauge.set(self._event_fetch_ongoing)
 
     def _fetch_event_list(
         self, conn: Connection, event_list: List[Tuple[List[str], defer.Deferred]]
@@ -977,6 +988,7 @@ class EventsWorkerStore(SQLBaseStore):
 
             if self._event_fetch_ongoing < EVENT_QUEUE_THREADS:
                 self._event_fetch_ongoing += 1
+                event_fetch_ongoing_gauge.set(self._event_fetch_ongoing)
                 should_start = True
             else:
                 should_start = False