Speed up unit tests when using PostgreSQL (#8450)

changelog.d/8450.misc

+Speed up unit tests when using PostgreSQL.

synapse/storage/databases/main/events_worker.py

@@ -74,6 +74,13 @@ class EventRedactBehaviour(Names):
 
 
 class EventsWorkerStore(SQLBaseStore):
+    # Whether to use dedicated DB threads for event fetching. This is only used
+    # if there are multiple DB threads available. When used will lock the DB
+    # thread for periods of time (so unit tests want to disable this when they
+    # run DB transactions on the main thread). See EVENT_QUEUE_* for more
+    # options controlling this.
+    USE_DEDICATED_DB_THREADS_FOR_EVENT_FETCHING = True
+
     def __init__(self, database: DatabasePool, db_conn, hs):
         super().__init__(database, db_conn, hs)
 
@@ -522,7 +529,11 @@ class EventsWorkerStore(SQLBaseStore):
 
                 if not event_list:
                     single_threaded = self.database_engine.single_threaded
-                    if single_threaded or i > EVENT_QUEUE_ITERATIONS:
+                    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:

tests/server.py

@@ -372,6 +372,10 @@ def setup_test_homeserver(cleanup_func, *args, **kwargs):
         pool.threadpool = ThreadPool(clock._reactor)
         pool.running = True
 
+    # We've just changed the Databases to run DB transactions on the same
+    # thread, so we need to disable the dedicated thread behaviour.
+    server.get_datastores().main.USE_DEDICATED_DB_THREADS_FOR_EVENT_FETCHING = False
+
     return server