Allow setting transaction limit for db connections (#10440)

Setting the value will help PostgreSQL free up memory by recycling
the connections in the connection pool.

Signed-off-by: Toni Spets <toni.spets@iki.fi>

tests/storage/test_txn_limit.py

@@ -0,0 +1,36 @@
+# Copyright 2014-2021 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# 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.
+
+from tests import unittest
+
+
+class SQLTransactionLimitTestCase(unittest.HomeserverTestCase):
+    """Test SQL transaction limit doesn't break transactions."""
+
+    def make_homeserver(self, reactor, clock):
+        return self.setup_test_homeserver(db_txn_limit=1000)
+
+    def test_config(self):
+        db_config = self.hs.config.get_single_database()
+        self.assertEqual(db_config.config["txn_limit"], 1000)
+
+    def test_select(self):
+        def do_select(txn):
+            txn.execute("SELECT 1")
+
+        db_pool = self.hs.get_datastores().databases[0]
+
+        # force txn limit to roll over at least once
+        for i in range(0, 1001):
+            self.get_success_or_raise(db_pool.runInteraction("test_select", do_select))