From dea577998f221297d3ff30bdf904f7147f3c3d8a Mon Sep 17 00:00:00 2001
From: Sean Quah <8349537+squahtx@users.noreply.github.com>
Date: Tue, 15 Mar 2022 15:40:34 +0000
Subject: [PATCH] Add tests for database transaction callbacks (#12198)

Signed-off-by: Sean Quah <seanq@element.io>
---
 changelog.d/12198.misc         |   1 +
 tests/storage/test_database.py | 104 ++++++++++++++++++++++++++++++++-
 2 files changed, 104 insertions(+), 1 deletion(-)
 create mode 100644 changelog.d/12198.misc

diff --git a/changelog.d/12198.misc b/changelog.d/12198.misc
new file mode 100644
index 0000000000..6b184a9053
--- /dev/null
+++ b/changelog.d/12198.misc
@@ -0,0 +1 @@
+Add tests for database transaction callbacks.
diff --git a/tests/storage/test_database.py b/tests/storage/test_database.py
index 8597867563..ae13bed086 100644
--- a/tests/storage/test_database.py
+++ b/tests/storage/test_database.py
@@ -12,7 +12,18 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from synapse.storage.database import make_tuple_comparison_clause
+from typing import Callable, Tuple
+from unittest.mock import Mock, call
+
+from twisted.test.proto_helpers import MemoryReactor
+
+from synapse.server import HomeServer
+from synapse.storage.database import (
+    DatabasePool,
+    LoggingTransaction,
+    make_tuple_comparison_clause,
+)
+from synapse.util import Clock
 
 from tests import unittest
 
@@ -22,3 +33,94 @@ class TupleComparisonClauseTestCase(unittest.TestCase):
         clause, args = make_tuple_comparison_clause([("a", 1), ("b", 2)])
         self.assertEqual(clause, "(a,b) > (?,?)")
         self.assertEqual(args, [1, 2])
+
+
+class CallbacksTestCase(unittest.HomeserverTestCase):
+    """Tests for transaction callbacks."""
+
+    def prepare(self, reactor: MemoryReactor, clock: Clock, hs: HomeServer) -> None:
+        self.store = hs.get_datastores().main
+        self.db_pool: DatabasePool = self.store.db_pool
+
+    def _run_interaction(
+        self, func: Callable[[LoggingTransaction], object]
+    ) -> Tuple[Mock, Mock]:
+        """Run the given function in a database transaction, with callbacks registered.
+
+        Args:
+            func: The function to be run in a transaction. The transaction will be
+                retried if `func` raises an `OperationalError`.
+
+        Returns:
+            Two mocks, which were registered as an `after_callback` and an
+            `exception_callback` respectively, on every transaction attempt.
+        """
+        after_callback = Mock()
+        exception_callback = Mock()
+
+        def _test_txn(txn: LoggingTransaction) -> None:
+            txn.call_after(after_callback, 123, 456, extra=789)
+            txn.call_on_exception(exception_callback, 987, 654, extra=321)
+            func(txn)
+
+        try:
+            self.get_success_or_raise(
+                self.db_pool.runInteraction("test_transaction", _test_txn)
+            )
+        except Exception:
+            pass
+
+        return after_callback, exception_callback
+
+    def test_after_callback(self) -> None:
+        """Test that the after callback is called when a transaction succeeds."""
+        after_callback, exception_callback = self._run_interaction(lambda txn: None)
+
+        after_callback.assert_called_once_with(123, 456, extra=789)
+        exception_callback.assert_not_called()
+
+    def test_exception_callback(self) -> None:
+        """Test that the exception callback is called when a transaction fails."""
+        _test_txn = Mock(side_effect=ZeroDivisionError)
+        after_callback, exception_callback = self._run_interaction(_test_txn)
+
+        after_callback.assert_not_called()
+        exception_callback.assert_called_once_with(987, 654, extra=321)
+
+    def test_failed_retry(self) -> None:
+        """Test that the exception callback is called for every failed attempt."""
+        # Always raise an `OperationalError`.
+        _test_txn = Mock(side_effect=self.db_pool.engine.module.OperationalError)
+        after_callback, exception_callback = self._run_interaction(_test_txn)
+
+        after_callback.assert_not_called()
+        exception_callback.assert_has_calls(
+            [
+                call(987, 654, extra=321),
+                call(987, 654, extra=321),
+                call(987, 654, extra=321),
+                call(987, 654, extra=321),
+                call(987, 654, extra=321),
+                call(987, 654, extra=321),
+            ]
+        )
+        self.assertEqual(exception_callback.call_count, 6)  # no additional calls
+
+    def test_successful_retry(self) -> None:
+        """Test callbacks for a failed transaction followed by a successful attempt."""
+        # Raise an `OperationalError` on the first attempt only.
+        _test_txn = Mock(
+            side_effect=[self.db_pool.engine.module.OperationalError, None]
+        )
+        after_callback, exception_callback = self._run_interaction(_test_txn)
+
+        # Calling both `after_callback`s when the first attempt failed is rather
+        # surprising (#12184). Let's document the behaviour in a test.
+        after_callback.assert_has_calls(
+            [
+                call(123, 456, extra=789),
+                call(123, 456, extra=789),
+            ]
+        )
+        self.assertEqual(after_callback.call_count, 2)  # no additional calls
+        exception_callback.assert_not_called()