Make sqlite database migrations transactional again, part two

#14910 fixed the regression introduced by #13873 where sqlite database
migrataions would no longer run inside a transaction. However, it
committed the transaction before Synapse updated its bookkeeping of
which migrations have been run, which means that migrations may be run
again after they have completed successfully.

Leave the transaction open at the end of `executescript`, to restore the
old, correct behaviour.

Fixes #14909.

Signed-off-by: Sean Quah <[email protected]>

changelog.d/14926.bugfix

@@ -0,0 +1 @@
+Fix a regression introduced in Synapse 1.69.0 which can result in database corruption when database migrations are interrupted on sqlite.

synapse/storage/engines/_base.py

@@ -133,8 +133,9 @@ def executescript(cursor: CursorType, script: str) -> None:
 
         This is not provided by DBAPI2, and so needs engine-specific support.
 
-        Some database engines may automatically COMMIT the ongoing transaction both
-        before and after executing the script.
+        Some database engines may automatically COMMIT the ongoing transaction before
+        executing the script in its own transaction. The script transaction is left
+        open and it is the responsibility of the caller to commit it.
         """
         ...
 

synapse/storage/engines/sqlite.py

@@ -135,14 +135,16 @@ def executescript(cursor: sqlite3.Cursor, script: str) -> None:
         > than one statement with it, it will raise a Warning. Use executescript() if
         > you want to execute multiple SQL statements with one call.
 
-        The script is wrapped in transaction control statemnets, since the docs for
+        The script is prefixed with a `BEGIN TRANSACTION`, since the docs for
         `executescript` warn:
 
         > If there is a pending transaction, an implicit COMMIT statement is executed
         > first. No other implicit transaction control is performed; any transaction
         > control must be added to sql_script.
         """
-        cursor.executescript(f"BEGIN TRANSACTION;\n{script}\nCOMMIT;")
+        # The implementation of `executescript` can be found at
+        # https://github.com/python/cpython/blob/3.11/Modules/_sqlite/cursor.c#L1035.
+        cursor.executescript(f"BEGIN TRANSACTION; {script}")
 
 
 # Following functions taken from: https://github.com/coleifer/peewee

tests/storage/test_database.py

@@ -22,6 +22,7 @@
 from synapse.server import HomeServer
 from synapse.storage.database import (
     DatabasePool,
+    LoggingDatabaseConnection,
     LoggingTransaction,
     make_tuple_comparison_clause,
 )
@@ -37,6 +38,101 @@ def test_native_tuple_comparison(self) -> None:
         self.assertEqual(args, [1, 2])
 
 
+class ExecuteScriptTestCase(unittest.HomeserverTestCase):
+    """Tests for `BaseDatabaseEngine.executescript` implementations."""
+
+    def prepare(self, reactor: MemoryReactor, clock: Clock, hs: HomeServer) -> None:
+        self.store = hs.get_datastores().main
+        self.db_pool: DatabasePool = self.store.db_pool
+        self.get_success(
+            self.db_pool.runInteraction(
+                "create",
+                lambda txn: txn.execute("CREATE TABLE foo (name TEXT PRIMARY KEY)"),
+            )
+        )
+
+    def test_transaction(self) -> None:
+        """Test that all statements are run in a single transaction."""
+
+        def run(conn: LoggingDatabaseConnection) -> None:
+            cur = conn.cursor(txn_name="test_transaction")
+            self.db_pool.engine.executescript(
+                cur,
+                ";".join(
+                    [
+                        "INSERT INTO foo (name) VALUES ('transaction test')",
+                        # This next statement will fail. When `executescript` is not
+                        # transactional, the previous row will be observed later.
+                        "INSERT INTO foo (name) VALUES ('transaction test')",
+                    ]
+                ),
+            )
+
+        self.get_failure(
+            self.db_pool.runWithConnection(run),
+            self.db_pool.engine.module.IntegrityError,
+        )
+
+        self.assertIsNone(
+            self.get_success(
+                self.db_pool.simple_select_one_onecol(
+                    "foo",
+                    keyvalues={"name": "transaction test"},
+                    retcol="name",
+                    allow_none=True,
+                )
+            ),
+            "executescript is not running statements inside a transaction",
+        )
+
+    def test_commit(self) -> None:
+        """Test that the script transaction remains open and can be committed."""
+
+        def run(conn: LoggingDatabaseConnection) -> None:
+            cur = conn.cursor(txn_name="test_commit")
+            self.db_pool.engine.executescript(
+                cur, "INSERT INTO foo (name) VALUES ('commit test')"
+            )
+            cur.execute("COMMIT")
+
+        self.get_success(self.db_pool.runWithConnection(run))
+
+        self.assertIsNotNone(
+            self.get_success(
+                self.db_pool.simple_select_one_onecol(
+                    "foo",
+                    keyvalues={"name": "commit test"},
+                    retcol="name",
+                    allow_none=True,
+                )
+            ),
+        )
+
+    def test_rollback(self) -> None:
+        """Test that the script transaction remains open and can be rolled back."""
+
+        def run(conn: LoggingDatabaseConnection) -> None:
+            cur = conn.cursor(txn_name="test_rollback")
+            self.db_pool.engine.executescript(
+                cur, "INSERT INTO foo (name) VALUES ('rollback test')"
+            )
+            cur.execute("ROLLBACK")
+
+        self.get_success(self.db_pool.runWithConnection(run))
+
+        self.assertIsNone(
+            self.get_success(
+                self.db_pool.simple_select_one_onecol(
+                    "foo",
+                    keyvalues={"name": "rollback test"},
+                    retcol="name",
+                    allow_none=True,
+                )
+            ),
+            "executescript is not leaving the script transaction open",
+        )
+
+
 class CallbacksTestCase(unittest.HomeserverTestCase):
     """Tests for transaction callbacks."""