Make sqlite database migrations transactional again (#14910)

#13873 introduced a regression which causes sqlite database migrations
to no longer run inside a transaction. Wrap them in a transaction again,
to avoid database corruption when migrations are interrupted.

Fixes #14909.

Signed-off-by: Sean Quah <seanq@matrix.org>

1 files changed, 3 insertions, 0 deletions

diff --git a/synapse/storage/engines/_base.py b/synapse/storage/engines/_base.py

index 70e594a68f..bc9ca3a53c 100644
--- a/synapse/storage/engines/_base.py
+++ b/synapse/storage/engines/_base.py
@@ -132,6 +132,9 @@ class BaseDatabaseEngine(Generic[ConnectionType, CursorType], metaclass=abc.ABCM
         """Execute a chunk of SQL containing multiple semicolon-delimited statements.
 
         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.
         """
         ...