Repository: matrix-org/synapse

Issue:
Re-apply making thread_id required in push rules tables
The migration `synapse/storage/schema/main/delta/74/03thread_notifications_not_null.sql.postgres` from #15350 took too long and caused a small outage on Matrix.org.

I've reverted it for now

We believe the statement doing this is one or both of the `ALTER TABLE` statements.

I hoped #15357 might fix it, but the Postgres docs don't mention it as a valid technique to improve performance and my testing shows the addition of the constraint takes the same amount of time without regard for the presence of the index, definitely not negligible.

The way forward is probably to add a `NOT VALID` constraint, which is enforced immediately for new rows, and then do a `VALIDATE CONSTRAINT` in the background. It's a shame we didn't know about this earlier since this could have been started ages ago, but ah well — as they say, the next best time is now.

> Scanning a large table to verify a new foreign key or check constraint can take a long time, and other updates to the table are locked out until the ALTER TABLE ADD CONSTRAINT command is committed. The main purpose of the NOT VALID constraint option is to reduce the impact of adding a constraint on concurrent updates. With NOT VALID, the ADD CONSTRAINT command does not scan the table and can be committed immediately. After that, a VALIDATE CONSTRAINT command can be issued to verify that existing rows satisfy the constraint. The validation step does not need to lock out concurrent updates, since it knows that other transactions will be enforcing the constraint for rows that they insert or update; only pre-existing rows need to be checked. Hence, validation acquires only a SHARE UPDATE EXCLUSIVE lock on the table being altered. (If the constraint is a foreign key then a ROW SHARE lock is also required on the table referenced by the constraint.) In addition to improving concurrency, it can be useful to use NOT VALID and VALIDATE CONSTRAINT in cases where the table is known to contain pre-existing violations. Once the constraint is in place, no new violations can be inserted, and the existing problems can be corrected at leisure until VALIDATE CONSTRAINT finally succeeds.

— [postgresql.org/docs/15/sql-altertable.html](https://www.postgresql.org/docs/15/sql-altertable.html)




Discussion:

Resolving patch:
diff --git a/synapse/storage/background_updates.py b/synapse/storage/background_updates.py
--- a/synapse/storage/background_updates.py
+++ b/synapse/storage/background_updates.py
@@ -561,6 +561,50 @@ async def updater(progress: JsonDict, batch_size: int) -> int:
             updater, oneshot=True
         )
 
+    def register_background_validate_constraint(
+        self, update_name: str, constraint_name: str, table: str
+    ) -> None:
+        """Helper for store classes to do a background validate constraint.
+
+        This only applies on PostgreSQL.
+
+        To use:
+
+        1. use a schema delta file to add a background update. Example:
+            INSERT INTO background_updates (update_name, progress_json) VALUES
+                ('validate_my_constraint', '{}');
+
+        2. In the Store constructor, call this method
+
+        Args:
+            update_name: update_name to register for
+            constraint_name: name of constraint to validate
+            table: table the constraint is applied to
+        """
+
+        def runner(conn: Connection) -> None:
+            c = conn.cursor()
+
+            sql = f"""
+            ALTER TABLE {table} VALIDATE CONSTRAINT {constraint_name};
+            """
+            logger.debug("[SQL] %s", sql)
+            c.execute(sql)
+
+        async def updater(progress: JsonDict, batch_size: int) -> int:
+            assert isinstance(
+                self.db_pool.engine, engines.PostgresEngine
+            ), "validate constraint background update registered for non-Postres database"
+
+            logger.info("Validating constraint %s to %s", constraint_name, table)
+            await self.db_pool.runWithConnection(runner)
+            await self._end_background_update(update_name)
+            return 1
+
+        self._background_update_handlers[update_name] = _BackgroundUpdateHandler(
+            updater, oneshot=True
+        )
+
     async def create_index_in_background(
         self,
         index_name: str,
diff --git a/synapse/storage/databases/main/event_push_actions.py b/synapse/storage/databases/main/event_push_actions.py
--- a/synapse/storage/databases/main/event_push_actions.py
+++ b/synapse/storage/databases/main/event_push_actions.py
@@ -289,179 +289,52 @@ def __init__(
             unique=True,
         )
 
-        self.db_pool.updates.register_background_update_handler(
-            "event_push_backfill_thread_id",
-            self._background_backfill_thread_id,
+        self.db_pool.updates.register_background_validate_constraint(
+            "event_push_actions_staging_thread_id",
+            constraint_name="event_push_actions_staging_thread_id",
+            table="event_push_actions_staging",
         )
-
-        # Indexes which will be used to quickly make the thread_id column non-null.
-        self.db_pool.updates.register_background_index_update(
-            "event_push_actions_thread_id_null",
-            index_name="event_push_actions_thread_id_null",
+        self.db_pool.updates.register_background_validate_constraint(
+            "event_push_actions_thread_id",
+            constraint_name="event_push_actions_thread_id",
             table="event_push_actions",
-            columns=["thread_id"],
-            where_clause="thread_id IS NULL",
         )
-        self.db_pool.updates.register_background_index_update(
-            "event_push_summary_thread_id_null",
-            index_name="event_push_summary_thread_id_null",
+        self.db_pool.updates.register_background_validate_constraint(
+            "event_push_summary_thread_id",
+            constraint_name="event_push_summary_thread_id",
             table="event_push_summary",
-            columns=["thread_id"],
-            where_clause="thread_id IS NULL",
         )
 
-        # Check ASAP (and then later, every 1s) to see if we have finished
-        # background updates the event_push_actions and event_push_summary tables.
-        self._clock.call_later(0.0, self._check_event_push_backfill_thread_id)
-        self._event_push_backfill_thread_id_done = False
-
-    @wrap_as_background_process("check_event_push_backfill_thread_id")
-    async def _check_event_push_backfill_thread_id(self) -> None:
-        """
-        Has thread_id finished backfilling?
-
-        If not, we need to just-in-time update it so the queries work.
-        """
-        done = await self.db_pool.updates.has_completed_background_update(
-            "event_push_backfill_thread_id"
+        self.db_pool.updates.register_background_update_handler(
+            "event_push_drop_null_thread_id_indexes",
+            self._background_drop_null_thread_id_indexes,
         )
 
-        if done:
-            self._event_push_backfill_thread_id_done = True
-        else:
-            # Reschedule to run.
-            self._clock.call_later(15.0, self._check_event_push_backfill_thread_id)
-
-    async def _background_backfill_thread_id(
+    async def _background_drop_null_thread_id_indexes(
         self, progress: JsonDict, batch_size: int
     ) -> int:
         """
-        Fill in the thread_id field for event_push_actions and event_push_summary.
-
-        This is preparatory so that it can be made non-nullable in the future.
-
-        Because all current (null) data is done in an unthreaded manner this
-        simply assumes it is on the "main" timeline. Since event_push_actions
-        are periodically cleared it is not possible to correctly re-calculate
-        the thread_id.
+        Drop the indexes used to find null thread_ids for event_push_actions and
+        event_push_summary.
         """
-        event_push_actions_done = progress.get("event_push_actions_done", False)
 
-        def add_thread_id_txn(
-            txn: LoggingTransaction, start_stream_ordering: int
-        ) -> int:
-            sql = """
-            SELECT stream_ordering
-            FROM event_push_actions
-            WHERE
-                thread_id IS NULL
-                AND stream_ordering > ?
-            ORDER BY stream_ordering
-            LIMIT ?
-            """
-            txn.execute(sql, (start_stream_ordering, batch_size))
-
-            # No more rows to process.
-            rows = txn.fetchall()
-            if not rows:
-                progress["event_push_actions_done"] = True
-                self.db_pool.updates._background_update_progress_txn(
-                    txn, "event_push_backfill_thread_id", progress
-                )
-                return 0
+        def drop_null_thread_id_indexes_txn(txn: LoggingTransaction) -> None:
+            sql = "DROP INDEX IF EXISTS event_push_actions_thread_id_null"
+            logger.debug("[SQL] %s", sql)
+            txn.execute(sql)
 
-            # Update the thread ID for any of those rows.
-            max_stream_ordering = rows[-1][0]
+            sql = "DROP INDEX IF EXISTS event_push_summary_thread_id_null"
+            logger.debug("[SQL] %s", sql)
+            txn.execute(sql)
 
-            sql = """
-            UPDATE event_push_actions
-            SET thread_id = 'main'
-            WHERE ? < stream_ordering AND stream_ordering <= ? AND thread_id IS NULL
-            """
-            txn.execute(
-                sql,
-                (
-                    start_stream_ordering,
-                    max_stream_ordering,
-                ),
-            )
-
-            # Update progress.
-            processed_rows = txn.rowcount
-            progress["max_event_push_actions_stream_ordering"] = max_stream_ordering
-            self.db_pool.updates._background_update_progress_txn(
-                txn, "event_push_backfill_thread_id", progress
-            )
-
-            return processed_rows
-
-        def add_thread_id_summary_txn(txn: LoggingTransaction) -> int:
-            min_user_id = progress.get("max_summary_user_id", "")
-            min_room_id = progress.get("max_summary_room_id", "")
-
-            # Slightly overcomplicated query for getting the Nth user ID / room
-            # ID tuple, or the last if there are less than N remaining.
-            sql = """
-            SELECT user_id, room_id FROM (
-                SELECT user_id, room_id FROM event_push_summary
-                WHERE (user_id, room_id) > (?, ?)
-                    AND thread_id IS NULL
-                ORDER BY user_id, room_id
-                LIMIT ?
-            ) AS e
-            ORDER BY user_id DESC, room_id DESC
-            LIMIT 1
-            """
-
-            txn.execute(sql, (min_user_id, min_room_id, batch_size))
-            row = txn.fetchone()
-            if not row:
-                return 0
-
-            max_user_id, max_room_id = row
-
-            sql = """
-            UPDATE event_push_summary
-            SET thread_id = 'main'
-            WHERE
-                (?, ?) < (user_id, room_id) AND (user_id, room_id) <= (?, ?)
-                AND thread_id IS NULL
-            """
-            txn.execute(sql, (min_user_id, min_room_id, max_user_id, max_room_id))
-            processed_rows = txn.rowcount
-
-            progress["max_summary_user_id"] = max_user_id
-            progress["max_summary_room_id"] = max_room_id
-            self.db_pool.updates._background_update_progress_txn(
-                txn, "event_push_backfill_thread_id", progress
-            )
-
-            return processed_rows
-
-        # First update the event_push_actions table, then the event_push_summary table.
-        #
-        # Note that the event_push_actions_staging table is ignored since it is
-        # assumed that items in that table will only exist for a short period of
-        # time.
-        if not event_push_actions_done:
-            result = await self.db_pool.runInteraction(
-                "event_push_backfill_thread_id",
-                add_thread_id_txn,
-                progress.get("max_event_push_actions_stream_ordering", 0),
-            )
-        else:
-            result = await self.db_pool.runInteraction(
-                "event_push_backfill_thread_id",
-                add_thread_id_summary_txn,
-            )
-
-            # Only done after the event_push_summary table is done.
-            if not result:
-                await self.db_pool.updates._end_background_update(
-                    "event_push_backfill_thread_id"
-                )
-
-        return result
+        await self.db_pool.runInteraction(
+            "drop_null_thread_id_indexes_txn",
+            drop_null_thread_id_indexes_txn,
+        )
+        await self.db_pool.updates._end_background_update(
+            "event_push_drop_null_thread_id_indexes"
+        )
+        return 0
 
     async def get_unread_counts_by_room_for_user(self, user_id: str) -> Dict[str, int]:
         """Get the notification count by room for a user. Only considers notifications,
@@ -711,25 +584,6 @@ def _get_thread(thread_id: str) -> NotifCounts:
             (ReceiptTypes.READ, ReceiptTypes.READ_PRIVATE),
         )
 
-        # First ensure that the existing rows have an updated thread_id field.
-        if not self._event_push_backfill_thread_id_done:
-            txn.execute(
-                """
-                UPDATE event_push_summary
-                SET thread_id = ?
-                WHERE room_id = ? AND user_id = ? AND thread_id is NULL
-                """,
-                (MAIN_TIMELINE, room_id, user_id),
-            )
-            txn.execute(
-                """
-                UPDATE event_push_actions
-                SET thread_id = ?
-                WHERE room_id = ? AND user_id = ? AND thread_id is NULL
-                """,
-                (MAIN_TIMELINE, room_id, user_id),
-            )
-
         # First we pull the counts from the summary table.
         #
         # We check that `last_receipt_stream_ordering` matches the stream ordering of the
@@ -1545,25 +1399,6 @@ def _handle_new_receipts_for_notifs_txn(self, txn: LoggingTransaction) -> bool:
                 (room_id, user_id, stream_ordering, *thread_args),
             )
 
-            # First ensure that the existing rows have an updated thread_id field.
-            if not self._event_push_backfill_thread_id_done:
-                txn.execute(
-                    """
-                    UPDATE event_push_summary
-                    SET thread_id = ?
-                    WHERE room_id = ? AND user_id = ? AND thread_id is NULL
-                    """,
-                    (MAIN_TIMELINE, room_id, user_id),
-                )
-                txn.execute(
-                    """
-                    UPDATE event_push_actions
-                    SET thread_id = ?
-                    WHERE room_id = ? AND user_id = ? AND thread_id is NULL
-                    """,
-                    (MAIN_TIMELINE, room_id, user_id),
-                )
-
             # Fetch the notification counts between the stream ordering of the
             # latest receipt and what was previously summarised.
             unread_counts = self._get_notif_unread_count_for_user_room(
@@ -1698,19 +1533,6 @@ def _rotate_notifs_before_txn(
             rotate_to_stream_ordering: The new maximum event stream ordering to summarise.
         """
 
-        # Ensure that any new actions have an updated thread_id.
-        if not self._event_push_backfill_thread_id_done:
-            txn.execute(
-                """
-                UPDATE event_push_actions
-                SET thread_id = ?
-                WHERE ? < stream_ordering AND stream_ordering <= ? AND thread_id IS NULL
-                """,
-                (MAIN_TIMELINE, old_rotate_stream_ordering, rotate_to_stream_ordering),
-            )
-
-        # XXX Do we need to update summaries here too?
-
         # Calculate the new counts that should be upserted into event_push_summary
         sql = """
             SELECT user_id, room_id, thread_id,
@@ -1773,20 +1595,6 @@ def _rotate_notifs_before_txn(
 
         logger.info("Rotating notifications, handling %d rows", len(summaries))
 
-        # Ensure that any updated threads have the proper thread_id.
-        if not self._event_push_backfill_thread_id_done:
-            txn.execute_batch(
-                """
-                UPDATE event_push_summary
-                SET thread_id = ?
-                WHERE room_id = ? AND user_id = ? AND thread_id is NULL
-                """,
-                [
-                    (MAIN_TIMELINE, room_id, user_id)
-                    for user_id, room_id, _ in summaries
-                ],
-            )
-
         self.db_pool.simple_upsert_many_txn(
             txn,
             table="event_push_summary",
diff --git a/synapse/storage/schema/__init__.py b/synapse/storage/schema/__init__.py
--- a/synapse/storage/schema/__init__.py
+++ b/synapse/storage/schema/__init__.py
@@ -110,6 +110,9 @@
     # Queries against `event_stream_ordering` columns in membership tables must
     # be disambiguated.
     #
+    # The threads_id column must written to with non-null values for the
+    # event_push_actions, event_push_actions_staging, and event_push_summary tables.
+    #
     # insertions to the column `full_user_id` of tables profiles and user_filters can no
     # longer be null
     76


Test patch: