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Unblock the stream_ordering bigint migration on PostgreSQL.

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A Postgres database older than schema version 60 could not be upgraded,
because the `stream_ordering` -> bigint migration deadlocked with the foreign
keys that delta 74/03 added on `events.stream_ordering`:

  * delta 79/04 tried to repoint those foreign keys at `stream_ordering2`, but
    a foreign key needs a unique index on the referenced column and that index
    is built by the `index_stream_ordering2` background update — which cannot
    have run, since every delta is applied before any background update. It
    failed with `InvalidForeignKey: there is no unique constraint matching
    given keys for referenced table "events"`.

  * without delta 79/04, the `replace_stream_ordering_column` background update
    failed instead, because Postgres will not drop a column that a foreign key
    depends on.

Move the responsibility into `replace_stream_ordering_column`, which now drops
the foreign keys before the column swap and recreates them from their saved
definitions afterwards, by which point `stream_ordering2` has been renamed into
place. The foreign keys are looked up rather than hardcoded: delta 79/04 only
knew about the three on the membership tables, and the sliding sync tables have
since added two more.

Delta 79/04 becomes a no-op, as it can no longer have any work to do.

Signed-off-by: Olivier 'reivilibre <oliverw@matrix.org>
This commit is contained in:
Olivier 'reivilibre
2026-08-13 18:09:54 +01:00
committed by Olivier 'reivilibre
parent 5bc1195e6f
commit 471ab8b9bc
3 changed files with 64 additions and 51 deletions
Download Patch File Download Diff File Expand all files Collapse all files
changelog.d/20101.bugfix
+1
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@@ -0,0 +1 @@
Fix the `stream_ordering` bigint migration being unable to complete on PostgreSQL databases created before schema version 60.
synapse/storage/databases/main/events_bg_updates.py
+43
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@@ -90,6 +90,28 @@ _REPLACE_STREAM_ORDERING_SQL_COMMANDS = (
"ALTER INDEX events_ts2 RENAME TO events_ts",
)
# Postgres refuses to drop a column that a foreign key depends on, so any foreign key
# referencing `events.stream_ordering` has to be dropped before the swap above and
# recreated afterwards (by which point `stream_ordering2` has been renamed into place,
# so the saved definitions still name the right column).
#
# We look these up rather than hardcoding them, because the set has grown over time:
# delta 74/03 added three on the membership tables, and the sliding sync tables later
# added two more.
#
# Foreign keys which already reference `stream_ordering2` — because delta
# 79/04 repointed them — are deliberately not matched here: the rename carries
# those over on its own.
_SELECT_STREAM_ORDERING_FOREIGN_KEYS_SQL = """
SELECT c.conrelid::regclass::text, quote_ident(c.conname), pg_get_constraintdef(c.oid)
FROM pg_constraint c
JOIN pg_attribute a
ON a.attrelid = c.confrelid AND a.attnum = ANY (c.confkey)
WHERE c.contype = 'f'
AND c.confrelid = 'events'::regclass
AND a.attname = 'stream_ordering'
"""
@attr.s(slots=True, frozen=True, auto_attribs=True)
class _CalculateChainCover:
@@ -1476,10 +1498,31 @@ class EventsBackgroundUpdatesStore(
"""Drop the old 'stream_ordering' column and rename 'stream_ordering2' into its place."""
def process(txn: Cursor) -> None:
txn.execute(_SELECT_STREAM_ORDERING_FOREIGN_KEYS_SQL)
foreign_keys = txn.fetchall()
for table, constraint, _definition in foreign_keys:
logger.info(
"dropping %s on %s so that stream_ordering can be replaced",
constraint,
table,
)
txn.execute(f"ALTER TABLE {table} DROP CONSTRAINT {constraint}")
for sql in _REPLACE_STREAM_ORDERING_SQL_COMMANDS:
logger.info("completing stream_ordering migration: %s", sql)
txn.execute(sql)
for table, constraint, definition in foreign_keys:
logger.info("restoring %s on %s", constraint, table)
# Constraints which were not `NOT VALID` are validated as they are
# added, which scans the table. That is acceptable here: a database
# still running this migration predates every table that holds such a
# constraint, so those tables are empty at this point.
txn.execute(
f"ALTER TABLE {table} ADD CONSTRAINT {constraint} {definition}"
)
# ANALYZE the new column to build stats on it, to encourage PostgreSQL to use the
# indexes on it.
await self.db_pool.runInteraction(
synapse/storage/schema/main/delta/79/04_mitigate_stream_ordering_update_race.py
+20 -51
View File
@@ -8,7 +8,7 @@
from synapse.storage.database import LoggingTransaction
from synapse.storage.engines import BaseDatabaseEngine, PostgresEngine
from synapse.storage.engines import BaseDatabaseEngine
def run_create(
@@ -16,55 +16,24 @@ def run_create(
database_engine: BaseDatabaseEngine,
) -> None:
"""
An attempt to mitigate a painful race between foreground and background updates
touching the `stream_ordering` column of the events table. More info can be found
This delta used to repoint the `event_stream_ordering_fkey` foreign keys added by
delta 74/03 at `events.stream_ordering2`, so that they would survive the column
swap performed by the `replace_stream_ordering_column` background update. It was an
attempt to mitigate a painful race between foreground and background updates
touching the `stream_ordering` column of the events table; more info can be found
at https://github.com/matrix-org/synapse/issues/15677.
It could never do so successfully, because the unique index that a foreign key on
`stream_ordering2` requires is itself built by a background update
(`index_stream_ordering2`), and every delta runs before any background update does.
Whenever this delta had work to do it therefore failed with
psycopg2.errors.InvalidForeignKey: there is no unique constraint matching
given keys for referenced table "events"
which left any Postgres database older than schema version 60 unable to upgrade.
`replace_stream_ordering_column` now drops and recreates these foreign keys itself,
which works no matter which column they currently reference, so there is nothing
left for this delta to do.
"""
# technically the bg update we're concerned with below should only have been added in
# postgres but it doesn't hurt to be extra careful
if isinstance(database_engine, PostgresEngine):
select_sql = """
SELECT 1 FROM background_updates
WHERE update_name = 'replace_stream_ordering_column'
"""
cur.execute(select_sql)
res = cur.fetchone()
# if the background update `replace_stream_ordering_column` is still pending, we need
# to drop the indexes added in 7403, and re-add them to the column `stream_ordering2`
# with the idea that they will be preserved when the column is renamed `stream_ordering`
# after the background update has finished
if res:
drop_cse_sql = """
ALTER TABLE current_state_events DROP CONSTRAINT IF EXISTS event_stream_ordering_fkey
"""
cur.execute(drop_cse_sql)
drop_lcm_sql = """
ALTER TABLE local_current_membership DROP CONSTRAINT IF EXISTS event_stream_ordering_fkey
"""
cur.execute(drop_lcm_sql)
drop_rm_sql = """
ALTER TABLE room_memberships DROP CONSTRAINT IF EXISTS event_stream_ordering_fkey
"""
cur.execute(drop_rm_sql)
add_cse_sql = """
ALTER TABLE current_state_events ADD CONSTRAINT event_stream_ordering_fkey
FOREIGN KEY (event_stream_ordering) REFERENCES events(stream_ordering2) NOT VALID;
"""
cur.execute(add_cse_sql)
add_lcm_sql = """
ALTER TABLE local_current_membership ADD CONSTRAINT event_stream_ordering_fkey
FOREIGN KEY (event_stream_ordering) REFERENCES events(stream_ordering2) NOT VALID;
"""
cur.execute(add_lcm_sql)
add_rm_sql = """
ALTER TABLE room_memberships ADD CONSTRAINT event_stream_ordering_fkey
FOREIGN KEY (event_stream_ordering) REFERENCES events(stream_ordering2) NOT VALID;
"""
cur.execute(add_rm_sql)