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Reject concurrent sequence refreshes.

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'ALTER SUBSCRIPTION ... REFRESH SEQUENCES' can race with an already
running sequence synchronization worker. If a second refresh request
resets the synchronization state while the worker has already fetched
sequence values from the publisher but has not yet applied them to the
subscriber, the worker can overwrite the subscriber with stale values
and mark the synchronization as complete.

Avoid this race by rejecting 'ALTER SUBSCRIPTION ... REFRESH SEQUENCES'
when a sequence synchronization worker is already running for the
subscription. The command reports an error asking the user to rerun it
after the current synchronization completes.

Also add a wait for the re-added 'regress_s4' sequence to finish
synchronizing in 036_sequences.pl, so the subsequent test does not race
against its sequencesync worker.

Reported-by: Noah Misch <noah@leadboat.com>
Author: vignesh C <vignesh21@gmail.com>
Reviewed-by: Shveta Malik <shveta.malik@gmail.com>
Reviewed-by: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Backpatch-through: 19, where it was introduced
Discussion: https://postgr.es/m/20260710045217.f0.noahmisch@microsoft.com
This commit is contained in:
Amit Kapila
2026-07-15 15:40:36 +05:30
parent 01f78a84bf
commit f38afa4abb
2 changed files with 31 additions and 0 deletions
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27
src/backend/commands/subscriptioncmds.c
View File

@@ -1363,6 +1363,33 @@ AlterSubscription_refresh_seq(Subscription *sub)
WalReceiverConn *wrconn;
bool must_use_password;
/*
* Disallow a concurrent REFRESH SEQUENCES while a sequence sync worker
* for this subscription is still running. This avoids a race where the
* publisher's sequence advances after the current worker has fetched its
* value but before it marks the sequence READY. A user may then issue
* another REFRESH SEQUENCES to synchronize the updated value. Since the
* affected sequences are already in the INIT state, the running worker
* has no indication that a new synchronization has been requested. It
* would then apply the stale value it already fetched and mark the
* sequence READY, causing the new synchronization request to be lost and
* preventing the updated publisher values from being synchronized.
*/
LWLockAcquire(LogicalRepWorkerLock, LW_SHARED);
if (logicalrep_worker_find(WORKERTYPE_SEQUENCESYNC, sub->oid, InvalidOid,
true))
{
LWLockRelease(LogicalRepWorkerLock);
ereport(ERROR,
errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
/* translator: %s is an SQL ALTER command */
errmsg("cannot execute %s while a sequence synchronization worker is running",
"ALTER SUBSCRIPTION ... REFRESH SEQUENCES"),
errhint("Try again after the current synchronization completes."));
}
LWLockRelease(LogicalRepWorkerLock);
/* Load the library providing us libpq calls. */
load_file("libpqwalreceiver", false);

4
src/test/subscription/t/036_sequences.pl
View File

@@ -232,6 +232,10 @@ $node_publisher->safe_psql(
CREATE SEQUENCE regress_s4 START 10 INCREMENT 2;
));
# Wait for the missing sequence added to be synced
$node_subscriber->poll_query_until('postgres', $synced_query)
or die "Timed out while waiting for subscriber to synchronize data";
##########
# Ensure that insufficient privileges on the publisher for a sequence
# are reported correctly as a permission issue, not as a missing sequence.