From 1061aa84efe3f04ff8cafa811d9ec61ce40132c8 Mon Sep 17 00:00:00 2001 From: Zhijie Hou Date: Fri, 21 Nov 2025 13:16:00 +0800 Subject: [PATCH v8_HEAD] Fix the race condition between slot wal reservation and checkpoint Previously, a race condition existed where a newly created slot selected a WAL position for reservation but had not yet updated the restart_lsn. Meanwhile, a checkpoint could occur, choosing a later WAL position as the redo pointer. Due to the lack of interlock between WAL reservation and checkpoints, this situation could lead to the premature removal of WALs reserved by the new slot during a checkpoint, causing the newly created slot to become invalidated. This commit fixes it by taking exclusive lock on ReplicationSlotAllocationLock when reserving WAL to serialize the minimum restart_lsn computation in checkpoint process and WAL reservation, ensuring that: * If the WAL reservation occurs first, the checkpoint must wait for the restart_lsn to be updated before proceeding with WAL removal. This guarantees that the most recent restart_lsn position is detected. * If the checkpoint calls CheckPointReplicationSlots() first, then any subsequent WAL reservation must take a position later than the redo pointer. --- src/backend/replication/slot.c | 100 ++++++++++++++++++--------------- 1 file changed, 54 insertions(+), 46 deletions(-) diff --git a/src/backend/replication/slot.c b/src/backend/replication/slot.c index 86ae99a3ca9..682eccd116c 100644 --- a/src/backend/replication/slot.c +++ b/src/backend/replication/slot.c @@ -1572,63 +1572,65 @@ void ReplicationSlotReserveWal(void) { ReplicationSlot *slot = MyReplicationSlot; + XLogSegNo segno; + XLogRecPtr restart_lsn; Assert(slot != NULL); Assert(!XLogRecPtrIsValid(slot->data.restart_lsn)); Assert(!XLogRecPtrIsValid(slot->last_saved_restart_lsn)); /* - * The replication slot mechanism is used to prevent removal of required - * WAL. As there is no interlock between this routine and checkpoints, WAL - * segments could concurrently be removed when a now stale return value of - * ReplicationSlotsComputeRequiredLSN() is used. In the unlikely case that - * this happens we'll just retry. + * The replication slot mechanism is used to prevent the removal of + * required WAL. + * + * Acquire an exclusive lock to prevent the checkpoint process from + * concurrently computing the minimum slot LSN (see + * CheckPointReplicationSlots). This ensures that the WAL reserved for + * replication cannot be removed during a checkpoint. + * + * The mechanism is reliable because if WAL reservation occurs first, the + * checkpoint must wait for the restart_lsn update before determining the + * minimum non-removable LSN. On the other hand, if the checkpoint happens + * first, subsequent WAL reservations will select positions at or beyond + * the redo pointer of that checkpoint. */ - while (true) - { - XLogSegNo segno; - XLogRecPtr restart_lsn; + LWLockAcquire(ReplicationSlotAllocationLock, LW_EXCLUSIVE); - /* - * For logical slots log a standby snapshot and start logical decoding - * at exactly that position. That allows the slot to start up more - * quickly. But on a standby we cannot do WAL writes, so just use the - * replay pointer; effectively, an attempt to create a logical slot on - * standby will cause it to wait for an xl_running_xact record to be - * logged independently on the primary, so that a snapshot can be - * built using the record. - * - * None of this is needed (or indeed helpful) for physical slots as - * they'll start replay at the last logged checkpoint anyway. Instead - * return the location of the last redo LSN. While that slightly - * increases the chance that we have to retry, it's where a base - * backup has to start replay at. - */ - if (SlotIsPhysical(slot)) - restart_lsn = GetRedoRecPtr(); - else if (RecoveryInProgress()) - restart_lsn = GetXLogReplayRecPtr(NULL); - else - restart_lsn = GetXLogInsertRecPtr(); + /* + * For logical slots log a standby snapshot and start logical decoding at + * exactly that position. That allows the slot to start up more quickly. + * But on a standby we cannot do WAL writes, so just use the replay + * pointer; effectively, an attempt to create a logical slot on standby + * will cause it to wait for an xl_running_xact record to be logged + * independently on the primary, so that a snapshot can be built using the + * record. + * + * None of this is needed (or indeed helpful) for physical slots as + * they'll start replay at the last logged checkpoint anyway. Instead, + * return the location of the last redo LSN, where a base backup has to + * start replay at. + */ + if (SlotIsPhysical(slot)) + restart_lsn = GetRedoRecPtr(); + else if (RecoveryInProgress()) + restart_lsn = GetXLogReplayRecPtr(NULL); + else + restart_lsn = GetXLogInsertRecPtr(); - SpinLockAcquire(&slot->mutex); - slot->data.restart_lsn = restart_lsn; - SpinLockRelease(&slot->mutex); + SpinLockAcquire(&slot->mutex); + slot->data.restart_lsn = restart_lsn; + SpinLockRelease(&slot->mutex); - /* prevent WAL removal as fast as possible */ - ReplicationSlotsComputeRequiredLSN(); + /* prevent WAL removal as fast as possible */ + ReplicationSlotsComputeRequiredLSN(); - /* - * If all required WAL is still there, great, otherwise retry. The - * slot should prevent further removal of WAL, unless there's a - * concurrent ReplicationSlotsComputeRequiredLSN() after we've written - * the new restart_lsn above, so normally we should never need to loop - * more than twice. - */ - XLByteToSeg(slot->data.restart_lsn, segno, wal_segment_size); - if (XLogGetLastRemovedSegno() < segno) - break; - } + /* Checkpoint shouldn't remove the required WAL. */ + XLByteToSeg(slot->data.restart_lsn, segno, wal_segment_size); + if (XLogGetLastRemovedSegno() >= segno) + elog(ERROR, "WAL required by replication slot %s has been removed concurrently", + NameStr(slot->data.name)); + + LWLockRelease(ReplicationSlotAllocationLock); if (!RecoveryInProgress() && SlotIsLogical(slot)) { @@ -2136,6 +2138,12 @@ CheckPointReplicationSlots(bool is_shutdown) * acquiring a slot we cannot take the control lock - but that's OK, * because holding ReplicationSlotAllocationLock is strictly stronger, and * enough to guarantee that nobody can change the in_use bits on us. + * + * Additionally, acquiring the Allocation lock is necessary to serialize + * the slot flush process with concurrent slot WAL reservation. This + * ensures that the WAL position being reserved is either flushed to disk + * or is beyond or equal to the redo pointer of the current checkpoint + * (See ReplicationSlotReserveWal for details). */ LWLockAcquire(ReplicationSlotAllocationLock, LW_SHARED); -- 2.51.1.windows.1