Hello Nick On 2026-Aug-21, Nick Ivanov wrote:
> We are encountering a possible race condition when executing several > `pg_basebackup --wal-method=stream --slot=... --create-slot` concurrently > while initialising streaming replicas. Our automation starts 3 > pg_basebackup processes on 3 future replica servers within 1 second of each > other. One of them almost always fails with "requested WAL segment ... has > already been removed". I think this is related to this thread here: https://www.postgresql.org/message-id/flat/5e045179-236f-4f8f-84f1-0f2566ba784c.mengjuan.cmj%40alibaba-inc.com and to this commit Author: Amit Kapila <[email protected]> Branch: master Release: REL_19_BR [006dd4b2e] 2025-12-08 05:21:22 +0000 Branch: REL_18_STABLE Release: REL_18_2 [d3ceb2084] 2025-12-08 05:33:14 +0000 Prevent invalidation of newly created replication slots. A race condition could cause a newly created replication slot to become invalidated between WAL reservation and a checkpoint. Previously, if the required WAL was removed, we retried the reservation process. However, the slot could still be invalidated before the retry if the WAL was not yet removed but the checkpoint advanced the redo pointer beyond the slot's intended restart LSN and computed the minimum LSN that needs to be preserved for the slots. The fix is to acquire an exclusive lock on ReplicationSlotAllocationLock during WAL reservation to serialize WAL reservation and checkpoint's minimum restart_lsn computation. This ensures that, if WAL reservation occurs first, the checkpoint waits until restart_lsn is updated before removing WAL. If the checkpoint runs first, subsequent WAL reservations pick a position at or after the latest checkpoint's redo pointer. We can't use the same fix for branch 17 and prior because commit 2090edc6f3 changed to compute to the minimum restart_LSN among slot's at the beginning of checkpoint (or restart point). The fix for 17 and prior branches is under discussion and will be committed separately. Reported-by: suyu.cmj <[email protected]> Author: Hou Zhijie <[email protected]> Reviewed-by: Vitaly Davydov <[email protected]> Reviewed-by: Masahiko Sawada <[email protected]> Reviewed-by: Amit Kapila <[email protected]> Backpatch-through: 18 Discussion: https://postgr.es/m/5e045179-236f-4f8f-84f1-0f2566ba784c.mengjuan....@alibaba-inc.com and to this other commit Author: Amit Kapila <[email protected]> Branch: REL_17_STABLE Release: REL_17_8 [3510ebeb0] 2026-01-08 07:17:56 +0000 Branch: REL_16_STABLE Release: REL_16_12 [24cce33c3] 2026-01-08 07:07:23 +0000 Branch: REL_15_STABLE Release: REL_15_16 [aae05622a] 2026-01-08 06:54:52 +0000 Branch: REL_14_STABLE Release: REL_14_21 [7406df605] 2026-01-08 06:44:28 +0000 Prevent invalidation of newly created replication slots. A race condition could cause a newly created replication slot to become invalidated between WAL reservation and a checkpoint. Previously, if the required WAL was removed, we retried the reservation process. However, the slot could still be invalidated before the retry if the WAL was not yet removed but the checkpoint advanced the redo pointer beyond the slot's intended restart LSN and computed the minimum LSN that needs to be preserved for the slots. The fix is to acquire an exclusive lock on ReplicationSlotAllocationLock during WAL reservation, and a shared lock during the minimum LSN calculation at checkpoints to serialize the process. This ensures that, if WAL reservation occurs first, the checkpoint waits until restart_lsn is updated before calculating the minimum LSN. If the checkpoint runs first, subsequent WAL reservations pick a position at or after the latest checkpoint's redo pointer. We used a similar fix in HEAD (via commit 006dd4b2e5) and 18. The difference is that in 17 and prior branches we need to additionally handle the race condition with slot's minimum LSN computation during checkpoints. Reported-by: suyu.cmj <[email protected]> Author: Hou Zhijie <[email protected]> Author: vignesh C <[email protected]> Reviewed-by: Hayato Kuroda <[email protected]> Reviewed-by: Masahiko Sawada <[email protected]> Reviewed-by: Amit Kapila <[email protected]> Backpatch-through: 14 Discussion: https://postgr.es/m/5e045179-236f-4f8f-84f1-0f2566ba784c.mengjuan....@alibaba-inc.com What version are you using? If you're using a version that contains these fixes, then we may have some slightly different bug ... -- Álvaro Herrera Breisgau, Deutschland — https://www.EnterpriseDB.com/ "No hay ausente sin culpa ni presente sin disculpa" (Prov. francés)
