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Fix assertion failure during decoding from synced slots.

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The slot synchronization skips updating the confirmed_flush LSN of the
local slot if the local slot has a newer catalog_xmin or restart_lsn, but
still allows updating the two_phase and two_phase_at fields of the slot.
This opens up a window for the prepared transactions between old
confirmed_flush LSN and two_phase_at to unexpectedly get decoded and sent
to the downstream after promotion. Then, while decoding the commit
prepared the assert will fail, which expects that the prepare hasn't been
sent to the downstream.

The fix is to skip updating the other slot fields when we are skipping to
update the confirmed_flush LSN of the slot.

We didn't backpatch this commit as two_phase_at was not synced in back
branches, which means prepared transactions won't be unexpectedly sent to
downstream.

We discovered this problem while analyzing BF failure reported in the
discussion link.

Reliably reproducing this issue without a debugger is difficult. Given
its rarity, adding specific injection point to test it doesn't seem
worthwhile, so we won't be adding a dedicated test case.

Author: Zhijie Hou <houzj.fnst@fujitsu.com>
Reviewed-by: shveta malik <shveta.malik@gmail.com>
Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Discussion: https://postgr.es/m/OS0PR01MB5716B44052000EB91EFAE60E94BC2@OS0PR01MB5716.jpnprd01.prod.outlook.com
This commit is contained in:
Amit Kapila 2025-04-29 12:52:05 +05:30
parent ef1811ac9a
commit 3ff2a1f0c9
1 changed files with 27 additions and 12 deletions
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39
src/backend/replication/logical/slotsync.c
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@ -196,14 +196,14 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
* restart_lsn or the initial xmin_horizon computed for the local slot * restart_lsn or the initial xmin_horizon computed for the local slot
* is ahead of the remote slot. * is ahead of the remote slot.
* *
* If the slot is persistent, restart_lsn of the synced slot could * If the slot is persistent, both restart_lsn and catalog_xmin of the
* still be ahead of the remote slot. Since we use slot advance * synced slot could still be ahead of the remote slot. Since we use
* functionality to keep snapbuild/slot updated, it is possible that * slot advance functionality to keep snapbuild/slot updated, it is
* the restart_lsn is advanced to a later position than it has on the * possible that the restart_lsn and catalog_xmin are advanced to a
* primary. This can happen when slot advancing machinery finds * later position than it has on the primary. This can happen when
* running xacts record after reaching the consistent state at a later * slot advancing machinery finds running xacts record after reaching
* point than the primary where it serializes the snapshot and updates * the consistent state at a later point than the primary where it
* the restart_lsn. * serializes the snapshot and updates the restart_lsn.
* *
* We LOG the message if the slot is temporary as it can help the user * We LOG the message if the slot is temporary as it can help the user
* to understand why the slot is not sync-ready. In the case of a * to understand why the slot is not sync-ready. In the case of a
@ -221,16 +221,25 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
if (remote_slot_precedes) if (remote_slot_precedes)
*remote_slot_precedes = true; *remote_slot_precedes = true;
/*
* Skip updating the configuration. This is required to avoid syncing
* two_phase_at without syncing confirmed_lsn. Otherwise, the prepared
* transaction between old confirmed_lsn and two_phase_at will
* unexpectedly get decoded and sent to the downstream after
* promotion. See comments in ReorderBufferFinishPrepared.
*/
return false;
} }
/* /*
* Attempt to sync LSNs and xmins only if remote slot is ahead of local * Attempt to sync LSNs and xmins only if remote slot is ahead of local
* slot. * slot.
*/ */
else if (remote_slot->confirmed_lsn > slot->data.confirmed_flush || if (remote_slot->confirmed_lsn > slot->data.confirmed_flush ||
remote_slot->restart_lsn > slot->data.restart_lsn || remote_slot->restart_lsn > slot->data.restart_lsn ||
TransactionIdFollows(remote_slot->catalog_xmin, TransactionIdFollows(remote_slot->catalog_xmin,
slot->data.catalog_xmin)) slot->data.catalog_xmin))
{ {
/* /*
* We can't directly copy the remote slot's LSN or xmin unless there * We can't directly copy the remote slot's LSN or xmin unless there
@ -294,6 +303,12 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
SpinLockRelease(&slot->mutex); SpinLockRelease(&slot->mutex);
updated_config = true; updated_config = true;
/*
* Ensure that there is no risk of sending prepared transactions
* unexpectedly after the promotion.
*/
Assert(slot->data.two_phase_at <= slot->data.confirmed_flush);
} }
/* /*