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RelationTruncate() must set DELAY_CHKPT_START.

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Previously, it set only DELAY_CHKPT_COMPLETE. That was important,
because it meant that if the XLOG_SMGR_TRUNCATE record preceded a
XLOG_CHECKPOINT_ONLINE record in the WAL, then the truncation would also
happen on disk before the XLOG_CHECKPOINT_ONLINE record was
written.

However, it didn't guarantee that the sync request for the truncation
was processed before the XLOG_CHECKPOINT_ONLINE record was written. By
setting DELAY_CHKPT_START, we guarantee that if an XLOG_SMGR_TRUNCATE
record is written to WAL before the redo pointer of a concurrent
checkpoint, the sync request queued by that operation must be processed
by that checkpoint, rather than being left for the following one.

This is a refinement of commit 412ad7a5563.  Back-patch to all supported
releases, like that commit.

Author: Robert Haas <robertmhaas@gmail.com>
Reported-by: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://postgr.es/m/CA%2BhUKG%2B-2rjGZC2kwqr2NMLBcEBp4uf59QT1advbWYF_uc%2B0Aw%40mail.gmail.com
This commit is contained in:
Thomas Munro 2024-12-03 09:27:05 +13:00
parent a963abd549
commit d4ffbf47b2
1 changed files with 20 additions and 7 deletions
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27
src/backend/catalog/storage.c
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@ -337,20 +337,33 @@ RelationTruncate(Relation rel, BlockNumber nblocks)
RelationPreTruncate(rel);
/*
* Make sure that a concurrent checkpoint can't complete while truncation
* is in progress.
* The code which follows can interact with concurrent checkpoints in two
* separate ways.
*
* The truncation operation might drop buffers that the checkpoint
* First, the truncation operation might drop buffers that the checkpoint
* otherwise would have flushed. If it does, then it's essential that the
* files actually get truncated on disk before the checkpoint record is
* written. Otherwise, if reply begins from that checkpoint, the
* to-be-truncated blocks might still exist on disk but have older
* contents than expected, which can cause replay to fail. It's OK for the
* blocks to not exist on disk at all, but not for them to have the wrong
* contents.
* contents. For this reason, we need to set DELAY_CHKPT_COMPLETE while
* this code executes.
*
* Second, the call to smgrtruncate() below will in turn call
* RegisterSyncRequest(). We need the sync request created by that call to
* be processed before the checkpoint completes. CheckPointGuts() will
* call ProcessSyncRequests(), but if we register our sync request after
* that happens, then the WAL record for the truncation could end up
* preceding the checkpoint record, while the actual sync doesn't happen
* until the next checkpoint. To prevent that, we need to set
* DELAY_CHKPT_START here. That way, if the XLOG_SMGR_TRUNCATE precedes
* the redo pointer of a concurrent checkpoint, we're guaranteed that the
* corresponding sync request will be processed before the checkpoint
* completes.
*/
Assert((MyProc->delayChkptFlags & DELAY_CHKPT_COMPLETE) == 0);
MyProc->delayChkptFlags |= DELAY_CHKPT_COMPLETE;
Assert((MyProc->delayChkptFlags & (DELAY_CHKPT_START | DELAY_CHKPT_COMPLETE)) == 0);
MyProc->delayChkptFlags |= DELAY_CHKPT_START | DELAY_CHKPT_COMPLETE;
/*
* We WAL-log the truncation before actually truncating, which means
@ -398,7 +411,7 @@ RelationTruncate(Relation rel, BlockNumber nblocks)
smgrtruncate(RelationGetSmgr(rel), forks, nforks, blocks);
/* We've done all the critical work, so checkpoints are OK now. */
MyProc->delayChkptFlags &= ~DELAY_CHKPT_COMPLETE;
MyProc->delayChkptFlags &= ~(DELAY_CHKPT_START | DELAY_CHKPT_COMPLETE);
/*
* Update upper-level FSM pages to account for the truncation. This is