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Commitd4ffbf4

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RelationTruncate() must set DELAY_CHKPT_START.
Previously, it set only DELAY_CHKPT_COMPLETE. That was important,because it meant that if the XLOG_SMGR_TRUNCATE record preceded aXLOG_CHECKPOINT_ONLINE record in the WAL, then the truncation would alsohappen on disk before the XLOG_CHECKPOINT_ONLINE record waswritten.However, it didn't guarantee that the sync request for the truncationwas processed before the XLOG_CHECKPOINT_ONLINE record was written. Bysetting DELAY_CHKPT_START, we guarantee that if an XLOG_SMGR_TRUNCATErecord is written to WAL before the redo pointer of a concurrentcheckpoint, the sync request queued by that operation must be processedby that checkpoint, rather than being left for the following one.This is a refinement of commit412ad7a. Back-patch to all supportedreleases, 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
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‎src/backend/catalog/storage.c

Lines changed: 20 additions & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -337,20 +337,33 @@ RelationTruncate(Relation rel, BlockNumber nblocks)
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RelationPreTruncate(rel);
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/*
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*Make sure that a concurrent checkpoint can't complete while truncation
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*is in progress.
340+
*The code which follows can interact with concurrent checkpoints in two
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*separate ways.
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*
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*The truncation operation might drop buffers that the checkpoint
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*First, the truncation operation might drop buffers that the checkpoint
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* otherwise would have flushed. If it does, then it's essential that the
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* files actually get truncated on disk before the checkpoint record is
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* written. Otherwise, if reply begins from that checkpoint, the
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* to-be-truncated blocks might still exist on disk but have older
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* contents than expected, which can cause replay to fail. It's OK for the
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* blocks to not exist on disk at all, but not for them to have the wrong
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* contents.
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* contents. For this reason, we need to set DELAY_CHKPT_COMPLETE while
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* this code executes.
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*
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* Second, the call to smgrtruncate() below will in turn call
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* RegisterSyncRequest(). We need the sync request created by that call to
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* be processed before the checkpoint completes. CheckPointGuts() will
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* call ProcessSyncRequests(), but if we register our sync request after
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* that happens, then the WAL record for the truncation could end up
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* preceding the checkpoint record, while the actual sync doesn't happen
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* until the next checkpoint. To prevent that, we need to set
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* DELAY_CHKPT_START here. That way, if the XLOG_SMGR_TRUNCATE precedes
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* the redo pointer of a concurrent checkpoint, we're guaranteed that the
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* corresponding sync request will be processed before the checkpoint
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* completes.
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*/
352-
Assert((MyProc->delayChkptFlags&DELAY_CHKPT_COMPLETE)==0);
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MyProc->delayChkptFlags |=DELAY_CHKPT_COMPLETE;
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Assert((MyProc->delayChkptFlags&(DELAY_CHKPT_START |DELAY_CHKPT_COMPLETE))==0);
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MyProc->delayChkptFlags |=DELAY_CHKPT_START |DELAY_CHKPT_COMPLETE;
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/*
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* We WAL-log the truncation before actually truncating, which means
@@ -398,7 +411,7 @@ RelationTruncate(Relation rel, BlockNumber nblocks)
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smgrtruncate(RelationGetSmgr(rel),forks,nforks,blocks);
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/* We've done all the critical work, so checkpoints are OK now. */
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MyProc->delayChkptFlags &= ~DELAY_CHKPT_COMPLETE;
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MyProc->delayChkptFlags &= ~(DELAY_CHKPT_START |DELAY_CHKPT_COMPLETE);
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/*
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* Update upper-level FSM pages to account for the truncation. This is

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