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 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

diff --git a/src/backend/catalog/storage.c b/src/backend/catalog/storage.c
index f56b3cc0f231b3072458c0153e408657456e819e..bb8c4d15612891f75b029dcc7cda0af0f1011c0c 100644 (file)
--- a/src/backend/catalog/storage.c
+++ b/src/backend/catalog/storage.c
@@ -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