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 8f74a3efdeb58c4db71570e8a830bc70a67ac6cd..b8baa3b822df045e945d5b2b03050df3c77e7973 100644 (file)
--- a/src/backend/catalog/storage.c
+++ b/src/backend/catalog/storage.c
@@ -326,20 +326,35 @@ 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
-    * 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
+    * 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 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. 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->delayChkpt);
+   MyProc->delayChkpt = true;          /* DELAY_CHKPT_START */
    Assert(!MyProc->delayChkptEnd);
-   MyProc->delayChkptEnd = true;
+   MyProc->delayChkptEnd = true;       /* DELAY_CHKPT_COMPLETE */
 
    /*
     * We WAL-log the truncation before actually truncating, which means
@@ -387,6 +402,7 @@ RelationTruncate(Relation rel, BlockNumber nblocks)
    smgrtruncate(RelationGetSmgr(rel), forks, nforks, blocks);
 
    /* We've done all the critical work, so checkpoints are OK now. */
+   MyProc->delayChkpt = false;
    MyProc->delayChkptEnd = false;
 
    /*