Use GetXLogInsertEndRecPtr in gistGetFakeLSN

The function used GetXLogInsertRecPtr() to generate the fake LSN. Most
of the time this is the same as what XLogInsert() would return, and so
it works fine with the XLogFlush() call. But if the last record ends at
a page boundary, GetXLogInsertRecPtr() returns LSN pointing after the
page header. In such case XLogFlush() fails with errors like this:

  ERROR: xlog flush request 0/01BD2018 is not satisfied --- flushed only to 0/01BD2000

Such failures are very hard to trigger, particularly outside aggressive
test scenarios.

Fixed by introducing GetXLogInsertEndRecPtr(), returning the correct LSN
without skipping the header. This is the same as GetXLogInsertRecPtr(),
except that it calls XLogBytePosToEndRecPtr().

Initial investigation by me, root cause identified by Andres Freund.

This is a long-standing bug in gistGetFakeLSN(), probably introduced by
c6b92041d38 in PG13. Backpatch to all supported versions.

Reported-by: Peter Geoghegan <pg@bowt.ie>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Noah Misch <noah@leadboat.com>
Discussion: https://postgr.es/m/vf4hbwrotvhbgcnknrqmfbqlu75oyjkmausvy66ic7x7vuhafx@e4rvwavtjswo
Backpatch-through: 14

diff --git a/src/backend/access/gist/gistutil.c b/src/backend/access/gist/gistutil.c
index 9c5597f614eae6f85181f2564639a610422c63b5..687cb7175d949cf551c00ac61cc75cc111d99613 100644 (file)
--- a/src/backend/access/gist/gistutil.c
+++ b/src/backend/access/gist/gistutil.c
@@ -1034,7 +1034,7 @@ gistGetFakeLSN(Relation rel)
                 * last call.
                 */
                static XLogRecPtr lastlsn = InvalidXLogRecPtr;
-               XLogRecPtr      currlsn = GetXLogInsertRecPtr();
+               XLogRecPtr      currlsn = GetXLogInsertEndRecPtr();
 
                /* Shouldn't be called for WAL-logging relations */
                Assert(!RelationNeedsWAL(rel));

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index c9da51f80ea1b13b2260e1db2aae7ae6f13d216d..03d26e24d2c86d8bea97b7f91caa57bde6b456e7 100644 (file)
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -9008,6 +9008,22 @@ GetXLogInsertRecPtr(void)
        return XLogBytePosToRecPtr(current_bytepos);
 }
 
+/*
+ * Get latest WAL record end pointer
+ */
+XLogRecPtr
+GetXLogInsertEndRecPtr(void)
+{
+       XLogCtlInsert *Insert = &XLogCtl->Insert;
+       uint64          current_bytepos;
+
+       SpinLockAcquire(&Insert->insertpos_lck);
+       current_bytepos = Insert->CurrBytePos;
+       SpinLockRelease(&Insert->insertpos_lck);
+
+       return XLogBytePosToEndRecPtr(current_bytepos);
+}
+
 /*
  * Get latest WAL write pointer
  */

diff --git a/src/include/access/xlog.h b/src/include/access/xlog.h
index ebb9eaade0ac9370de2960f018ec1b551ff57197..6fd5b3dfa6625f89186770852aba8032bc0a84ca 100644 (file)
--- a/src/include/access/xlog.h
+++ b/src/include/access/xlog.h
@@ -219,6 +219,7 @@ extern bool RecoveryInProgress(void);
 extern RecoveryState GetRecoveryState(void);
 extern bool XLogInsertAllowed(void);
 extern XLogRecPtr GetXLogInsertRecPtr(void);
+extern XLogRecPtr GetXLogInsertEndRecPtr(void);
 extern XLogRecPtr GetXLogWriteRecPtr(void);
 
 extern uint64 GetSystemIdentifier(void);