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 d4bf0c7563de8617e05b8c37f9f4cea8ff427d0d..a639768256318c993a9ccf786a231c91224167cd 100644 (file)
--- a/src/backend/access/gist/gistutil.c
+++ b/src/backend/access/gist/gistutil.c
@@ -1043,7 +1043,7 @@ 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 7aa2a354f2d59689547f3bd293161222fb271c44..f79961633a6251283656357da0ab824a45c94961 100644 (file)
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -8932,6 +8932,22 @@ 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 e81a9d6aec954390ba2b89542ea76ba361dfd973..af3ce8579717440966f5c6c714b2c0be26fbcb5e 100644 (file)
--- a/src/include/access/xlog.h
+++ b/src/include/access/xlog.h
@@ -218,6 +218,7 @@ 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);