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
c6b92041d3 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

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));

src/backend/access/transam/xlog.c

@@ -9595,6 +9595,22 @@ 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
  */

src/include/access/xlog.h

@@ -238,6 +238,7 @@ 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);