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 7844c6074fefa5d0f663c3a64c15eed39040684f..382154d5d48631b2b518953a5572977930d028b3 100644 (file)
--- a/src/backend/access/gist/gistutil.c
+++ b/src/backend/access/gist/gistutil.c
@@ -1033,7 +1033,7 @@ 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 948cb1fc7038fb50d3990a6d51d49e1fc19c44ab..745276908d2faf0f38cef8272069ae9e8e61a612 100644 (file)
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -9500,6 +9500,22 @@ 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 3d8bc98886146aac1bf3359b1782995ec8f11fce..30f5aa6f15185b336b9a7f58596231ffae395971 100644 (file)
--- a/src/include/access/xlog.h
+++ b/src/include/access/xlog.h
@@ -224,6 +224,7 @@ 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);