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 27972fad2b003a51b57ac3bd50c00ff26fd8d0eb..e7e4b273e196b62b0e0b8a4140a45ce9e14b0984 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 b9b678f37229094fd026303453360e089519c3ba..f760291e10c0ef17f77e69a7d962295124f2238a 100644 (file)
--- a/src/backend/access/transam/xlog.c
+++ b/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
  */

diff --git a/src/include/access/xlog.h b/src/include/access/xlog.h
index fdfb572467be4cda49a2e3d6f9c66eafac4540da..958f39edda403b74be4cad590dd17ecf2b8bf70c 100644 (file)
--- a/src/include/access/xlog.h
+++ b/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);