Fix some more bugs in GIN's WAL replay logic.

In commit 4016bde I fixed a bunch of
ginxlog.c bugs having to do with not handling XLogReadBuffer failures
correctly.  However, in ginRedoUpdateMetapage and ginRedoDeleteListPages,
I unaccountably thought that failure to read the metapage would be
impossible and just put in an elog(PANIC) call.  This is of course wrong:
failure is exactly what will happen if the index got dropped (or rebuilt)
between creation of the WAL record and the crash we're trying to recover
from.  I believe this explains Nicholas Wilson's recent report of these
errors getting reached.

Also, fix memory leak in forgetIncompleteSplit.  This wasn't of much
concern when the code was written, but in a long-running standby server
page split records could be expected to accumulate indefinitely.

Back-patch to 8.4 --- before that, GIN didn't have a metapage.

Author: tglsfdc

src/backend/access/gin/ginxlog.c

@@ -59,9 +59,12 @@ forgetIncompleteSplit(RelFileNode node, BlockNumber leftBlkno, BlockNumber updat
 	{
 		ginIncompleteSplit *split = (ginIncompleteSplit *) lfirst(l);
 
-		if (RelFileNodeEquals(node, split->node) && leftBlkno == split->leftBlkno && updateBlkno == split->rightBlkno)
+		if (RelFileNodeEquals(node, split->node) &&
+			leftBlkno == split->leftBlkno &&
+			updateBlkno == split->rightBlkno)
 		{
 			incomplete_splits = list_delete_ptr(incomplete_splits, split);
+			pfree(split);
 			break;
 		}
 	}
@@ -487,7 +490,7 @@ ginRedoUpdateMetapage(XLogRecPtr lsn, XLogRecord *record)
 
 	metabuffer = XLogReadBuffer(data->node, GIN_METAPAGE_BLKNO, false);
 	if (!BufferIsValid(metabuffer))
-		elog(PANIC, "GIN metapage disappeared");
+		return;					/* assume index was deleted, nothing to do */
 	metapage = BufferGetPage(metabuffer);
 
 	if (!XLByteLE(lsn, PageGetLSN(metapage)))
@@ -632,7 +635,7 @@ ginRedoDeleteListPages(XLogRecPtr lsn, XLogRecord *record)
 
 	metabuffer = XLogReadBuffer(data->node, GIN_METAPAGE_BLKNO, false);
 	if (!BufferIsValid(metabuffer))
-		elog(PANIC, "GIN metapage disappeared");
+		return;					/* assume index was deleted, nothing to do */
 	metapage = BufferGetPage(metabuffer);
 
 	if (!XLByteLE(lsn, PageGetLSN(metapage)))