Move to BufferGetLSNAtomic where spinlock is needed

Certain callers of PageGetLSN weren't correctly holding the buffer
spinlock; it is needed whenever the buffer content lock is not held in
exclusive mode.

For heapam.c, also ensure that we don't access the LSN after releasing
the lock.
Signed-off-by: NAsim R P <apraveen@pivotal.io>

src/backend/access/gist/gist.c

@@ -562,7 +562,7 @@ gistfindleaf(GISTInsertState *state, GISTSTATE *giststate)
 		state->stack->page = (Page) BufferGetPage(state->stack->buffer);
 		opaque = GistPageGetOpaque(state->stack->page);
 
-		state->stack->lsn = PageGetLSN(state->stack->page);
+		state->stack->lsn = BufferGetLSNAtomic(state->stack->buffer);
 		Assert(state->r->rd_istemp || !XLogRecPtrIsInvalid(state->stack->lsn));
 
 		if (state->stack->blkno != GIST_ROOT_BLKNO &&
@@ -699,7 +699,7 @@ gistFindPath(Relation r, BlockNumber child)
 			break;
 		}
 
-		top->lsn = PageGetLSN(page);
+		top->lsn = BufferGetLSNAtomic(buffer);
 
 		if (top->parent && XLByteLT(top->parent->lsn, GistPageGetOpaque(page)->nsn) &&
 			GistPageGetOpaque(page)->rightlink != InvalidBlockNumber /* sanity check */ )

src/backend/access/gist/gistget.c

@@ -43,7 +43,7 @@ killtuple(Relation r, GISTScanOpaque so, ItemPointer iptr)
 	gistcheckpage(r, so->curbuf);
 	p = (Page) BufferGetPage(so->curbuf);
 
-	if (XLByteEQ(so->stack->lsn, PageGetLSN(p)))
+	if (XLByteEQ(so->stack->lsn, BufferGetLSNAtomic(so->curbuf)))
 	{
 		/* page unchanged, so all is simple */
 		offset = ItemPointerGetOffsetNumber(iptr);
@@ -240,7 +240,7 @@ gistnext(IndexScanDesc scan, ScanDirection dir, ItemPointer tids,
 		opaque = GistPageGetOpaque(p);
 
 		/* remember lsn to identify page changed for tuple's killing */
-		so->stack->lsn = PageGetLSN(p);
+		so->stack->lsn = BufferGetLSNAtomic(so->curbuf);
 
 		/* check page split, occured from last visit or visit to parent */
 		if (!XLogRecPtrIsInvalid(so->stack->parentlsn) &&

src/backend/access/heap/heapam.c

@@ -5241,11 +5241,6 @@ heap_xlog_delete(XLogRecPtr lsn, XLogRecord *record)
 
 	if (XLByteLE(lsn, PageGetLSN(page)))		/* changes are applied */
 	{
-		UnlockReleaseBuffer(buffer);
-		
-		MIRROREDLOCK_BUFMGR_UNLOCK;
-		// -------- MirroredLock ----------
-		
 		if (Debug_print_qd_mirroring)
 		{
 			elog(LOG, "delete already appplied: lsn (%X,%X), page (%X,%X)",
@@ -5254,6 +5249,12 @@ heap_xlog_delete(XLogRecPtr lsn, XLogRecord *record)
 				 PageGetLSN(page).xlogid,
 				 PageGetLSN(page).xrecoff);
 		}
+
+		UnlockReleaseBuffer(buffer);
+
+		MIRROREDLOCK_BUFMGR_UNLOCK;
+		// -------- MirroredLock ----------
+
 		return;
 	}