Fix yet another crash in page split during GiST index creation.

Commit a7ee7c85 fixed a bug in GiST page split during index creation,
where we failed to re-find the position of a downlink after the page
containing it was split. However, that fix was incomplete; the other call
to gistinserttuples() in the same function needs to also clear
'downlinkoffnum'.

Fixes bug #16134 reported by Alexander Lakhin, for real this time. The
previous fix was enough to fix the crash with the reproducer script for
bug #16162, but the original script for #16134 was still crashing.

Backpatch to v12, like the previous incomplete fix.

Discussion: https://www.postgresql.org/message-id/d869f537-abe4-d2ea-0510-38cd053f5152%40gmail.com

src/backend/access/gist/gist.c

@@ -1325,10 +1325,9 @@ gistfinishsplit(GISTInsertState *state, GISTInsertStack *stack,
 	 * downlink for the original page as one operation.
 	 */
 	LockBuffer(stack->parent->buffer, GIST_EXCLUSIVE);
-	gistFindCorrectParent(state->r, stack);
 
 	/*
-	 * insert downlinks for the siblings from right to left, until there are
+	 * Insert downlinks for the siblings from right to left, until there are
 	 * only two siblings left.
 	 */
 	for (int pos = list_length(splitinfo) - 1; pos > 1; pos--)
@@ -1336,17 +1335,17 @@ gistfinishsplit(GISTInsertState *state, GISTInsertStack *stack,
 		right = (GISTPageSplitInfo *) list_nth(splitinfo, pos);
 		left = (GISTPageSplitInfo *) list_nth(splitinfo, pos - 1);
 
+		gistFindCorrectParent(state->r, stack);
 		if (gistinserttuples(state, stack->parent, giststate,
 							 &right->downlink, 1,
 							 InvalidOffsetNumber,
 							 left->buf, right->buf, false, false))
 		{
 			/*
-			 * If the parent page was split, need to relocate the original
-			 * parent pointer.
+			 * If the parent page was split, the existing downlink might
+			 * have moved.
 			 */
 			stack->downlinkoffnum = InvalidOffsetNumber;
-			gistFindCorrectParent(state->r, stack);
 		}
 		/* gistinserttuples() released the lock on right->buf. */
 	}
@@ -1361,13 +1360,21 @@ gistfinishsplit(GISTInsertState *state, GISTInsertStack *stack,
 	 */
 	tuples[0] = left->downlink;
 	tuples[1] = right->downlink;
-	gistinserttuples(state, stack->parent, giststate,
-					 tuples, 2,
-					 stack->downlinkoffnum,
-					 left->buf, right->buf,
-					 true,		/* Unlock parent */
-					 unlockbuf	/* Unlock stack->buffer if caller wants that */
-		);
+	gistFindCorrectParent(state->r, stack);
+	if (gistinserttuples(state, stack->parent, giststate,
+						 tuples, 2,
+						 stack->downlinkoffnum,
+						 left->buf, right->buf,
+						 true,		/* Unlock parent */
+						 unlockbuf	/* Unlock stack->buffer if caller wants that */
+			))
+	{
+		/*
+		 * If the parent page was split, the downlink might have moved.
+		 */
+		stack->downlinkoffnum = InvalidOffsetNumber;
+	}
+
 	Assert(left->buf == stack->buffer);
 
 	/*