Relax assertion in finding correct GiST parent

Commit 28d3c2ddcf introduced an assertion that if the memorized
downlink location in the insertion stack isn't valid, the parent's
LSN should've changed too. Turns out that was too strict. In
gistFindCorrectParent(), if we walk right, we update the parent's
block number and clear its memorized 'downlinkoffnum'. That triggered
the assertion on next call to gistFindCorrectParent(), if the parent
needed to be split too. Relax the assertion, so that it's OK if
downlinkOffnum is InvalidOffsetNumber.

Backpatch to v13-, all supported versions. The assertion was added in
commit 28d3c2ddcf in v12.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Reviewed-by: Tender Wang <tndrwang@gmail.com>
Discussion: https://www.postgresql.org/message-id/18396-03cac9beb2f7aac3@postgresql.org

src/backend/access/gist/gist.c

@@ -1043,12 +1043,19 @@ gistFindCorrectParent(Relation r, GISTInsertStack *child, bool is_build)
 	/*
 	 * The page has changed since we looked. During normal operation, every
 	 * update of a page changes its LSN, so the LSN we memorized should have
-	 * changed too. During index build, however, we don't WAL-log the changes
-	 * until we have built the index, so the LSN doesn't change. There is no
-	 * concurrent activity during index build, but we might have changed the
-	 * parent ourselves.
+	 * changed too.
+	 *
+	 * During index build, however, we don't WAL-log the changes until we have
+	 * built the index, so the LSN doesn't change. There is no concurrent
+	 * activity during index build, but we might have changed the parent
+	 * ourselves.
+	 *
+	 * We will also get here if child->downlinkoffnum is invalid. That happens
+	 * if 'parent' had been updated by an earlier call to this function on its
+	 * grandchild, which had to move right.
 	 */
-	Assert(parent->lsn != PageGetLSN(parent->page) || is_build);
+	Assert(parent->lsn != PageGetLSN(parent->page) || is_build ||
+		   child->downlinkoffnum == InvalidOffsetNumber);
 
 	/*
 	 * Scan the page to re-find the downlink. If the page was split, it might