Relax assertion in finding correct GiST parent

Commit 28d3c2d 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. Back to v12, like commit 28d3c2d Discussion: https://www.postgresql.org/message-id/[email protected]

Author: hlinnaka

src/backend/access/gist/gist.c

@@ -1053,7 +1053,8 @@ gistFindCorrectParent(Relation r, GISTInsertStack *child, bool is_build)
 * concurrent activity during index build, but we might have changed the
 * parent ourselves.
 */
-Assert(parent->lsn != PageGetLSN(parent->page) || is_build);
+Assert(child->downlinkoffnum == InvalidOffsetNumber ||
+parent->lsn != PageGetLSN(parent->page) || is_build);
 
 /*
 * Scan the page to re-find the downlink. If the page was split, it might