Fix bug in detecting concurrent page splits in GiST insert

In commit 9eb5607, I got the condition on checking for split or
deleted page wrong: I used && instead of ||. The comment correctly said
"concurrent split _or_ deletion".

As a result, GiST insertion could miss a concurrent split, and insert to
wrong page. Duncan Sands demonstrated this with a test script that did a
lot of concurrent inserts.

Backpatch to v12, where this was introduced. REINDEX is required to fix
indexes that were affected by this bug.

Backpatch-through: 12
Reported-by: Duncan Sands
Discussion: https://www.postgresql.org/message-id/a9690483-6c6c-3c82-c8ba-dc1a40848f11%40deepbluecap.com

Author: hlinnaka

src/backend/access/gist/gist.c

@@ -246,6 +246,9 @@ gistplacetopage(Relation rel, Size freespace, GISTSTATE *giststate,
 	if (GistFollowRight(page))
 		elog(ERROR, "concurrent GiST page split was incomplete");
 
+	/* should never try to insert to a deleted page */
+	Assert(!GistPageIsDeleted(page));
+
 	*splitinfo = NIL;
 
 	/*
@@ -861,7 +864,7 @@ gistdoinsert(Relation r, IndexTuple itup, Size freespace,
 					 */
 				}
 				else if ((GistFollowRight(stack->page) ||
-						  stack->parent->lsn < GistPageGetNSN(stack->page)) &&
+						  stack->parent->lsn < GistPageGetNSN(stack->page)) ||
 						 GistPageIsDeleted(stack->page))
 				{
 					/*