Fix amcheck for page checks concurrent to replay of btree page deletion

amcheck expects at least hikey to always exist on leaf page even if it is
deleted page.  But replica reinitializes page during replay of page deletion,
causing deleted page to have no items.  Thus, replay of page deletion can
cause an error in concurrent amcheck run.

This commit relaxes amcheck expectation making it tolerate deleted page with
no items.

Reported-by: Konstantin Knizhnik
Discussion: https://postgr.es/m/CAPpHfdt_OTyQpXaPJcWzV2N-LNeNJseNB-K_A66qG%3DL518VTFw%40mail.gmail.com
Author: Alexander Korotkov
Reviewed-by: Peter Geoghegan
Backpatch-through: 11

contrib/amcheck/verify_nbtree.c

@@ -2864,7 +2864,11 @@ palloc_btree_page(BtreeCheckState *state, BlockNumber blocknum)
 	 * As noted at the beginning of _bt_binsrch(), an internal page must have
 	 * children, since there must always be a negative infinity downlink
 	 * (there may also be a highkey).  In the case of non-rightmost leaf
-	 * pages, there must be at least a highkey.
+	 * pages, there must be at least a highkey.  Deleted pages on replica
+	 * might contain no items, because page unlink re-initializes
+	 * page-to-be-deleted.  Deleted pages with no items might be on primary
+	 * too due to preceding recovery, but on primary new deletions can't
+	 * happen concurrently to amcheck.
 	 *
 	 * This is correct when pages are half-dead, since internal pages are
 	 * never half-dead, and leaf pages must have a high key when half-dead
@@ -2884,13 +2888,13 @@ palloc_btree_page(BtreeCheckState *state, BlockNumber blocknum)
 						blocknum, RelationGetRelationName(state->rel),
 						MaxIndexTuplesPerPage)));
 
-	if (!P_ISLEAF(opaque) && maxoffset < P_FIRSTDATAKEY(opaque))
+	if (!P_ISLEAF(opaque) && !P_ISDELETED(opaque) && maxoffset < P_FIRSTDATAKEY(opaque))
 		ereport(ERROR,
 				(errcode(ERRCODE_INDEX_CORRUPTED),
 				 errmsg("internal block %u in index \"%s\" lacks high key and/or at least one downlink",
 						blocknum, RelationGetRelationName(state->rel))));
 
-	if (P_ISLEAF(opaque) && !P_RIGHTMOST(opaque) && maxoffset < P_HIKEY)
+	if (P_ISLEAF(opaque) && !P_ISDELETED(opaque) && !P_RIGHTMOST(opaque) && maxoffset < P_HIKEY)
 		ereport(ERROR,
 				(errcode(ERRCODE_INDEX_CORRUPTED),
 				 errmsg("non-rightmost leaf block %u in index \"%s\" lacks high key item",