amcheck: Fix parent key check in gin_index_check()

The checks introduced by commit 14ffaece0f did not get the parent key
checks quite right, missing some data corruption cases. In particular:

* The "rightlink" check was not working as intended, because rightlink
  is a BlockNumber, and InvalidBlockNumber is 0xFFFFFFFF, so

    !GinPageGetOpaque(page)->rightlink

  almost always evaluates to false (except for rightlink=0). So in most
  cases parenttup was left NULL, preventing any checks against parent.

* Use GinGetDownlink() to retrieve child blkno to avoid triggering
  Assert, same as the core GIN code.

Issues reported by Arseniy Mukhin, along with a proposed patch. Review
by Andrey M. Borodin, cleanup and improvements by me.

Author: Arseniy Mukhin <arseniy.mukhin.dev@gmail.com>
Reviewed-by: Andrey M. Borodin <x4mmm@yandex-team.ru>
Discussion: https://postgr.es/m/CAE7r3MJ611B9TE=YqBBncewp7-k64VWs+sjk7XF6fJUX77uFBA@mail.gmail.com

contrib/amcheck/t/006_verify_gin.pl

@@ -34,6 +34,8 @@ $node->safe_psql(
 invalid_entry_order_leaf_page_test();
 invalid_entry_order_inner_page_test();
 invalid_entry_columns_order_test();
+inconsistent_with_parent_key__parent_key_corrupted_test();
+inconsistent_with_parent_key__child_key_corrupted_test();
 
 sub invalid_entry_order_leaf_page_test
 {
@@ -159,6 +161,82 @@ sub invalid_entry_columns_order_test
 	like($stderr, qr/$expected/);
 }
 
+sub inconsistent_with_parent_key__parent_key_corrupted_test
+{
+	my $relname = "test";
+	my $indexname = "test_gin_idx";
+
+	# fill the table until we have a split
+	$node->safe_psql(
+		'postgres', qq(
+		DROP TABLE IF EXISTS $relname;
+		CREATE TABLE $relname (a text[]);
+		INSERT INTO $relname (a) VALUES (('{' || 'llllllllll' || random_string($filler_size) ||'}')::text[]);
+		INSERT INTO $relname (a) VALUES (('{' || 'mmmmmmmmmm' || random_string($filler_size) ||'}')::text[]);
+		INSERT INTO $relname (a) VALUES (('{' || 'nnnnnnnnnn' || random_string($filler_size) ||'}')::text[]);
+		INSERT INTO $relname (a) VALUES (('{' || 'xxxxxxxxxx' || random_string($filler_size) ||'}')::text[]);
+		INSERT INTO $relname (a) VALUES (('{' || 'yyyyyyyyyy' || random_string($filler_size) ||'}')::text[]);
+		CREATE INDEX $indexname ON $relname USING gin (a);
+	));
+	my $relpath = relation_filepath($indexname);
+
+	$node->stop;
+
+	my $blkno = 1;  # root
+
+	# we have nnnnnnnnnn... as parent key in the root, so replace it with something smaller then child's keys
+	string_replace_block(
+		$relpath,
+		'nnnnnnnnnn',
+		'aaaaaaaaaa',
+		$blkno
+	);
+
+	$node->start;
+
+	my ($result, $stdout, $stderr) = $node->psql('postgres', qq(SELECT gin_index_check('$indexname')));
+	my $expected = "index \"$indexname\" has inconsistent records on page 3 offset 3";
+	like($stderr, qr/$expected/);
+}
+
+sub inconsistent_with_parent_key__child_key_corrupted_test
+{
+	my $relname = "test";
+	my $indexname = "test_gin_idx";
+
+	# fill the table until we have a split
+	$node->safe_psql(
+		'postgres', qq(
+		DROP TABLE IF EXISTS $relname;
+		CREATE TABLE $relname (a text[]);
+		INSERT INTO $relname (a) VALUES (('{' || 'llllllllll' || random_string($filler_size) ||'}')::text[]);
+		INSERT INTO $relname (a) VALUES (('{' || 'mmmmmmmmmm' || random_string($filler_size) ||'}')::text[]);
+		INSERT INTO $relname (a) VALUES (('{' || 'nnnnnnnnnn' || random_string($filler_size) ||'}')::text[]);
+		INSERT INTO $relname (a) VALUES (('{' || 'xxxxxxxxxx' || random_string($filler_size) ||'}')::text[]);
+		INSERT INTO $relname (a) VALUES (('{' || 'yyyyyyyyyy' || random_string($filler_size) ||'}')::text[]);
+		CREATE INDEX $indexname ON $relname USING gin (a);
+	 ));
+	my $relpath = relation_filepath($indexname);
+
+	$node->stop;
+
+	my $blkno = 3;  # leaf
+
+	# we have nnnnnnnnnn... as parent key in the root, so replace child key with something bigger
+	string_replace_block(
+		$relpath,
+		'nnnnnnnnnn',
+		'pppppppppp',
+		$blkno
+	);
+
+	$node->start;
+
+	my ($result, $stdout, $stderr) = $node->psql('postgres', qq(SELECT gin_index_check('$indexname')));
+	my $expected = "index \"$indexname\" has inconsistent records on page 3 offset 3";
+	like($stderr, qr/$expected/);
+}
+
 # Returns the filesystem path for the named relation.
 sub relation_filepath
 {

contrib/amcheck/verify_gin.c

@@ -608,10 +608,10 @@ gin_check_parent_keys_consistency(Relation rel,
 				ptr = (GinScanItem *) palloc(sizeof(GinScanItem));
 				ptr->depth = stack->depth + 1;
 				/* last tuple in layer has no high key */
-				if (i != maxoff && !GinPageGetOpaque(page)->rightlink)
-					ptr->parenttup = CopyIndexTuple(idxtuple);
-				else
+				if (i == maxoff && rightlink == InvalidBlockNumber)
 					ptr->parenttup = NULL;
+				else
+					ptr->parenttup = CopyIndexTuple(idxtuple);
 				ptr->parentblk = stack->blkno;
 				ptr->blkno = GinGetDownlink(idxtuple);
 				ptr->next = stack->next;
@@ -748,7 +748,7 @@ gin_refind_parent(Relation rel, BlockNumber parentblkno,
 		ItemId		p_iid = PageGetItemIdCareful(rel, parentblkno, parentpage, o);
 		IndexTuple	itup = (IndexTuple) PageGetItem(parentpage, p_iid);
 
-		if (ItemPointerGetBlockNumber(&(itup->t_tid)) == childblkno)
+		if (GinGetDownlink(itup) == childblkno)
 		{
 			/* Found it! Make copy and return it */
 			result = CopyIndexTuple(itup);