dlist_check: add check for nodes pointing to themselves

Currently, postgresql will enter an infinite loop when calling dlist_push_head the second time.
After applying this patch, it will report ERROR: doubly linked list is corrupted.

test case:
./configure --enable-debug CFLAGS='-O0 -DILIST_DEBUG' --enable-cassert

PG_FUNCTION_INFO_V1(test_dlist);
Datum
test_dlist(PG_FUNCTION_ARGS)
{
dlist_head head;
dlist_node *n;

n = palloc0(sizeof(dlist_node));
dlist_init(&head);
dlist_push_head(&head, n);
dlist_push_head(&head, n);

PG_RETURN_NULL();
}

regards,
Lei Yang

From: 杨磊
Date: 2026-03-17 20:56
To: pgsql-hackers
Subject: dlist_check: add check for nodes pointing to themselves
Hi hackers,

I accidentally called the function dlist_push_head consecutively on the same node,
inserting it into a doubly linked list, which caused an infinite loop.
After adding the compile parameter -DILIST_DEBUG, an infinite loop occurred in the dlist_check function,
revealing that the next pointer of the current node was pointing to itself.
I am attempting to fix this issue with the following patch.

regards,
Lei Yang