Freeciv

/* Recursion detection: Maintain a stack of requirements that are

* currently being checked, and abort if we see the same req twice.

* Note that we're only storing the reqs themselves; we're not making

* an exception for the same req being evaluated against multiple

* different contexts. */

struct recur_node {

const struct recur_node *next;

const struct requirement *req;

};

/* The current deepest level of recursion */

static const struct recur_node *recur_head = nullptr;

/* Stack node representing this call to tri_req_present() */

const struct recur_node recur_current = {

.next = recur_head,

.req = req,

};

const struct recur_node *recur_i;

enum fc_tristate result;

for (recur_i = recur_head; recur_i != nullptr; recur_i = recur_i->next) {

if (are_requirements_equal(req, recur_i->req)) {

/* Infinite recursion! Abort and return TRI_MAYBE. */

struct astring astr;

log_warn("Infinite recursion when evaluating requirement: %s",

req_to_fstring(req, &astr));

astr_free(&astr);

return TRI_MAYBE;

}

}

/* push */

recur_head = &recur_current;

result = req_definitions[req->source.kind].cb(nmap, context,

other_context, req);

/* pop */

recur_head = recur_current.next;

return result;