relay: Look at our cache when looking for an IP change

Regularly, tor looks if its IP has changed. It does the entire auto discovery
process again. However, it is possible that it does not find anything.

Instead of thinking the IP changed to an unknown address, look at our cache
and see if that value has changed.

The reason for this is because if tor gets its address as a suggestion from a
directory authority, it is because the auto discovery failed and thus that
address should be consider for the IP change check.

Related to #40071

Signed-off-by: David Goulet <dgoulet@torproject.org>

src/feature/relay/router.c

@@ -2679,8 +2679,21 @@ check_descriptor_ipaddress_changed(time_t now)
 
     /* Ignore returned value because we want to notice not only an address
      * change but also if an address is lost (current == UNSPEC). */
-    find_my_address(get_options(), family, LOG_INFO, &current, &method,
-                    &hostname);
+    bool found = find_my_address(get_options(), family, LOG_INFO, &current,
+                                 &method, &hostname);
+    if (!found) {
+      /* Address was possibly not found because it is simply not configured or
+       * discoverable. Fallback to our cache, which includes any suggestion
+       * sent by a trusted directory server. */
+      found = relay_find_addr_to_publish(get_options(), family,
+                                         RELAY_FIND_ADDR_CACHE_ONLY,
+                                         &current);
+    }
+
+    /* The "current" address might be UNSPEC meaning it was not discovered nor
+     * found in our current cache. If we had an address before and we have
+     * none now, we consider this an IP change since it appears the relay lost
+     * its address. */
 
     if (!tor_addr_eq(previous, &current)) {
       char *source;