Merge remote-tracking branch 'tor-github/pr/1346' into maint-0.4.1

src/feature/relay/router.c

@@ -3355,6 +3355,10 @@ router_free_all(void)
   crypto_pk_free(server_identitykey);
   crypto_pk_free(client_identitykey);
 
+  /* Destroying a locked mutex is undefined behaviour. This mutex may be
+   * locked, because multiple threads can access it. But we need to destroy
+   * it, otherwise re-initialisation will trigger undefined behaviour.
+   * See #31735 for details. */
   tor_mutex_free(key_lock);
   routerinfo_free(desc_routerinfo);
   extrainfo_free(desc_extrainfo);