Add warning if firewall blocks loopback (closes #2621)

src/mongoose.c

@@ -3459,14 +3459,17 @@ void mg_broadcast(struct mg_mgr *mgr, mg_event_handler_t cb, void *data,
    */
   if (mgr->ctl[0] != INVALID_SOCKET && data != NULL &&
       len < sizeof(ctl_msg.message)) {
-    size_t dummy;
+    size_t ret;
 
     ctl_msg.callback = cb;
     memcpy(ctl_msg.message, data, len);
-    dummy = MG_SEND_FUNC(mgr->ctl[0], (char *) &ctl_msg,
+    ret = MG_SEND_FUNC(mgr->ctl[0], (char *) &ctl_msg,
                          offsetof(struct ctl_msg, message) + len, 0);
-    dummy = MG_RECV_FUNC(mgr->ctl[0], (char *) &len, 1, 0);
-    (void) dummy; /* https://gcc.gnu.org/bugzilla/show_bug.cgi?id=25509 */
+    if (ret < 0)
+      perror("mg_broadcast() send failed, check UDP loopback device");
+    ret = MG_RECV_FUNC(mgr->ctl[0], (char *) &len, 1, 0);
+    if (ret < 0)
+      perror("mg_broadcast() recv failed, check firewall UDP loopback rules");
   }
 }
 #endif /* MG_ENABLE_BROADCAST */
@@ -4116,9 +4119,12 @@ static void mg_mgr_handle_ctl_sock(struct mg_mgr *mgr) {
   struct ctl_msg ctl_msg;
   int len =
       (int) MG_RECV_FUNC(mgr->ctl[1], (char *) &ctl_msg, sizeof(ctl_msg), 0);
-  size_t dummy = MG_SEND_FUNC(mgr->ctl[1], ctl_msg.message, 1, 0);
+  if (len < 0)
+    perror("mg_mgr_handle_ctl_sock() recv failed, check firewall UDP loopback rules");
+  size_t ret = MG_SEND_FUNC(mgr->ctl[1], ctl_msg.message, 1, 0);
+  if (ret < 0)
+    perror("mg_mgr_handle_ctl_sock() send failed, check UDP loopback device");
   DBG(("read %d from ctl socket", len));
-  (void) dummy; /* https://gcc.gnu.org/bugzilla/show_bug.cgi?id=25509 */
   if (len >= (int) sizeof(ctl_msg.callback) && ctl_msg.callback != NULL) {
     struct mg_connection *nc;
     for (nc = mg_next(mgr, NULL); nc != NULL; nc = mg_next(mgr, nc)) {

src/mongoose_broadcast.patch

@@ -0,0 +1,41 @@
+diff --git a/src/mongoose.c b/src/mongoose.c
+index c9c40863..55bd01e1 100644
+--- a/src/mongoose.c
++++ b/src/mongoose.c
+@@ -3459,14 +3459,17 @@ void mg_broadcast(struct mg_mgr *mgr, mg_event_handler_t cb, void *data,
+    */
+   if (mgr->ctl[0] != INVALID_SOCKET && data != NULL &&
+       len < sizeof(ctl_msg.message)) {
+-    size_t dummy;
++    size_t ret;
+
+     ctl_msg.callback = cb;
+     memcpy(ctl_msg.message, data, len);
+-    dummy = MG_SEND_FUNC(mgr->ctl[0], (char *) &ctl_msg,
++    ret = MG_SEND_FUNC(mgr->ctl[0], (char *) &ctl_msg,
+                          offsetof(struct ctl_msg, message) + len, 0);
+-    dummy = MG_RECV_FUNC(mgr->ctl[0], (char *) &len, 1, 0);
+-    (void) dummy; /* https://gcc.gnu.org/bugzilla/show_bug.cgi?id=25509 */
++    if (ret < 0)
++      perror("mg_broadcast() send failed, check UDP loopback device");
++    ret = MG_RECV_FUNC(mgr->ctl[0], (char *) &len, 1, 0);
++    if (ret < 0)
++      perror("mg_broadcast() recv failed, check firewall UDP loopback rules");
+   }
+ }
+ #endif /* MG_ENABLE_BROADCAST */
+@@ -4116,9 +4119,12 @@ static void mg_mgr_handle_ctl_sock(struct mg_mgr *mgr) {
+   struct ctl_msg ctl_msg;
+   int len =
+       (int) MG_RECV_FUNC(mgr->ctl[1], (char *) &ctl_msg, sizeof(ctl_msg), 0);
+-  size_t dummy = MG_SEND_FUNC(mgr->ctl[1], ctl_msg.message, 1, 0);
++  if (len < 0)
++    perror("mg_mgr_handle_ctl_sock() recv failed, check firewall UDP loopback rules");
++  size_t ret = MG_SEND_FUNC(mgr->ctl[1], ctl_msg.message, 1, 0);
++  if (ret < 0)
++    perror("mg_mgr_handle_ctl_sock() send failed, check UDP loopback device");
+   DBG(("read %d from ctl socket", len));
+-  (void) dummy; /* https://gcc.gnu.org/bugzilla/show_bug.cgi?id=25509 */
+   if (len >= (int) sizeof(ctl_msg.callback) && ctl_msg.callback != NULL) {
+     struct mg_connection *nc;
+     for (nc = mg_next(mgr, NULL); nc != NULL; nc = mg_next(mgr, nc)) {