Fix handling of WS handshake error response

Check response code, make sure it's 101.
Pass http_message to the client to keep it appraised.
This represents a slight change in the API -
in case of an error MG_EV_WEBSOCKET_HANDSHAKE_DONE will now be delivered where previosuly connection would just hang.
Clients that do not examine the argument may for a moment think handshake has succeeded but in fact connection will be closed immediately.

CL: mg: Fix handling of WS handshake error response

PUBLISHED_FROM=645a43d9e5bee216e54411f85827c9b974e9a7d1

src/mg_http.c

@@ -827,16 +827,23 @@ void mg_http_handler(struct mg_connection *nc, int ev,
       /* Do nothing, request is not yet fully buffered */
     }
 #if MG_ENABLE_HTTP_WEBSOCKET
-    else if (nc->listener == NULL &&
-             mg_get_http_header(hm, "Sec-WebSocket-Accept")) {
+    else if (nc->listener == NULL && (nc->flags & MG_F_IS_WEBSOCKET)) {
       /* We're websocket client, got handshake response from server. */
-      /* TODO(lsm): check the validity of accept Sec-WebSocket-Accept */
-      mbuf_remove(io, req_len);
-      nc->proto_handler = mg_ws_handler;
-      nc->flags |= MG_F_IS_WEBSOCKET;
-      mg_call(nc, nc->handler, nc->user_data, MG_EV_WEBSOCKET_HANDSHAKE_DONE,
-              NULL);
-      mg_ws_handler(nc, MG_EV_RECV, ev_data MG_UD_ARG(user_data));
+      DBG(("%p WebSocket upgrade code %d", nc, hm->resp_code));
+      if (hm->resp_code == 101 &&
+          mg_get_http_header(hm, "Sec-WebSocket-Accept")) {
+        /* TODO(lsm): check the validity of accept Sec-WebSocket-Accept */
+        mg_call(nc, nc->handler, nc->user_data, MG_EV_WEBSOCKET_HANDSHAKE_DONE,
+                hm);
+        mbuf_remove(io, req_len);
+        nc->proto_handler = mg_ws_handler;
+        mg_ws_handler(nc, MG_EV_RECV, ev_data MG_UD_ARG(user_data));
+      } else {
+        mg_call(nc, nc->handler, nc->user_data, MG_EV_WEBSOCKET_HANDSHAKE_DONE,
+                hm);
+        nc->flags |= MG_F_CLOSE_IMMEDIATELY;
+        mbuf_remove(io, req_len);
+      }
     } else if (nc->listener != NULL &&
                (vec = mg_get_http_header(hm, "Sec-WebSocket-Key")) != NULL) {
       struct mg_http_endpoint *ep;
@@ -867,7 +874,7 @@ void mg_http_handler(struct mg_connection *nc, int ev,
           mg_ws_handshake(nc, vec, hm);
         }
         mg_call(nc, nc->handler, nc->user_data, MG_EV_WEBSOCKET_HANDSHAKE_DONE,
-                NULL);
+                hm);
         mg_ws_handler(nc, MG_EV_RECV, ev_data MG_UD_ARG(user_data));
       }
     }

src/mg_http.h

@@ -106,7 +106,7 @@ struct mg_ssi_call_ctx {
 
 #if MG_ENABLE_HTTP_WEBSOCKET
 #define MG_EV_WEBSOCKET_HANDSHAKE_REQUEST 111 /* struct http_message * */
-#define MG_EV_WEBSOCKET_HANDSHAKE_DONE 112    /* NULL */
+#define MG_EV_WEBSOCKET_HANDSHAKE_DONE 112    /* struct http_message * */
 #define MG_EV_WEBSOCKET_FRAME 113             /* struct websocket_message * */
 #define MG_EV_WEBSOCKET_CONTROL_FRAME 114     /* struct websocket_message * */
 #endif
@@ -145,7 +145,9 @@ struct mg_ssi_call_ctx {
  * - MG_EV_WEBSOCKET_HANDSHAKE_REQUEST: server has received the WebSocket
  *   handshake request. `ev_data` contains parsed HTTP request.
  * - MG_EV_WEBSOCKET_HANDSHAKE_DONE: server has completed the WebSocket
- *   handshake. `ev_data` is `NULL`.
+ *   handshake. `ev_data` is a `struct http_message` containing the
+ *   client's request (server mode) or server's response (client).
+ *   In client mode handler can examine `resp_code`, which should be 101.
  * - MG_EV_WEBSOCKET_FRAME: new WebSocket frame has arrived. `ev_data` is
  *   `struct websocket_message *`
  *

src/mg_http_websocket.c

@@ -478,6 +478,8 @@ void mg_send_websocket_handshake3v(struct mg_connection *nc,
   }
   mg_printf(nc, "\r\n");
 
+  nc->flags |= MG_F_IS_WEBSOCKET;
+
   mbuf_free(&auth);
 }