client: add CONNECTING protocol cb

include/libwebsockets/lws-callbacks.h

@@ -826,6 +826,15 @@ enum lws_callback_reasons {
 	 * destroyed.  in is the child wsi.
 	 */
 
+	LWS_CALLBACK_CONNECTING					= 105,
+	/**< Called before a socketfd is about to connect().  In is the
+	 * socketfd, cast to a (void *), if on a platform where the socketfd
+	 * is an int, recover portably using (lws_sockfd_type)(intptr_t)in.
+	 *
+	 * It's also called in SOCKS5 or http_proxy cases where the socketfd is
+	 * going to try to connect to its proxy.
+	 */
+
 	/* ---------------------------------------------------------------------
 	 * ----- Callbacks related to TLS certificate management -----
 	 */

lib/core-net/client/connect3.c

@@ -437,13 +437,22 @@ lws_client_connect_3_connect(struct lws *wsi, const char *ads,
 
 	wsi->socket_is_permanently_unusable = 0;
 
+	if (lws_fi(&wsi->fic, "conn_cb_rej") ||
+	    user_callback_handle_rxflow(wsi->a.protocol->callback, wsi,
+			LWS_CALLBACK_CONNECTING, wsi->user_space,
+			(void *)(intptr_t)wsi->desc.sockfd, 0)) {
+		lwsl_info("%s: CONNECTION CB closed\n", __func__);
+		goto failed1;
+	}
+
 #if defined(LWS_WITH_SYS_FAULT_INJECTION)
 	cfail = lws_fi(&wsi->fic, "connfail");
 	if (cfail)
 		m = -1;
 	else
 #endif
-		m = connect(wsi->desc.sockfd, (const struct sockaddr *)psa, (unsigned int)n);
+		m = connect(wsi->desc.sockfd, (const struct sockaddr *)psa,
+			    (unsigned int)n);
 
 #if defined(LWS_WITH_CONMON)
 	wsi->conmon_datum = lws_now_usecs();