fix bug where attaching to dead session won't give underlying exit code

If dvtm dies, and you run `:|abduco -a session`, abduco will often exit
without giving you the exit code, and without removing the socket from
~/.abduco.

We resolve this by first sending the client either an EXIT or an empty
CONTENT packet.

Only after the client recieves it, we set the socket to non-blocking.

Fixes: https://github.com/martanne/abduco/issues/44

abduco.c

@@ -518,8 +518,6 @@ static bool attach_session(const char *name, const bool terminate) {
 		close(server.socket);
 	if ((server.socket = session_connect(name)) == -1)
 		return false;
-	if (server_set_socket_non_blocking(server.socket) == -1)
-		return false;
 
 	struct sigaction sa;
 	sa.sa_flags = 0;
@@ -529,9 +527,25 @@ static bool attach_session(const char *name, const bool terminate) {
 	sa.sa_handler = SIG_IGN;
 	sigaction(SIGPIPE, &sa, NULL);
 
-	client_setup_terminal();
+	Packet pkt = {
-	int status = client_mainloop();
+		.type = MSG_ATTACH,
-	client_restore_terminal();
+		.u.i = client.flags,
+		.len = sizeof(pkt.u.i),
+	};
+	client_send_packet(&pkt);
+
+	int status;
+	if (client_recv_packet(&pkt) && pkt.type == MSG_PID) {
+		if (server_set_socket_non_blocking(server.socket) == -1)
+			return false;
+		client_setup_terminal();
+		status = client_mainloop();
+		client_restore_terminal();
+	} else if (pkt.type == MSG_EXIT) {
+		status = pkt.u.i;
+		client_send_packet(&pkt);
+		close(server.socket);
+	}
 	if (status == -1) {
 		info("detached");
 	} else if (status == -EIO) {

client.c

@@ -63,13 +63,6 @@ static int client_mainloop(void) {
 	sigprocmask(SIG_BLOCK, &blockset, NULL);
 
 	client.need_resize = true;
-	Packet pkt = {
-		.type = MSG_ATTACH,
-		.u.i = client.flags,
-		.len = sizeof(pkt.u.i),
-	};
-	client_send_packet(&pkt);
-
 	while (server.running) {
 		fd_set fds;
 		FD_ZERO(&fds);

server.c

@@ -222,11 +222,26 @@ static void server_mainloop(void) {
 				case MSG_CONTENT:
 					server_write_pty(&client_packet);
 					break;
-				case MSG_ATTACH:
+				case MSG_ATTACH: {
+					Packet pkt;
+					if (!server.running && server.exit_status != -1) {
+						pkt = (Packet) {
+							.type = MSG_EXIT,
+							.u.i = server.exit_status,
+							.len = sizeof(pkt.u.i),
+						};
+					} else {
+						pkt = (Packet) {
+							.type = MSG_CONTENT,
+							.len = 0,
+						};
+					}
+					server_send_packet(c, &pkt);
 					c->flags = client_packet.u.i;
 					if (c->flags & CLIENT_LOWPRIORITY)
 						server_sink_client();
 					break;
+				}
 				case MSG_RESIZE:
 					c->state = STATE_ATTACHED;
 					if (!(c->flags & CLIENT_READONLY) && c == server.clients) {