BUG/MEDIUM: mworker: report status, if daemonized master fails

As daemonization fork happens now very early and before the master-worker
fork, if master or worker processes fail during the initialization, some
critical errors can't be reported to stdout. The launching (parent) process in
such cases exits with 0. This makes an impression, that master and his worker
have successfully started at background, which really complicates the
operations.

In the previous commit a pipe was added to make daemonized child communicate
with his parent. Let's add the same logic to master-worker mode. Up to
receiving the READY message from the worker, master will "forward" it via the
pipe to the launching process. Launching process can obtain master's exit
status, if the master fails to start and nothing has been written in the pipe.

This fix should be backported only in 3.1.

src/cli.c

@@ -2494,6 +2494,7 @@ static int _send_status(char **args, char *payload, struct appctx *appctx, void
 {
 	struct listener *mproxy_li;
 	struct mworker_proc *proc;
+	char *msg = "READY\n";
 	int pid;
 
 	BUG_ON((strcmp(args[0], "_send_status") != 0),
@@ -2537,6 +2538,15 @@ static int _send_status(char **args, char *payload, struct appctx *appctx, void
 		nb_oldpids = tell_old_pids(oldpids_sig);
 	}
 
+	if (daemon_fd[1] != -1) {
+		if (write(daemon_fd[1], msg, strlen(msg)) < 0) {
+			ha_alert("[%s.main()] Failed to write into pipe with parent process: %s\n", progname, strerror(errno));
+			exit(1);
+		}
+		close(daemon_fd[1]);
+		daemon_fd[1] = -1;
+	}
+
 	load_status = 1;
 	ha_notice("Loading success.\n");
 

src/mworker.c

@@ -1250,6 +1250,11 @@ void mworker_apply_master_worker_mode(void)
 
 		exit(EXIT_FAILURE);
 	case 0:
+		if (daemon_fd[1] >= 0) {
+			close(daemon_fd[1]);
+			daemon_fd[1] = -1;
+		}
+
 		/* This one must not be exported, it's internal! */
 		unsetenv("HAPROXY_MWORKER_REEXEC");
 		ha_random_jump96(1);