Better handle premature Xorg exit

Register proper signal handler for SIGCHLD, and collect the Xorg's
zombie in it.

This has two effects:
1. The main loop can explicitly exit on Xorg termination, not only via
   receiving EOF on the socket.
2. Due to not ignoring SIGCHLD anymore, accept() in mkghandles will also
   notice Xorg early exit and not wait indefinitely (it will fail with
   EINTR). For this case, improve error message.

There is still a small race on startup, if Xorg exits before reaching
accept() (or listen()) call. Handle this by checking just before
accept() call. It isn't perfect (there is still a few instructions
window where it wouldn't notice it in time), but it's good enough for
practical purposes.

QubesOS/qubes-issues#8060

gui-agent/vmside.c

@@ -1558,9 +1558,16 @@ static void wait_for_unix_socket(Ghandles *g)
 
     addrlen = sizeof(peer);
     fprintf (stderr, "Waiting on %s socket...\n", SOCKET_ADDRESS);
+    if (g->x_pid == (pid_t)-1) {
+        fprintf(stderr, "Xorg exited in the meantime, aborting\n");
+        exit(1);
+    }
     g->xserver_fd = accept(g->xserver_listen_fd, (struct sockaddr *) &peer, &addrlen);
     if (g->xserver_fd == -1) {
-        perror("unix accept");
+        if (errno == EINTR && g->x_pid == (pid_t)-1)
+            fprintf(stderr, "Xorg exited in the meantime, aborting\n");
+        else
+            perror("unix accept");
         exit(1);
     }
     fprintf (stderr, "Ok, somebody connected.\n");
@@ -2317,6 +2324,21 @@ static _Noreturn void handle_sigterm(int UNUSED(sig),
     exit(0);
 }
 
+static void handle_sigchld(int UNUSED(sig),
+        siginfo_t *UNUSED(info), void *UNUSED(context))
+{
+    Ghandles *g = ghandles_for_vchan_reinitialize;
+    if (g->x_pid != (pid_t)-1) {
+        int status;
+        pid_t pid = waitpid(g->x_pid, &status, WNOHANG);
+        if (pid == g->x_pid && (WIFEXITED(status) || WIFSIGNALED(status)))
+            /* TODO: consider saving also exit status, but right now gui-agent
+             * would handle it the same regardless, so maybe later, just for
+             * logging purposes */
+            g->x_pid = -1;
+    }
+}
+
 static void usage(void)
 {
     fprintf(stderr, "Usage: qubes_gui [options]\n");
@@ -2456,7 +2478,13 @@ int main(int argc, char **argv)
     vchan_register_at_eof(handle_guid_disconnect);
 
     ghandles_for_vchan_reinitialize = &g;
-    signal(SIGCHLD, SIG_IGN);
+    struct sigaction sigchld_handler = {
+        .sa_sigaction = handle_sigchld,
+        .sa_flags = SA_SIGINFO,
+    };
+    sigemptyset(&sigchld_handler.sa_mask);
+    if (sigaction(SIGCHLD, &sigchld_handler, NULL))
+        err(1, "sigaction");
     struct sigaction sigterm_handler = {
         .sa_sigaction = handle_sigterm,
         .sa_flags = SA_SIGINFO,
@@ -2544,6 +2572,11 @@ int main(int argc, char **argv)
     for (;;) {
         int busy;
 
+        if (g.x_pid == -1) {
+            fprintf(stderr, "Xorg exited prematurely\n");
+            exit(1);
+        }
+
         fds[0].fd = libvchan_fd_for_select(g.vchan);
         wait_for_vchan_or_argfd(g.vchan, fds, QUBES_ARRAY_SIZE(fds));
         /* first process possible qubes_drv reconnection, otherwise we may be