simple-xinit: Avoid hanging if the server fails to set up the display fd.

Early fatal errors may leave us with nothing in the displayfd pipe,
and we'd block forever.

Signed-off-by: Eric Anholt <eric@anholt.net>

test/simple-xinit.c

@@ -60,6 +60,15 @@ usage(int argc, char **argv)
     exit(1);
 }
 
+static int server_displayfd;
+static const char *server_dead = "server_dead";
+
+static void
+handle_sigchld(int sig)
+{
+    write(server_displayfd, server_dead, strlen(server_dead));
+}
+
 /* Starts the X server, returning its pid. */
 static int
 start_server(char *const *server_args)
@@ -71,6 +80,17 @@ start_server(char *const *server_args)
         exit(1);
     } else if (server_pid != 0) {
         /* Continue along the main process that will exec the client. */
+
+        struct sigaction sa;
+        sa.sa_handler = handle_sigchld;
+        sigemptyset(&sa.sa_mask);
+        sa.sa_flags = SA_RESTART | SA_NOCLDSTOP;
+        if (sigaction(SIGCHLD, &sa, 0) == -1) {
+            fprintf(stderr, "Failed to set up signal handler: %s\n",
+                    strerror(errno));
+            exit(1);
+        }
+
         return server_pid;
     }
 
@@ -84,7 +104,7 @@ start_server(char *const *server_args)
 static int
 get_display(int displayfd)
 {
-    char display_string[10];
+    char display_string[20];
     ssize_t ret;
 
     ret = read(displayfd, display_string, sizeof(display_string) - 1);
@@ -97,6 +117,12 @@ get_display(int displayfd)
      * '\n', but not '\0'.  Cap it and parse the number.
      */
     display_string[ret] = '\0';
+
+    if (strncmp(display_string, server_dead, strlen(server_dead)) == 0) {
+        fprintf(stderr, "Server failed to start before setting up displayfd\n");
+        exit(1);
+    }
+
     return atoi(display_string);
 }
 
@@ -222,7 +248,8 @@ main(int argc, char **argv)
         exit(1);
     }
 
-    parse_args(argc, argv, &client_args, &server_args, displayfd_pipe[1]);
+    server_displayfd = displayfd_pipe[1];
+    parse_args(argc, argv, &client_args, &server_args, server_displayfd);
     server_pid = start_server(server_args);
     display = get_display(displayfd_pipe[0]);
     ret = start_client(client_args, display);