disallow -daemonize usage of stdio (curses display, -nographic, -serial stdio etc)

Curses display requires stdin/out to stay on the terminal,
so -daemonize makes no sense in this case.  Instead of
leaving display uninitialized like is done since 995ee2bf,
explicitly detect this case earlier and error out.

-nographic can actually be used with -daemonize, by redirecting
everything to a null device, but the problem is that according
to documentation and historical behavour, -nographic redirects
guest ports to stdin/out, which, again, makes no sense in case
of -daemonize.  Since -nographic is a legacy option, don't bother
fixing this case (to allow -nographic and -daemonize by redirecting
guest ports to null instead of stdin/out in this case), but disallow
it completely instead, to stop garbling host terminal.

If no display display needed and user wants to use -nographic,
the right way to go is to use
  -serial null -parallel null -monitor none -display none -vga none
instead of -nographic.

Also prevent the same issue -- it was possible to get garbled
host tty after

  -nographic -daemonize

and it is still possible to have it by using

  -serial stdio -daemonize

Fix this by disallowing opening stdio chardev when -daemonize
is specified.
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

qemu-char.c

@@ -772,6 +772,10 @@ static CharDriverState *qemu_chr_open_stdio(QemuOpts *opts)
     if (stdio_nb_clients >= STDIO_MAX_CLIENTS) {
         return NULL;
     }
+    if (is_daemonized()) {
+        error_report("cannot use stdio with -daemonize");
+        return NULL;
+    }
     if (stdio_nb_clients == 0) {
         old_fd0_flags = fcntl(0, F_GETFL);
         tcgetattr (0, &oldtty);

vl.c

@@ -3637,6 +3637,30 @@ int main(int argc, char **argv, char **envp)
         default_sdcard = 0;
     }
 
+    if (is_daemonized()) {
+        /* According to documentation and historically, -nographic redirects
+         * serial port, parallel port and monitor to stdio, which does not work
+         * with -daemonize.  We can redirect these to null instead, but since
+         * -nographic is legacy, let's just error out.
+         * We disallow -nographic only if all other ports are not redirected
+         * explicitly, to not break existing legacy setups which uses
+         * -nographic _and_ redirects all ports explicitly - this is valid
+         * usage, -nographic is just a no-op in this case.
+         */
+        if (display_type == DT_NOGRAPHIC
+            && (default_parallel || default_serial
+                || default_monitor || default_virtcon)) {
+            fprintf(stderr, "-nographic can not be used with -daemonize\n");
+            exit(1);
+        }
+#ifdef CONFIG_CURSES
+        if (display_type == DT_CURSES) {
+            fprintf(stderr, "curses display can not be used with -daemonize\n");
+            exit(1);
+        }
+#endif
+    }
+
     if (display_type == DT_NOGRAPHIC) {
         if (default_parallel)
             add_device_config(DEV_PARALLEL, "null");
@@ -3903,9 +3927,7 @@ int main(int argc, char **argv, char **envp)
         break;
 #if defined(CONFIG_CURSES)
     case DT_CURSES:
-        if (!is_daemonized()) {
-            curses_display_init(ds, full_screen);
-        }
+        curses_display_init(ds, full_screen);
         break;
 #endif
 #if defined(CONFIG_SDL)