ui: Make the DisplayType enum entries conditional

Libvirt's "domcapabilities" command has a way to state whether certain
graphic frontends are available in QEMU or not. Originally, libvirt
looked at the "--help" output of the QEMU binary to determine whether
SDL was available or not (by looking for the "-sdl" parameter in the
help text), but since libvirt stopped doing this analysis of the help
text, the detection of SDL is currently broken, see:

 https://bugzilla.redhat.com/show_bug.cgi?id=1790902

QEMU should provide a way via the QMP interface instead. A simple way,
without introducing additional commands, is to make the DisplayType
enum entries conditional, so that the enum only contains the entries if
the corresponding CONFIG_xxx switches have been set. This of course
only gives an indication which possibilities have been enabled during
compile-time of QEMU (and does not take into account whether modules
are later available or not for example - for this we'd need a separate
command), but anyway, this should already be good enough for the above
bug ticket, and it's a good idea anyway to make the QMP interface
conditional here, so let's simply do it.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20210615090439.70926-1-thuth@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

qapi/ui.json

@@ -1130,9 +1130,16 @@
 #
 ##
 { 'enum'    : 'DisplayType',
-  'data'    : [ 'default', 'none', 'gtk', 'sdl',
-                'egl-headless', 'curses', 'cocoa',
-                'spice-app'] }
+  'data'    : [
+    { 'name': 'default' },
+    { 'name': 'none' },
+    { 'name': 'gtk', 'if': 'defined(CONFIG_GTK)' },
+    { 'name': 'sdl', 'if': 'defined(CONFIG_SDL)' },
+    { 'name': 'egl-headless',
+              'if': 'defined(CONFIG_OPENGL) && defined(CONFIG_GBM)' },
+    { 'name': 'curses', 'if': 'defined(CONFIG_CURSES)' },
+    { 'name': 'cocoa', 'if': 'defined(CONFIG_COCOA)' },
+    { 'name': 'spice-app', 'if': 'defined(CONFIG_SPICE)'} ] }
 
 ##
 # @DisplayOptions:
@@ -1156,9 +1163,13 @@
                 '*show-cursor'   : 'bool',
                 '*gl'            : 'DisplayGLMode' },
   'discriminator' : 'type',
-  'data'    : { 'gtk'            : 'DisplayGTK',
-                'curses'         : 'DisplayCurses',
-                'egl-headless'   : 'DisplayEGLHeadless'} }
+  'data'    : {
+      'gtk': { 'type': 'DisplayGTK', 'if': 'defined(CONFIG_GTK)' },
+      'curses': { 'type': 'DisplayCurses', 'if': 'defined(CONFIG_CURSES)' },
+      'egl-headless': { 'type': 'DisplayEGLHeadless',
+                        'if': 'defined(CONFIG_OPENGL) && defined(CONFIG_GBM)' }
+  }
+}
 
 ##
 # @query-display-options:

softmmu/vl.c

@@ -1068,6 +1068,7 @@ static void parse_display(const char *p)
          *     Not clear yet what happens to them long-term.  Should
          *     replaced by something better or deprecated and dropped.
          */
+#if defined(CONFIG_SDL)
         dpy.type = DISPLAY_TYPE_SDL;
         while (*opts) {
             const char *nextopt;
@@ -1131,6 +1132,10 @@ static void parse_display(const char *p)
             }
             opts = nextopt;
         }
+#else
+        error_report("SDL display supported is not available in this binary");
+        exit(1);
+#endif
     } else if (strstart(p, "vnc", &opts)) {
         /*
          * vnc isn't a (local) DisplayType but a protocol for remote
@@ -1867,13 +1872,22 @@ static void qemu_apply_machine_options(void)
 static void qemu_create_early_backends(void)
 {
     MachineClass *machine_class = MACHINE_GET_CLASS(current_machine);
+#if defined(CONFIG_SDL)
+    const bool use_sdl = (dpy.type == DISPLAY_TYPE_SDL);
+#else
+    const bool use_sdl = false;
+#endif
+#if defined(CONFIG_GTK)
+    const bool use_gtk = (dpy.type == DISPLAY_TYPE_GTK);
+#else
+    const bool use_gtk = false;
+#endif
 
-    if ((alt_grab || ctrl_grab) && dpy.type != DISPLAY_TYPE_SDL) {
+    if ((alt_grab || ctrl_grab) && !use_sdl) {
         error_report("-alt-grab and -ctrl-grab are only valid "
                      "for SDL, ignoring option");
     }
-    if (dpy.has_window_close &&
-        (dpy.type != DISPLAY_TYPE_GTK && dpy.type != DISPLAY_TYPE_SDL)) {
+    if (dpy.has_window_close && !use_gtk && !use_sdl) {
         error_report("-no-quit is only valid for GTK and SDL, "
                      "ignoring option");
     }

ui/console.c

@@ -2370,13 +2370,19 @@ void qemu_display_register(QemuDisplay *ui)
 bool qemu_display_find_default(DisplayOptions *opts)
 {
     static DisplayType prio[] = {
+#if defined(CONFIG_GTK)
         DISPLAY_TYPE_GTK,
+#endif
+#if defined(CONFIG_SDL)
         DISPLAY_TYPE_SDL,
+#endif
+#if defined(CONFIG_COCOA)
         DISPLAY_TYPE_COCOA
+#endif
     };
     int i;
 
-    for (i = 0; i < ARRAY_SIZE(prio); i++) {
+    for (i = 0; i < (int)ARRAY_SIZE(prio); i++) {
         if (dpys[prio[i]] == NULL) {
             ui_module_load_one(DisplayType_str(prio[i]));
         }