x11: actually disable screensaver

We already use 2 screensaver APIs when attempting to disable the
screensaver: XResetScreenSaver() (from xlib) and XScreenSaverSuspend
(from the X11 Screen Saver extension). None of these actually work.

On modern desktop Linux, we are expected to make dbus calls using some
freedesktop-defined protocol (and possibly we'd have to fallback to a
Gnome specific one). At least xscreensaver doesn't respect the "old"
APIs either.

Solve this by running the xdg-screensaver script. It's a terrible, ugly
piece of shit (just read the script if you disagree), but at least it
appears to work everywhere. It's also simpler than involving various
dbus client libraries.

I hope this can replace the --heartbeat-cmd option, and maybe we could
remove our own DPMS/XSS code too.

video/out/x11_common.c

@@ -38,6 +38,10 @@
 #include "vo.h"
 #include "win_state.h"
 #include "osdep/timer.h"
+#include "osdep/subprocess.h"
+
+// Specifically for mp_cancel
+#include "stream/stream.h"
 
 #include <X11/Xmd.h>
 #include <X11/Xlib.h>
@@ -470,6 +474,30 @@ static void vo_x11_get_bounding_monitors(struct vo_x11_state *x11, long b[4])
 #endif
 }
 
+static void *screensaver_thread(void *arg)
+{
+    struct vo_x11_state *x11 = arg;
+
+    for (;;) {
+        sem_wait(&x11->screensaver_sem);
+        // don't queue multiple wakeups
+        while (!sem_trywait(&x11->screensaver_sem)) {}
+
+        if (mp_cancel_test(x11->screensaver_terminate))
+            break;
+
+        char *args[] = {"xdg-screensaver", "reset", NULL};
+        if (mp_subprocess(args, x11->screensaver_terminate, NULL, NULL,
+                          NULL, &(char*){0}))
+        {
+            MP_WARN(x11, "Disabling screensaver failed.\n");
+            break;
+        }
+    }
+
+    return NULL;
+}
+
 int vo_x11_init(struct vo *vo)
 {
     struct mp_vo_opts *opts = vo->opts;
@@ -487,6 +515,14 @@ int vo_x11_init(struct vo *vo)
     };
     vo->x11 = x11;
 
+    x11->screensaver_terminate = mp_cancel_new(x11);
+    sem_init(&x11->screensaver_sem, 0, 0);
+    if (pthread_create(&x11->screensaver_thread, NULL, screensaver_thread, x11)) {
+        x11->screensaver_terminate = NULL;
+        sem_destroy(&x11->screensaver_sem);
+        goto error;
+    }
+
     x11_error_output = x11->log;
     XSetErrorHandler(x11_errorhandler);
 
@@ -498,13 +534,7 @@ int vo_x11_init(struct vo *vo)
     if (!x11->display) {
         MP_MSG(x11, vo->probing ? MSGL_V : MSGL_ERR,
                "couldn't open the X11 display (%s)!\n", dispName);
-
-        x11_error_output = NULL;
-        XSetErrorHandler(NULL);
-
-        talloc_free(x11);
-        vo->x11 = NULL;
-        return 0;
+        goto error;
     }
     x11->screen = DefaultScreen(x11->display);  // screen ID
     x11->rootwin = RootWindow(x11->display, x11->screen);   // root window ID
@@ -543,6 +573,10 @@ int vo_x11_init(struct vo *vo)
     vo_x11_update_geometry(vo);
 
     return 1;
+
+error:
+    vo_x11_uninit(vo);
+    return 0;
 }
 
 static const struct mp_keymap keymap[] = {
@@ -691,9 +725,18 @@ void vo_x11_uninit(struct vo *vo)
     MP_VERBOSE(x11, "uninit ...\n");
     if (x11->xim)
         XCloseIM(x11->xim);
-    x11_error_output = NULL;
-    XSetErrorHandler(NULL);
-    XCloseDisplay(x11->display);
+    if (x11->display) {
+        x11_error_output = NULL;
+        XSetErrorHandler(NULL);
+        XCloseDisplay(x11->display);
+    }
+
+    if (x11->screensaver_terminate) {
+        mp_cancel_trigger(x11->screensaver_terminate);
+        sem_post(&x11->screensaver_sem);
+        pthread_join(x11->screensaver_thread, NULL);
+        sem_destroy(&x11->screensaver_sem);
+    }
 
     talloc_free(x11);
     vo->x11 = NULL;
@@ -1766,7 +1809,7 @@ static void xscreensaver_heartbeat(struct vo_x11_state *x11)
         (time - x11->screensaver_time_last) >= 10)
     {
         x11->screensaver_time_last = time;
-
+        sem_post(&x11->screensaver_sem);
         XResetScreenSaver(x11->display);
     }
 }
@@ -1790,7 +1833,7 @@ static int xss_suspend(Display *mDisplay, Bool suspend)
 static void set_screensaver(struct vo_x11_state *x11, bool enabled)
 {
     Display *mDisplay = x11->display;
-    if (x11->screensaver_enabled == enabled)
+    if (!mDisplay || x11->screensaver_enabled == enabled)
         return;
     MP_VERBOSE(x11, "%s screensaver.\n", enabled ? "Enabling" : "Disabling");
     x11->screensaver_enabled = enabled;

video/out/x11_common.h

@@ -20,9 +20,12 @@
 
 #include <stdint.h>
 #include <stdbool.h>
+#include <pthread.h>
 #include <X11/Xlib.h>
 #include <X11/Xutil.h>
 
+#include "osdep/semaphore.h"
+
 #include "common/common.h"
 
 struct vo;
@@ -57,6 +60,9 @@ struct vo_x11_state {
     bool screensaver_enabled;
     bool dpms_touched;
     double screensaver_time_last;
+    pthread_t screensaver_thread;
+    sem_t screensaver_sem;
+    struct mp_cancel *screensaver_terminate;
 
     XIM xim;
     XIC xic;