Xlib: check if we can call XInitThreads() before we do so

Parental advisory: naked kludge and explicit hack
(cherry picked from commit abbdcb63d9f4161055e24faec1c7edcc9b83e44c)

include/vlc_xlib.h

@@ -21,7 +21,10 @@
 #ifndef VLC_XLIB_H
 # define VLC_XLIB_H 1
 
+# include <stdio.h>
+# include <stdlib.h>
 # include <X11/Xlib.h>
+# include <X11/Xlibint.h>
 
 static inline bool vlc_xlib_init (vlc_object_t *obj)
 {
@@ -30,8 +33,21 @@ static inline bool vlc_xlib_init (vlc_object_t *obj)
     if (var_InheritBool (obj, "xlib"))
     {
         /* XInitThreads() can be called multiple times,
-         * but it is not reentrant. */
+         * but it is not reentrant, so we need this global lock. */
         vlc_global_lock (VLC_XLIB_MUTEX);
+
+        if (_Xglobal_lock == NULL && unlikely(_XErrorFunction != NULL))
+        {
+            /* (_Xglobal_lock == NULL) => Xlib threads not initialized */
+            /* (_XErrorFunction != NULL) => Xlib already in use */
+            fprintf (stderr, "%s:%u:%u: Xlib not initialized for threads.\n"
+                     "This process is probably using LibVLC incorrectly.\n"
+                     "Pass \"--no-xlib\" to libvlc_new() to fix this.\n",
+                     __FILE__, __LINE__, __func__);
+            /* Initiate core meltdown */
+            abort ();
+        }
+        else
             ok = XInitThreads () != 0;
         vlc_global_unlock (VLC_XLIB_MUTEX);
     }