Handle windows closing gracefully under X11 (#42)

Request and process WM_DELETE_WINDOW events, so that clicking on the "close"
button is equivalent to calling `Graphics.close_graph`.  Subsequent graphical
operations will raise the `Graphic_failure` exception instead of killing
the program on an I/O error.

In passing, use `XPending` + `XNextEvent` instead of `XCheckMaskEvent(-1)`,
as the latter ignores client events such as WM_DELETE_WINDOW.

Co-authored-by: David Allsopp <[email protected]>

src/unix/events.c

@@ -140,6 +140,12 @@ void caml_gr_handle_event(XEvent * event)
     caml_gr_enqueue_event(event->type, event->xmotion.x, event->xmotion.y,
                      BUTTON_STATE(event->xmotion.state), 0);
     break;
+
+  case ClientMessage:
+    if ((Atom) event->xclient.data.l[0] == caml_wm_delete_window) {
+      caml_gr_close_graph();
+    }
+    break;
   }
 }
 
@@ -165,8 +171,10 @@ static value caml_gr_wait_event_poll(void)
   unsigned int i;
 
   /* Process pending X events before polling */
-  while (XCheckMaskEvent(caml_gr_display, -1 /*all events*/, &grevent)) {
+  while (XPending(caml_gr_display)) {
+    XNextEvent(caml_gr_display, &grevent);
     caml_gr_handle_event(&grevent);
+    caml_gr_check_open();
   }
   /* Poll the mouse state */
   if (XQueryPointer(caml_gr_display, caml_gr_window.win,
@@ -232,8 +240,9 @@ static value caml_gr_wait_event_blocking(long mask)
   /* Replenish our event queue from that of X11 */
   caml_gr_ignore_sigio = True;
   while (1) {
-    if (XCheckMaskEvent(caml_gr_display, -1 /*all events*/, &event)) {
+    if (XPending(caml_gr_display)) {
       /* One event available: add it to our queue */
+      XNextEvent(caml_gr_display, &event);
       caml_gr_handle_event(&event);
       /* See if we now have a matching event */
       res = caml_gr_wait_event_in_queue(mask);
@@ -245,8 +254,8 @@ static value caml_gr_wait_event_blocking(long mask)
       caml_enter_blocking_section();
       select(FD_SETSIZE, &readfds, NULL, NULL, NULL);
       caml_leave_blocking_section();
-      caml_gr_check_open(); /* in case another thread closed the display */
     }
+    caml_gr_check_open(); /* in case the display was closed in the meantime */
   }
   caml_gr_ignore_sigio = False;
 

src/unix/libgraph.h

@@ -40,6 +40,7 @@ extern int caml_gr_color;        /* Current *CAML* drawing color (can be -1) */
 extern XFontStruct * caml_gr_font;    /* Current font */
 extern long caml_gr_selected_events;  /* Events we are interested in */
 extern Bool caml_gr_ignore_sigio;     /* Whether to consume events on sigio */
+extern Atom caml_wm_delete_window;    /* "WM_DELETE_WINDOW" atom */
 
 extern Bool caml_gr_direct_rgb;
 extern int caml_gr_byte_order;
@@ -87,3 +88,4 @@ extern void caml_gr_handle_event(XEvent *e);
 extern void caml_gr_init_color_cache(void);
 extern void caml_gr_init_direct_rgb_to_pixel(void);
 extern value caml_gr_id_of_window( Window w );
+extern value caml_gr_close_graph(void);

src/unix/open.c

@@ -43,6 +43,7 @@ long caml_gr_selected_events;
 Bool caml_gr_ignore_sigio = False;
 static Bool caml_gr_initialized = False;
 static char * window_name = NULL;
+Atom caml_wm_delete_window;
 
 static int caml_gr_error_handler(Display *display, XErrorEvent *error);
 static int caml_gr_ioerror_handler(Display *display);
@@ -121,6 +122,12 @@ value caml_gr_open_graph(value arg)
     /* What not use XSetWMProperties? */
     XSetStandardProperties(caml_gr_display, caml_gr_window.win, p, p,
                            None, NULL, 0, &hints);
+    /* Handle "please delete window" requests from window manager */
+    caml_wm_delete_window =
+      XInternAtom(caml_gr_display, "WM_DELETE_WINDOW", False);
+    XSetWMProtocols(caml_gr_display, caml_gr_window.win,
+                    &caml_wm_delete_window, 1);
+
     caml_gr_window.gc = XCreateGC(caml_gr_display, caml_gr_window.win, 0, NULL);
     XSetBackground(caml_gr_display, caml_gr_window.gc, caml_gr_background);
     XSetForeground(caml_gr_display, caml_gr_window.gc, caml_gr_black);
@@ -353,7 +360,8 @@ value caml_gr_sigio_handler(void)
   XEvent grevent;
 
   if (caml_gr_initialized && !caml_gr_ignore_sigio) {
-    while (XCheckMaskEvent(caml_gr_display, -1 /*all events*/, &grevent)) {
+    while (XPending(caml_gr_display)) {
+      XNextEvent(caml_gr_display, &grevent);
       caml_gr_handle_event(&grevent);
     }
   }