wmaker: replace and be replaced (ICCCM protocol)

Use the same logic used by xfwm4, metacity et al to replace an existing
window manager on the screen and allow other window managers to replace
us, as defined by the ICCCM 2.0:

http://www.x.org/releases/X11R7.6/doc/xorg-docs/specs/ICCCM/icccm.html
  Communication with the Window Manager by Means of Selections

By convention those window managers try to become the selection owner of
the WM_Sn atom where n is the screen number.  If the atom is owned by
another window manager and the --replace argument was not given to wmaker
we fail to start.  If the argument was given we try to become the new
owner and wait for the existing window manger to exit.

After a successful startup we watch for SelectionClear events on the
atom and initiate a shutdown if one arrives, as that implies that
another window manager was started with --replace.

src/screen.c

@@ -66,6 +66,8 @@
     |SubstructureRedirectMask|ButtonPressMask|ButtonReleaseMask\
     |KeyPressMask|KeyReleaseMask)
 
+#define REPLACE_WM_TIMEOUT 15
+
 #define STIPPLE_WIDTH 2
 #define STIPPLE_HEIGHT 2
 static char STIPPLE_DATA[] = { 0x02, 0x01 };
@@ -90,6 +92,106 @@ static void make_keys(void)
 	dDrawers = WMCreatePLString("Drawers");
 }
 
+/*
+ * Support for ICCCM 2.0: Window Manager Replacement protocol
+ * See: http://www.x.org/releases/X11R7.6/doc/xorg-docs/specs/ICCCM/icccm.html
+ *
+ * Basically, user should be able to dynamically change its window manager; this is done
+ * cooperatively through a special Selection ("WM_Sn" where 'n' is the X screen number)
+ *
+ * This function does 2 things:
+ *  - it checks if this selection is already owned, which means that another window
+ * manager is running. If it is the case and user specified '--replace' on the command
+ * line, then it asks the WM to shut down;
+ *  - when ok, it sets the selection ownership to ourself, so another window manager
+ * may ask us to shut down (this is handled in "event.c")
+ */
+static Bool replace_existing_wm(WScreen *scr)
+{
+	char atomName[16];
+	Window wm;
+	XSetWindowAttributes attribs;
+	XClientMessageEvent event;
+	unsigned long current_time;
+	int ret;
+
+	/* Try to acquire the atom named WM_S<screen> */
+	ret = snprintf(atomName, sizeof(atomName), "WM_S%d", scr->screen);
+	if (ret < 0 || ret == sizeof(atomName)) {
+		werror("out of memory trying to allocate window manager selection atom for screen %d", scr->screen);
+		return False;
+	}
+
+	scr->sn_atom = XInternAtom(dpy, atomName, False);
+	if (! scr->sn_atom)
+		return False;
+
+	/* Check if an existing window manager owns the selection */
+	wm = XGetSelectionOwner(dpy, scr->sn_atom);
+	if (wm) {
+		if (!wPreferences.flags.replace) {
+			wmessage(_("another window manager is running"));
+			wwarning(_("use the --replace flag to replace it"));
+			return False;
+		}
+
+		attribs.event_mask = StructureNotifyMask;
+		if (!XChangeWindowAttributes(dpy, wm, CWEventMask, &attribs))
+			wm = None;
+	}
+
+	/* for our window manager info notice board and the selection owner */
+	scr->info_window = XCreateSimpleWindow(dpy, scr->root_win, 0, 0, 10, 10, 0, 0, 0);
+
+	/* Try to acquire the selection */
+	current_time = CurrentTime;
+	ret = XSetSelectionOwner(dpy, scr->sn_atom, scr->info_window, current_time);
+	if (ret == BadAtom || ret == BadWindow)
+		return False;
+
+	/* Wait for other window manager to exit */
+	if (wm) {
+		unsigned long wait = 0;
+		unsigned long timeout = REPLACE_WM_TIMEOUT * 1000000L;
+		XEvent event;
+
+		while (wait < timeout) {
+			if (!(wait % 1000000))
+				wmessage(_("waiting %lus for other window manager to exit"), (timeout - wait) / 1000000L);
+
+			if (XCheckWindowEvent(dpy, wm, StructureNotifyMask, &event))
+				if (event.type == DestroyNotify)
+					break;
+
+			wusleep(100000);
+			wait += 100000;
+		}
+
+		if (wait >= timeout) {
+			wwarning(_("other window manager hasn't exited!"));
+			return False;
+		}
+
+		wmessage(_("replacing the other window manager"));
+	}
+
+	if (XGetSelectionOwner(dpy, scr->sn_atom) != scr->info_window)
+		return False;
+
+	event.type = ClientMessage;
+	event.message_type = scr->sn_atom;
+	event.format = 32;
+	event.data.l[0] = (long) current_time;
+	event.data.l[1] = (long) scr->sn_atom;
+	event.data.l[2] = (long) scr->info_window;
+	event.data.l[3] = (long) 0L;
+	event.data.l[4] = (long) 0L;
+	event.window = scr->root_win;
+	XSendEvent(dpy, scr->root_win, False, StructureNotifyMask, (XEvent *) &event);
+
+	return True;
+}
+
 /*
  *----------------------------------------------------------------------
  * alreadyRunningError--
@@ -523,6 +625,13 @@ WScreen *wScreenInit(int screen_number)
 	CantManageScreen = 0;
 	oldHandler = XSetErrorHandler(alreadyRunningError);
 
+	/* Do we need to replace an existing window manager? */
+	if (!replace_existing_wm(scr)) {
+		XDestroyWindow(dpy, scr->info_window);
+		wfree(scr);
+		return NULL;
+	}
+
 	event_mask = EVENT_MASK;
 	XSelectInput(dpy, scr->root_win, event_mask);
 
@@ -616,11 +725,6 @@ WScreen *wScreenInit(int screen_number)
 	/* create GCs with default values */
 	allocGCs(scr);
 
-	/* for our window manager info notice board. Need to
-	 * create before reading the defaults, because it will be used there.
-	 */
-	scr->info_window = XCreateSimpleWindow(dpy, scr->root_win, 0, 0, 10, 10, 0, 0, 0);
-
 	/* read defaults for this screen */
 	wReadDefaults(scr, w_global.domain.wmaker->dictionary);