xwm: clarify props[] in weston_wm_window_read_properties()

The props array contained offsets to struct members. It is convenient
for writing static const arrays as you only store a constant offset and
compute the pointer later. However, the array was not static to begin
with, the atoms are not build time constants. We can as well just store
the pointer directly in the array.

Entries that did not use the offset had bogus offsets, producing
pointers to arbitrary fields. They are changed to have a NULL pointer.
If the code unintentionally used the pointer, it will now explode rather
than corrupt memory.

Also explain the use of the #defined constants and #undef them when they
get out of scope. This clearly documents that they are just a convenient
hack to avoid lots of special cases in the function.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>

xwayland/window-manager.c

@@ -439,7 +439,10 @@ read_and_dump_property(struct weston_wm *wm,
 	free(reply);
 }
 
-/* We reuse some predefined, but otherwise useles atoms */
+/* We reuse some predefined, but otherwise useles atoms
+ * as local type placeholders that never touch the X11 server,
+ * to make weston_wm_window_read_properties() less exceptional.
+ */
 #define TYPE_WM_PROTOCOLS	XCB_ATOM_CUT_BUFFER0
 #define TYPE_MOTIF_WM_HINTS	XCB_ATOM_CUT_BUFFER1
 #define TYPE_NET_WM_STATE	XCB_ATOM_CUT_BUFFER2
@@ -452,22 +455,22 @@ weston_wm_window_read_properties(struct weston_wm_window *window)
 	const struct weston_desktop_xwayland_interface *xwayland_interface =
 		wm->server->compositor->xwayland_interface;
 
-#define F(field) offsetof(struct weston_wm_window, field)
+#define F(field) (&window->field)
 	const struct {
 		xcb_atom_t atom;
 		xcb_atom_t type;
-		int offset;
+		void *ptr;
 	} props[] = {
 		{ XCB_ATOM_WM_CLASS,           XCB_ATOM_STRING,            F(class) },
 		{ XCB_ATOM_WM_NAME,            XCB_ATOM_STRING,            F(name) },
 		{ XCB_ATOM_WM_TRANSIENT_FOR,   XCB_ATOM_WINDOW,            F(transient_for) },
-		{ wm->atom.wm_protocols, TYPE_WM_PROTOCOLS, F(protocols) },
+		{ wm->atom.wm_protocols,       TYPE_WM_PROTOCOLS,          NULL },
-		{ wm->atom.wm_normal_hints, TYPE_WM_NORMAL_HINTS, F(protocols) },
+		{ wm->atom.wm_normal_hints,    TYPE_WM_NORMAL_HINTS,       NULL },
-		{ wm->atom.net_wm_state, TYPE_NET_WM_STATE },
+		{ wm->atom.net_wm_state,       TYPE_NET_WM_STATE,          NULL },
 		{ wm->atom.net_wm_window_type, XCB_ATOM_ATOM,              F(type) },
 		{ wm->atom.net_wm_name,        XCB_ATOM_STRING,            F(name) },
 		{ wm->atom.net_wm_pid,         XCB_ATOM_CARDINAL,          F(pid) },
-		{ wm->atom.motif_wm_hints, TYPE_MOTIF_WM_HINTS, 0 },
+		{ wm->atom.motif_wm_hints,     TYPE_MOTIF_WM_HINTS,        NULL },
 		{ wm->atom.wm_client_machine,  XCB_ATOM_WM_CLIENT_MACHINE, F(machine) },
 	};
 #undef F
@@ -507,7 +510,7 @@ weston_wm_window_read_properties(struct weston_wm_window *window)
 			continue;
 		}
 
-		p = ((char *) window + props[i].offset);
+		p = props[i].ptr;
 
 		switch (props[i].type) {
 		case XCB_ATOM_WM_CLIENT_MACHINE:
@@ -601,6 +604,11 @@ weston_wm_window_read_properties(struct weston_wm_window *window)
 		xwayland_interface->set_pid(window->shsurf, window->pid);
 }
 
+#undef TYPE_WM_PROTOCOLS
+#undef TYPE_MOTIF_WM_HINTS
+#undef TYPE_NET_WM_STATE
+#undef TYPE_WM_NORMAL_HINTS
+
 static void
 weston_wm_window_get_frame_size(struct weston_wm_window *window,
 				int *width, int *height)