Misc: Add properties to the registration window

Add WM_CLASS, COMPTON_VERSION, and _NET_WM_PID properties to the
registration window, to ease the development of pcman's compton-conf.

compton.c

@@ -4543,8 +4543,31 @@ register_cm(session_t *ps) {
   if (ps->redirected)
     XCompositeUnredirectWindow(ps->dpy, ps->reg_win, CompositeRedirectManual);
 
+  {
+    XClassHint *h = XAllocClassHint();
+    if (h) {
+      h->res_name = "compton";
+      h->res_class = "xcompmgr";
+    }
     Xutf8SetWMProperties(ps->dpy, ps->reg_win, "xcompmgr", "xcompmgr",
-      NULL, 0, NULL, NULL, NULL);
+        NULL, 0, NULL, NULL, h);
+    cxfree(h);
+  }
+
+  // Set _NET_WM_PID
+  {
+    long pid = getpid();
+    if (!XChangeProperty(ps->dpy, ps->reg_win,
+          get_atom(ps, "_NET_WM_PID"), XA_CARDINAL, 32, PropModeReplace,
+          (unsigned char *) &pid, 1)) {
+      printf_errf("(): Failed to set _NET_WM_PID.");
+    }
+  }
+
+  // Set COMPTON_VERSION
+  if (!wid_set_text_prop(ps, ps->reg_win, get_atom(ps, "COMPTON_VERSION"), COMPTON_VERSION)) {
+    printf_errf("(): Failed to set COMPTON_VERSION.");
+  }
 
   {
     unsigned len = strlen(REGISTER_PROP) + 2;

compton.h

@@ -337,9 +337,7 @@ isdamagenotify(session_t *ps, const XEvent *ev) {
  */
 static inline XTextProperty *
 make_text_prop(session_t *ps, char *str) {
-  XTextProperty *pprop = malloc(sizeof(XTextProperty));
+  XTextProperty *pprop = cmalloc(1, XTextProperty);
-  if (!pprop)
-    printf_errfq(1, "(): Failed to allocate memory.");
 
   if (XmbTextListToTextProperty(ps->dpy, &str, 1,  XStringStyle, pprop)) {
     cxfree(pprop->value);
@@ -350,6 +348,25 @@ make_text_prop(session_t *ps, char *str) {
   return pprop;
 }
 
+
+/**
+ * Set a single-string text property on a window.
+ */
+static inline bool
+wid_set_text_prop(session_t *ps, Window wid, Atom prop_atom, char *str) {
+  XTextProperty *pprop = make_text_prop(ps, str);
+  if (!pprop) {
+    printf_errf("(\"%s\"): Failed to make text property.", str);
+    return false;
+  }
+
+  XSetTextProperty(ps->dpy, wid, pprop, prop_atom);
+  cxfree(pprop->value);
+  cxfree(pprop);
+
+  return true;
+}
+
 static void
 run_fade(session_t *ps, win *w, unsigned steps);