window: Add get_client_pid() method

It is often useful to identify the client process that created
a particular window, however the existing meta_window_get_pid()
method relies on _NET_WM_PID, which is only available on X11 and
depends on applications to set it correctly (which may not even
be possible when the app runs in its own PID namespace as Flatpak
apps do). So add a get_client_pid() method that uses windowing
system facilities to resolve the PID associated with a particular
window.

https://bugzilla.gnome.org/show_bug.cgi?id=772613

configure.ac

@@ -85,6 +85,7 @@ MUTTER_PC_MODULES="
    xrender
    x11-xcb
    xcb-randr
+   xcb-res
 "
 
 GLIB_GSETTINGS

src/core/window-private.h

@@ -524,6 +524,7 @@ struct _MetaWindowClass
   gboolean (*update_icon)        (MetaWindow       *window,
                                   cairo_surface_t **icon,
                                   cairo_surface_t **mini_icon);
+  uint32_t (*get_client_pid)     (MetaWindow *window);
   void (*update_main_monitor)    (MetaWindow *window);
   void (*main_monitor_changed)   (MetaWindow *window,
                                   const MetaMonitorInfo *old);
@@ -705,6 +706,8 @@ void meta_window_handle_leave (MetaWindow  *window);
 void meta_window_handle_ungrabbed_event (MetaWindow         *window,
                                          const ClutterEvent *event);
 
+uint32_t meta_window_get_client_pid (MetaWindow *window);
+
 void meta_window_get_client_area_rect (const MetaWindow      *window,
                                        cairo_rectangle_int_t *rect);
 void meta_window_get_titlebar_rect (MetaWindow    *window,

src/core/window.c

@@ -253,6 +253,12 @@ meta_window_real_update_icon (MetaWindow       *window,
   return FALSE;
 }
 
+static uint32_t
+meta_window_real_get_client_pid (MetaWindow *window)
+{
+  return 0;
+}
+
 static void
 meta_window_finalize (GObject *object)
 {
@@ -420,6 +426,7 @@ meta_window_class_init (MetaWindowClass *klass)
   klass->update_struts = meta_window_real_update_struts;
   klass->get_default_skip_hints = meta_window_real_get_default_skip_hints;
   klass->update_icon = meta_window_real_update_icon;
+  klass->get_client_pid = meta_window_real_get_client_pid;
 
   obj_props[PROP_TITLE] =
     g_param_spec_string ("title",
@@ -7055,6 +7062,21 @@ meta_window_get_transient_for (MetaWindow *window)
     return NULL;
 }
 
+/**
+ * meta_window_get_client_pid:
+ * @window: a #MetaWindow
+ *
+ * Returns the pid of the process that created this window, if available
+ * to the windowing system.
+ *
+ * Return value: the pid, or 0 if not known.
+ */
+uint32_t
+meta_window_get_client_pid (MetaWindow *window)
+{
+  return META_WINDOW_GET_CLASS (window)->get_client_pid (window);
+}
+
 /**
  * meta_window_get_pid:
  * @window: a #MetaWindow

src/wayland/meta-window-wayland.c

@@ -453,6 +453,17 @@ meta_window_wayland_main_monitor_changed (MetaWindow *window,
   meta_window_emit_size_changed (window);
 }
 
+static uint32_t
+meta_window_wayland_get_client_pid (MetaWindow *window)
+{
+  MetaWaylandSurface *surface = window->surface;
+  struct wl_resource *resource = surface->resource;
+  pid_t pid;
+
+  wl_client_get_credentials (wl_resource_get_client (resource), &pid, NULL, NULL);
+  return (uint32_t)pid;
+}
+
 static void
 appears_focused_changed (GObject    *object,
                          GParamSpec *pspec,
@@ -487,6 +498,7 @@ meta_window_wayland_class_init (MetaWindowWaylandClass *klass)
   window_class->move_resize_internal = meta_window_wayland_move_resize_internal;
   window_class->update_main_monitor = meta_window_wayland_update_main_monitor;
   window_class->main_monitor_changed = meta_window_wayland_main_monitor_changed;
+  window_class->get_client_pid = meta_window_wayland_get_client_pid;
 }
 
 MetaWindow *

src/x11/window-x11.c

@@ -28,6 +28,9 @@
 #include <string.h>
 #include <X11/Xatom.h>
 #include <X11/Xlibint.h> /* For display->resource_mask */
+#include <X11/Xlib-xcb.h>
+
+#include <xcb/res.h>
 
 #include <X11/extensions/shape.h>
 
@@ -1481,6 +1484,42 @@ meta_window_x11_main_monitor_changed (MetaWindow *window,
 {
 }
 
+static uint32_t
+meta_window_x11_get_client_pid (MetaWindow *window)
+{
+  xcb_connection_t *xcb = XGetXCBConnection (window->display->xdisplay);
+  xcb_res_client_id_spec_t spec = { 0 };
+  xcb_res_query_client_ids_cookie_t cookie;
+  xcb_res_query_client_ids_reply_t *reply = NULL;
+
+  spec.client = window->xwindow;
+  spec.mask = XCB_RES_CLIENT_ID_MASK_LOCAL_CLIENT_PID;
+
+  cookie = xcb_res_query_client_ids (xcb, 1, &spec);
+  reply = xcb_res_query_client_ids_reply (xcb, cookie, NULL);
+
+  if (reply == NULL)
+    return 0;
+
+  uint32_t pid = 0, *value;
+  xcb_res_client_id_value_iterator_t it;
+  for (it = xcb_res_query_client_ids_ids_iterator (reply);
+       it.rem;
+       xcb_res_client_id_value_next (&it))
+    {
+      spec = it.data->spec;
+      if (spec.mask & XCB_RES_CLIENT_ID_MASK_LOCAL_CLIENT_PID)
+        {
+          value = xcb_res_client_id_value_value (it.data);
+          pid = *value;
+          break;
+        }
+    }
+
+  free (reply);
+  return pid;
+}
+
 static void
 meta_window_x11_class_init (MetaWindowX11Class *klass)
 {
@@ -1501,6 +1540,7 @@ meta_window_x11_class_init (MetaWindowX11Class *klass)
   window_class->update_icon = meta_window_x11_update_icon;
   window_class->update_main_monitor = meta_window_x11_update_main_monitor;
   window_class->main_monitor_changed = meta_window_x11_main_monitor_changed;
+  window_class->get_client_pid = meta_window_x11_get_client_pid;
 }
 
 void