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window: Eliminate a potential race condition with _NET_WM_MOVERESIZE

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Clients using _NET_WM_MOVERESIZE to start a drag operation may encounter
a race condition if the user presses and releases a mouse button very
fast, getting "stuck" in a grab state. While this is easily fixed with
the user pressing the button or hitting Escape as the EWMH spec suggests,
its's still a bit of annoyance for users.

After starting a grab operation, check that the button is actually pressed
by the client, and if not, cancel the grab operation. This prevents the
stuck grab in a race-free way, although it requires an extra round-trip
to the server.

With client-side decorations becoming more popular, the use of
_NET_WM_MOVERESIZE is on the rise, thus this bug is seen more frequently
than before.

https://bugzilla.gnome.org/show_bug.cgi?id=699777
This commit is contained in:
Jasper St. Pierre 2013-05-06 15:45:24 -04:00 committed by Ray Strode
parent 74fb5a83dd
commit ca4e1fd4c9
1 changed files with 78 additions and 41 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

111
src/core/window.c
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@ -6634,6 +6634,41 @@ meta_window_change_workspace_by_index (MetaWindow *window,
#define _NET_WM_MOVERESIZE_MOVE_KEYBOARD 10
#define _NET_WM_MOVERESIZE_CANCEL 11
static int
query_pressed_buttons (MetaWindow *window)
{
double x, y, query_root_x, query_root_y;
Window root, child;
XIButtonState buttons;
XIModifierState mods;
XIGroupState group;
int button = 0;
meta_error_trap_push (window->display);
XIQueryPointer (window->display->xdisplay,
META_VIRTUAL_CORE_POINTER_ID,
window->xwindow,
&root, &child,
&query_root_x, &query_root_y,
&x, &y,
&buttons, &mods, &group);
if (meta_error_trap_pop_with_return (window->display) != Success)
goto out;
if (XIMaskIsSet (buttons.mask, Button1))
button |= 1 << 1;
if (XIMaskIsSet (buttons.mask, Button2))
button |= 1 << 2;
if (XIMaskIsSet (buttons.mask, Button3))
button |= 1 << 3;
free (buttons.mask);
out:
return button;
}
gboolean
meta_window_client_message (MetaWindow *window,
XEvent *event)
@ -6992,44 +7027,8 @@ meta_window_client_message (MetaWindow *window,
(op != META_GRAB_OP_MOVING &&
op != META_GRAB_OP_KEYBOARD_MOVING))))
{
/*
* the button SHOULD already be included in the message
*/
if (button == 0)
{
double x, y, query_root_x, query_root_y;
Window root, child;
XIButtonState buttons;
XIModifierState mods;
XIGroupState group;
int button_mask;
/* The race conditions in this _NET_WM_MOVERESIZE thing
* are mind-boggling
*/
meta_error_trap_push (window->display);
XIQueryPointer (window->display->xdisplay,
META_VIRTUAL_CORE_POINTER_ID,
window->xwindow,
&root, &child,
&query_root_x, &query_root_y,
&x, &y,
&buttons, &mods, &group);
meta_error_trap_pop (window->display);
if (XIMaskIsSet (buttons.mask, Button1))
button = 1;
else if (XIMaskIsSet (buttons.mask, Button2))
button = 2;
else if (XIMaskIsSet (buttons.mask, Button3))
button = 3;
else
button = 0;
free (buttons.mask);
}
if (button != 0)
{
meta_topic (META_DEBUG_WINDOW_OPS,
"Beginning move/resize with button = %d\n", button);
meta_display_begin_grab_op (window->display,
@ -7042,6 +7041,44 @@ meta_window_client_message (MetaWindow *window,
timestamp,
x_root,
y_root);
button_mask = query_pressed_buttons (window);
if (button == 0)
{
/*
* the button SHOULD already be included in the message
*/
if ((button_mask & (1 << 1)) != 0)
button = 1;
else if ((button_mask & (1 << 2)) != 0)
button = 2;
else if ((button_mask & (1 << 3)) != 0)
button = 3;
if (button != 0)
window->display->grab_button = button;
else
meta_display_end_grab_op (window->display,
timestamp);
}
else
{
/* There is a potential race here. If the user presses and
* releases their mouse button very fast, it's possible for
* both the ButtonPress and ButtonRelease to be sent to the
* client before it can get a chance to send _NET_WM_MOVERESIZE
* to us. When that happens, we'll become stuck in a grab
* state, as we haven't received a ButtonRelease to cancel the
* grab.
*
* We can solve this by querying after we take the explicit
* pointer grab -- if the button isn't pressed, we cancel the
* drag immediately.
*/
if ((button_mask & (1 << button)) == 0)
meta_display_end_grab_op (window->display, timestamp);
}
}