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present: Walk all children when setting window pixmaps 

We currently skip setting the window pixmap on any window
not using its parent's pixmap. That does not work correctly
in the presence of reparenting.

Consider the following scenario:
1. window A is created as child of B
2. present starts flipping and sets the whole window
   tree to use pixmap X
3. window C is created (uses the screen pixmap by default)
4. window A is reparented to C
5. present stops flipping and attempts to set the
   whole window tree back to the screen pixmap,
   except the walk terminates at window C
   since it's using an unexpected pixmap, and
   window A is left with the stale pixmap X
6. pixmap X is destroyed
7. the X server segfaults on a rendering operation
   on window A due the stale pixmap

I managed to hit this with mpv (doing present flips)
and crack-attack (keeps alternating between a menu
window and an actual game window):
1. start both applications
2. start a game in crack-attack
3. fullscreen mpv
4. end the game in crack attack
5. unfullscreen mpv
6. the crack-attack menu window has appeared, but
   might be corrupted and doing stuff on it segfaults
   the X server

I suppose the other option might be to make new windows
automatically inherit their parent's pixmap instead
of using the screen pixmap. But I've not looked into
how that would affect eg. composite.

Signed-off-by: notbabaisyou <though-went-some-simple@proton.me>
This commit is contained in:
Ville Syrjälä 2025-06-24 22:37:08 +02:00 committed by notbabaisyou
parent f5ea9069e1
commit dcf8371a81
1 changed files with 8 additions and 3 deletions
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11
present/present.c
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@ -120,9 +120,14 @@ present_set_tree_pixmap_visit(WindowPtr window, void *data)
struct pixmap_visit *visit = data;
ScreenPtr screen = window->drawable.pScreen;
if ((*screen->GetWindowPixmap)(window) != visit->old)
return WT_DONTWALKCHILDREN;
(*screen->SetWindowPixmap)(window, visit->new);
if ((*screen->GetWindowPixmap)(window) == visit->old)
(*screen->SetWindowPixmap)(window, visit->new);
/*
* Walk the entire tree in case windows using the
* the old pixmap have been reparented to newly
* created windows using the screen pixmap instead.
*/
return WT_WALKCHILDREN;
}