Bug 92455 - Need to flush twice to detach snapshots
Summary: Need to flush twice to detach snapshots
Alias: None
Product: cairo
Classification: Unclassified
Component: xcb backend (show other bugs)
Version: unspecified
Hardware: Other All
: medium normal
Assignee: Chris Wilson
QA Contact: cairo-bugs mailing list
Depends on:
Blocks: cairo-1.16
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Reported: 2015-10-14 07:29 UTC by Uli Schlachter
Modified: 2018-08-25 13:26 UTC (History)
0 users

See Also:
i915 platform:
i915 features:

Reproducer (1.48 KB, text/plain)
2015-10-14 07:29 UTC, Uli Schlachter

Description Uli Schlachter 2015-10-14 07:29:59 UTC
Created attachment 118865 [details]

This comes originally from https://github.com/i3/i3/pull/1990.

The attached test program has the following behavior:

$ gcc -Wall -Wextra test.c $(pkg-config --cflags --libs cairo xcb) &&  ./a.out 
a.out: ../../../../src/cairo-surface.c:1652: cairo_surface_mark_dirty_rectangle: Assertion `! _cairo_surface_has_snapshots (surface)' failed.
[1]    11287 abort      ./a.out
$ gcc -Wall -Wextra test.c $(pkg-config --cflags --libs cairo xcb) -DUSE_WORKAROUND &&  ./a.out 

This program creates a cairo-xcb surface (in a way so that RENDER is not used), draws to it (this causes a fallback) and then calls cairo_surface_flush(surf); cairo_surface_mark_dirty(surf);. As the failed assertion shows, the surface has snapshots attached during cairo_surface_mark_dirty() which we explicitly disallow via an assertion.

In the second run, the USE_WORKAROUND preprocessor symbol is defined. The only difference is that this program now calls cairo_surface_flush() twice in a row so that the second call can detach the snapshot that the first flush added. This snapshot is added in _cairo_xcb_surface_flush() and is used to save the fallback surface for later use.

As far as I can see, this bug always existed. No idea why no one noticed it so far (well, how many people don't have RENDER?).

@Chris: Why are we detaching snapshots in cairo_surface_flush() anyway? Wouldn't it make more sense to detach them in cairo_surface_mark_dirty(), because that's the point when the snapshots become invalid. Changing this would break _cairo_surface_snapshot(), because it uses a snapshot to do a copy-on-write, but that might count as misuse of the API. This could in turn be fixed with some kind of "modification callback chain". What do you think?

Short term, I would just remove the call to _cairo_surface_attach_snapshot() from _cairo_xcb_surface_flush() to fix the symptom of this problem, but flush() has (to me) more of a "make my drawing visible"-semantics and not a "I will definitely draw to things directly now".
Comment 1 GitLab Migration User 2018-08-25 13:26:38 UTC
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