[cairo-bugs] [Bug 74355] Line drawing not thread-safe

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=74355

Uli Schlachter <psychon at znc.in> changed:

           What    |Removed                     |Added
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             Status|NEW                         |NEEDINFO

--- Comment #1 from Uli Schlachter <psychon at znc.in> ---
(In reply to comment #0)
> 1. share a single cairo_surface_t between the threads, and have each thread
> draw using its own cairo_t. This crashes, but maybe I'm hoping for too much
> (although an image surface is essentially just a big array of bytes that
> should be writable from multiple threads). 

Sure, just an array. And this works as long as you expect anything like useful
results. Cairo is supposed to be thread-safe as long as the threads don't share
any state (well, this is an oversimplification, but your first approach isn't
supposed to work).

> 2. create a cairo_surface_t and cairo_t for each thread, and merge the
> results after drawing has completed. This crashes in pixman. The only Cairo
> calls that happen on a per-thread basis are cairo_set_line_cap,
> cairo_set_source_rgba, cairo_set_line_width, cairo_set_dash, cairo_move_to,
> cairo_line_to, and cairo_stroke.

Since this is so simple, do you have some small, self-contained C code that
only depends on some threading API and cairo and that reproduces this problem?

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