Re: Thread-safety and library versions

Hi,

On 17.09.2013 14:16, Weeble wrote:
[...]
> More specifically, I'm running libcairo 1.12.14 and libfontconfig
> 2.10.2. When Mono repeatedly creates cairo drawing contexts, calls
> cairo_select_font_face, and destroys the drawing contexts; (with each
> context accessed only from a single thread, but more than one thread
> running at once;) sooner or later it crashes when
> _cairo_toy_font_face_destroy calls _cairo_hash_table_remove and raises
> an assertion, apparently because the key is missing from the hash
> table.

I was thinking "it can't be that simple" and produced the attached test program
which does exactly this. The result with current cairo from git:

a.out: cairo-hash.c:506: _cairo_hash_table_lookup_exact_key: Assertion
`!"reached"' failed.

Could someone with more clue about how things are supposed to work take a look
at this? Chris? Behdad?

> Could this crash be caused by having an old version of
> fontconfig? There's no obvious involvement of fontconfig in this stack
> trace, but if this combination of libraries is known to have
> thread-safety issues I wonder if it could be due to memory corruption.

Just my 5c, but I don't think that this is due to fontconfig either.

> (Much more rarely, it crashes in FcPatternDestroy, with an assertion
> about a double-free or memory corruption, so I do have reason to
> suspect fontconfig, although I am not certain these crashes have the
> same cause.)
[...]

Haven't seen such a crash yet.

Cheers,
Uli
-- 
<alanc> I think someone had a Xprint version of glxgears at one point,
    but benchmarking how many GL pages you can print per second
    was deemed too silly to merge

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#include <cairo.h>
#include <stdio.h>
#include <pthread.h>

#define NTHREADS 2

static cairo_surface_t *surf;

static void *thread_start(void *unused)
{
	while (1) {
		cairo_t *cr = cairo_create(surf);
		cairo_select_font_face(cr, "serif", CAIRO_FONT_SLANT_NORMAL,
				CAIRO_FONT_WEIGHT_NORMAL);
		cairo_destroy(cr);
	}
}

int main(void)
{
	int i;
	pthread_t threads[NTHREADS];

	surf = cairo_image_surface_create(CAIRO_FORMAT_ARGB32, 100, 100);

	for (i = 0; i < NTHREADS; i++) {
		if (pthread_create(&threads[i], NULL, thread_start, NULL) != 0)
			perror("pthread_create");
	}

	for (i = 0; i < NTHREADS; i++) {
		if (pthread_join(threads[i], NULL) != 0)
			perror("pthread_join");
	}

	cairo_surface_destroy(surf);

	return 0;
}