[cairo] Thread-safety and library versions

[Weeble]

I'm investigating a bug[1] in (or at least affecting) Mono, which
exhibits as one of several different crashes in libcairo when trying
to perform drawing operations on different threads simultaneously,
even when no objects are (knowingly) shared between those threads.
>From reading the NEWS file, I understand that for libcairo 1.12.10,
Behdad Eshfabod fixed a large number of thread-safety issues around
font-handling. As far as I can tell, this also involved a number of
fixes to fontconfig, which appear in releases numbered 2.10.91 and up,
and are not yet in any stable release. (The last stable release is
2.10.2.)

Does that mean that at the current time it's not generally possible to
use libcairo from multiple threads, unless you can guarantee users
will have a development or future version of fontconfig? Or is there
some kind of workaround in place for old fontconfigs? Under what
circumstances, if any, is it safe to use libcairo from multiple
threads?

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. 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.
(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.)

Regards,

Weeble.

[1] - https://bugzilla.xamarin.com/show_bug.cgi?id=2426