[Fontconfig] Streamlining fontconfig scanning

[Akira TAGOH]

Speeding up the scanning is really nice but IMHO we can't remove the
lock as long as fontconfig has a possibility to update caches from
multiple processes at the same time. improving the speed of updating
caches may reduces the chance to see the issue, but that's it. in
other words, there are still an opportunity to see it if the condition
is met. in fact, that issue was there since quite a long time ago. but
it bagan to happen these days because of processing the updates of
caches in parallel by the package manager etc. so I don't want to do
that unless we have any idea to detect that breakage before writing or
any way to do update cache safely. otherwise we'll see some wired
issues in the future.

On Tue, Aug 8, 2017 at 2:30 AM, Behdad Esfahbod <behdad at behdad.org> wrote:
> Hi everyone,
>
> I have a proposed patchset speeding up fontconfig scanning by 10x, simply by
> not loading glyphs at all, and trusting fonts having correct cmap:
>
>   https://bugs.freedesktop.org/show_bug.cgi?id=64766#c56
>
> If no one has comments, I like to merge this and get it out for testing in
> the wild.
>
> After this, I think we should fix the relocatable feature to not touch
> mmaped cache files.  Here's my proposed approach:
>
>   https://bugs.freedesktop.org/show_bug.cgi?id=101889#c17
>
> After that, we should clean up the cache race patches and remove the
> locking.  We should accept that cache updates will always remain racy,
> simply because we don't have or want to use the kinds of synchronization
> primitives that guarantee no race.  With scanning 10x faster this shouldn't
> be a problem in practice.  I like to get back to each process trying to
> update the cache and possibly discarding its result if another process
> already did...
>
> Cheers,
>
> --
> behdad
> http://behdad.org/
>
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>



-- 
Akira TAGOH