[Fontconfig] Missing font issue due to broken caches

[Nick Alcock]

On 29 Jul 2015, Akira TAGOH said:

> Hi,
>
> Since fontconfig validates a cache during loading it, you'll see your
> concern after this change right.
> I think we need to implement another way or API etc for that use case and
> web font support instead of disabling this check. even checking mtime in
> this case shouldn't be wanted. let me think more.

Yeah. I've noticed extreme slowdowns with an NFS-mounted font directory
too (10+ seconds to start each fontconfig user after the NFS attribute
cache expires) -- fontconfig's statting the whole thing every time,
which is wholly pointless because will never be updated without running
fc-cache on all NFS-connected machines. And that directory has thousands
of fonts in it...

Having an optional off-by-default way to quiesce this revalidation would
be nice. (I was going to just implement it in an ugly local hack, but
then this came up... so clearly this sort of thing not affecting just
me.)

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