[Fontconfig-bugs] [Bug 101889] Make fontconfig cache relocatable

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=101889

--- Comment #15 from Behdad Esfahbod <freedesktop at behdad.org> ---
(In reply to Alexander Larsson from comment #13)
> Yes, we could do it lazily, true. However, I don't understand what you mean
> about sharing with others.

When the mapped file is not modified, the backing memory is shared across
processes.  They are copy-on-write.  If we can completely avoid changing is
even better.  I have a scheme in mind that does that.


> Here is what i imagine:
> 
> [Shared mmaped cache data]
> ...
> FontName="TheFont"
> FileName=offset 0
> ...
> FontName="OtherFont"
> FileName=offset 1
> ...
> FileNameTable
> [offset 0]
> [offset 12]
> FileNameStrings
> "TheFont.ttf\0OtherFont.ttf\0"
> 
> This would be readonly and shared by all clients.
> To go from a cache element to a filename you would normally
> go from offset in FileNameTable to offset in FileNameStrings, all shared by
> everyone. However, in the case where the cache is loaded in a non-canonical
> location, you'd allocate an in-memory (non-shared) FileNameTable and
> FileNameStrings array, which would would rewrite and use instead.
> 
> Such a setup could be lazy, either doing everything the first time you get a
> filename, or it could allocate filenames one-by-one lazily. (Although then
> you create a lot of small allocations instead of a single large one.

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