[Fontconfig] robustness of the mmap cache branch

[Patrick Lam]

Matthias Clasen wrote:
> I think FcGlobalCacheLoad() needs some robustness love. Just
> copy any text file to ~/.fonts.cache-2 and watch FcGlobalCacheLoad()
> go in an infinite loop:

Actually, FcCacheSkipToArch was looping.  I've fixed this by ensuring
that FcCacheSkipToArch always makes progress:

	if (FcCacheReadString (fd, candidate_arch_machine_name_count,
		sizeof (candidate_arch_machine_name_count)) == 0)
            return -1;
	if (!strlen(candidate_arch_machine_name_count))
	    return -1;
	bs = strtol(candidate_arch_machine_name_count, &candidate_arch, 16);

+	if (!bs || bs < strlen (candidate_arch_machine_name_count))
+	    return -1;

As you suggested, I also check the return values on read and lseek;
although they weren't the problem in this particular case, it's probably
still better to fix them.

Garbage .fonts.cache-2 files no longer trash fontconfig; it now
correctly regenerates the global cache file.

pat