Check directory permissions before creating / deleting .uuid files
Gentoo packages are built under dedicated non-root user and in an LD_PRELOAD-based sandbox which catches attempts to write outside of temporary build directory and stops build process.
Since Fontconfig 2.13.1, when some Fontconfig tool or library is used and empty directories in /usr/share/fonts exist which are not writable by non-root users, then Fontconfig will try to create and next delete .uuid files in these directories, which is not permitted by sandbox.
For example, output of building of libical with -DICAL_BUILD_DOCS=yes might contain:
...
Running dot...
Generating dot graphs using 3 parallel threads...
Running dot for graph 1/103
Running dot for graph 2/103
Running dot for graph 3/103
* ACCESS DENIED: mkostemp: /usr/share/fonts/100dpi/.uuid.TMP-XXXXXX
* ACCESS DENIED: mkostemp: /usr/share/fonts/100dpi/.uuid.TMP-XXXXXX
* ACCESS DENIED: mkostemp: /usr/share/fonts/100dpi/.uuid.TMP-XXXXXX
* * ACCESS DENIED: unlink: /usr/share/fonts/100dpi/.uuid
* ACCESS DENIED: unlink: /usr/share/fonts/100dpi/.uuid
ACCESS DENIED: unlink: /usr/share/fonts/100dpi/.uuid
* ACCESS DENIED: mkostemp: /usr/share/fonts/75dpi/.uuid.TMP-XXXXXX
* ACCESS DENIED: mkostemp: /usr/share/fonts/75dpi/.uuid.TMP-XXXXXX
* ACCESS DENIED: mkostemp: /usr/share/fonts/75dpi/.uuid.TMP-XXXXXX
* ACCESS DENIED: unlink: /usr/share/fonts/75dpi/.uuid
* ACCESS DENIED: unlink: /usr/share/fonts/75dpi/.uuid
* ACCESS DENIED: unlink: /usr/share/fonts/75dpi/.uuid
...I think that the proper solution is to make Fontconfig check for writeability of directories before trying to create/delete .uuid files. Additionally there is no need to try to delete .uuid files when they do not exist.
I attach the patch which implements this solution in FcDirCacheCreateUUID() and FcDirCacheDeleteUUID() functions.
(Also I should mention that I have backported commits ff5b49be and 5f5ec567, but they do not help in this specific issue.)
Example Gentoo bug: https://bugs.gentoo.org/666540