Here's a patch to fix it. I don't know if it's the right fix or not.
Created attachment 49 [details] [review] patch
I meant read-only, obviously.
I think the real problem is that $HOME isn't set. Fontconfig is supposed to survive this case, but the attached patch demonstrates a case where this isn't true. I found another issue with this same case.
I've audited uses of config->cache and other uses of $HOME and I think this was the only remaining problem. I've committed patches for this to both HEAD and the 2.2 branch.
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