base_dir does not play well with Clang relocatable PCH

[vient]

How to reproduce

1. touch pch.h
2. clang-16 -isysroot ${PWD} --relocatable-pch -x c++-header pch.h -o pch1.pch
3. CCACHE_BASEDIR=${PWD} ccache clang-16 -isysroot ${PWD} --relocatable-pch -x c++-header pch.h -o pch2.pch

Actual behavior

pch1.pch is relocatable relative to current directory, i.e. it contains relative path pch.h. pch2.pch is not relocatable because ccache modified isysroot argument to . which Clang then failed to match header path against.

$ grep -r "$(realpath pch.h)" .
Binary file ./pch2.pch matches

I am not 100% sure that this is ccache's fault instead of clang's. I think there might be situations where you may want PCH to be relocated against relative path, will clarify it with LLVM devs.

Expected behavior

Produced PCH files are binary identical (which they are if you do not pass CCACHE_BASEDIR).

Environment

ccache version 4.8

[vient]

LLVM confirmed that isysroot is not resolved in any way so ccache should not modify it when "relocatable-pch" is seen.

It seems that proper way would be to use relative isysroot in caching key though, so the same PCH will get a cache hit when compiled from different folder but changing isysroot will not result in false cache hit.

[jrosdahl]

Would you be interested in working on a proposed fix?

[vient]

Yes, I'm already doing it but it may take some time.