'-MF /dev/null' fails due to file rename: ccache: error: Failed to create temporary file for /dev/null.tmp: Permission denied

[trofi]

Minimal reproducer:

$ ccache /usr/bin/x86_64-pc-linux-gnu-gcc -O2 -pipe -c -MF /dev/null -MQ /dev/null -MMD -MP -x c /dev/null -o /dev/null
ccache: error: Failed to create temporary file for /dev/null.tmp: Permission denied

$ /usr/bin/x86_64-pc-linux-gnu-gcc -O2 -pipe -c -MF /dev/null -MQ /dev/null -MMD -MP -x c /dev/null -o /dev/null
<ok>

Originally observed when built git-2.21.0 using ccache-3.7. git seems to probe -MF feature support this way:
https://github.com/git/git/blob/14c0f8d3ab6c36672189cd2dd217f4617d12ccba/Makefile#L1220-L1224

[afbjorklund]

We have some checks for -o /dev/null, probably need to extend those for -MF /dev/null too

[afbjorklund]

Even better would be to have some release tests for high-profile projects, such as kernel or git.

[jrosdahl]

Thanks for the report.

Just a note on the reproducer: ccache /usr/bin/gcc -O2 -pipe -c -MF /dev/null -MQ /dev/null -MMD -MP -x c /dev/null -o /dev/null is not enough to trigger the problem with an empty ccache, but ccache /usr/bin/gcc -O2 -pipe -c -MF /dev/null -MQ /dev/null -MMD -MP -x c /dev/null (without -o /dev/null) is.

[afbjorklund]

There might be a slight problem with the implementation, I think we are skipping the name of the output file when we hash the arguments ? That might mean that we could get a cache hit, that is missing the dep ?

[afbjorklund]

If that is indeed the case, then we should make sure to hash "/dev/null" (as a special case). Will add a test for it to verify, just something that I missed when I looked it through - similar to that logging and relpath...