musl: introduce musl_stat_timespec cfg for struct stat

[heiher]

Description

Some musl targets use out-of-line timespec fields in struct stat. This property is independent of the musl_v1_2_3 switch.

Add a new musl_stat_timespec cfg and use it to control the presence of inline time fields in musl struct stat definitions, avoiding incorrect API assumptions based on musl version alone.

Fixes #4939

Sources

• https://github.com/kraj/musl/tree/v1.2.5

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI

[JohnTitor]

The key idea sounds good to me, @tgross35 any thoughts on this? I haven't catched up the time64 change deeply and would like to hear your views.

[tgross35]

Thanks for the fix, sorry that this broke.

Quite honestly I'm feeling like we should just delete the timespec fields and leave st_atime/st_atime_nsec (et. al.) with a comment explaining why we don't exactly match musl source, i.e. more or less revert 55fa65b. Musl allows using the st_atime names via macros https://github.com/kraj/musl/blob/ff441c9ddfefbb94e5881ddd5112b24a944dc36c/include/sys/stat.h#L32C18-L34 so we'd just have a mismatch on the _nsec fields which I think is fine. I'm assuming musl keeps _nsec a bit more hidden since they're not part of posix https://pubs.opengroup.org/onlinepubs/009695299/basedefs/sys/stat.h.html.

That's also nicer for our users who won't need to do a different config for musl and glibc.

Cc @xbjfk for your thoughts as you authored the commit in question.