add musl and glibc wrappers for getdents{,64}

[cosmicexplorer]

Description

The method posix_getdents() has recently been added in POSIX issue 8 (from 2024). This method offers a standard interface to retrieve multiple directory entries at once into a preallocated buffer. This typically translates directly to a syscall using the existing SYS_getdents{,64} key.

However, as posix_getdents() is so new, it has not been universally implemented yet. musl added it immediately last year, but glibc does not yet have it. Instead, glibc >= 2.30 has the method getdents64(), which is Linux-only.

While musl appears to implement posix_getdents() for all supported platforms, it appears that the version of musl rust pulls in is not from the local machine, but from its own bundled copy. This copy does not have posix_getdents(), but instead simply has getdents64() to match glibc on linux.

So this PR only adds getdents64() on glibc linux, and getdents{,64}() on musl linux.

Sources

• musl (posix_getdents()): https://github.com/bminor/musl/blob/86373b4999bfd9a9379bc4a3ca877b1c80a2a340/src/dirent/posix_getdents.c#L6
• musl (getdents()): https://github.com/bminor/musl/blob/86373b4999bfd9a9379bc4a3ca877b1c80a2a340/src/linux/getdents.c#L6
• musl (getdents64()): https://github.com/bminor/musl/blob/86373b4999bfd9a9379bc4a3ca877b1c80a2a340/include/dirent.h#L80
• glibc (getdents64()): https://github.com/bminor/glibc/blob/7eed691cc2b6c5dbb6066ee1251606a744c7f05c/sysdeps/unix/sysv/linux/bits/dirent_ext.h#L29

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

@rustbot label +stable-nominated

[cosmicexplorer]

I do not understand why freebsd tests would be failing if I have not modified any code or semver tests for freebsd.

It appears the tests are failing on methods I have not modified (https://cirrus-ci.com/task/5573619788546048):

warning: [email protected]: /tmp/cirrus-ci-build/target/x86_64-unknown-freebsd/debug/build/libc-test-ec9ac07491fdefaa/out/main.c:33518:59: error: invalid application of 'sizeof' to an incomplete type 'struct xktls_session_onedir'

[cosmicexplorer]

Ok, now I'm getting a failure in a musl shard (aarch64-unknown-linux-musl) which I'm not sure about (https://github.com/rust-lang/libc/actions/runs/16041997001/job/45265389790?pr=4522):

warning: [email protected]: /checkout/target/aarch64-unknown-linux-musl/debug/build/libc-test-c7eda42317ad320c/out/main.c: In function '__test_size_posix_dent':
warning: [email protected]: /checkout/target/aarch64-unknown-linux-musl/debug/build/libc-test-c7eda42317ad320c/out/main.c:39376:56: error: invalid application of 'sizeof' to incomplete type 'struct posix_dent'
warning: [email protected]: 39376 |  uint64_t __test_size_posix_dent(void) { return sizeof(struct posix_dent); }

The posix_dent struct is definitely defined in musl (https://github.com/bminor/musl/blob/86373b4999bfd9a9379bc4a3ca877b1c80a2a340/include/dirent.h#L21-L27). I did add posix_dent to libc-test/semver/linux-musl.txt -- I'm not sure if I'm supposed to do that or not for struct definitions. I was not able to get the local musl testing working on my machine due to a failure in build.rs for libc-test (see OP), so I'm having difficulty debugging this on my own.

Any pointers on how one would typically debug this would be appreciated. I know the header linux/can/j1939.h exists within /usr/include on my machine, but the build.rs is not picking it up, causing a failure to build main.c when running cargo test within the libc-test subdir.

[cosmicexplorer]

Ok, after testing this locally against a separate project (since the libc-test crate build script continues to be broken and unable to find headers), I found that posix_getdents() was not found in the musl rust was linking against for some reason, even though my local musl definitely has posix_getdents(). I'm assuming rust is pulling in an earlier version of musl that's not the one on my machine, so I simply removed it for now. That means this only adds getdents64() on glibc for linux, and getdents{,64}() on musl for linux. I'll update the OP to match this.

[cosmicexplorer]

Ok, it looks like the freebsd shards are failing on main too, so I'm going to assume that's not because of my changes.