implement better availability probing for copy_file_range

[the8472]

Followup to #75428 (comment)

Previously syscall detection was overly pessimistic. Any attempt to copy to an immutable file (EPERM) would disable copy_file_range support for the whole process.

The change tries to copy_file_range on invalid file descriptors which will never run into the immutable file case and thus we can clearly distinguish syscall availability.

[rust-highfive]

r? @withoutboats

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[Dylan-DPC-zz]

r? @nagisa

[nagisa]

Broadly LGTM. See inline comments for small nits. The constant magic number thing is the only blocker.

[nagisa]

Can you add a short blurb explaining why -1 is the descriptor we want to use here (and why it is guaranteed to be invalid)? Maybe even extract the magic number into constant? Maybe like this:

/// Invalid file descriptor.////// Valid file descriptors on UNIX are specified to be positive as per e.g. documentation of `open`.constINVALID_FD:_ = -1;

[nagisa]

Style nit: This match can probably be more concisely written as such:

let available = matches!(cvt(...).map_err(|e| e.raw_os_error()),Err(Some(libc::EBADF)))

If you decide you want to continue using match, I would suggest extracting the expression passed to match into a separate variable, because match COMPLEX_EXPRESSION { ... } tends to look pretty bad when auto-formatted.

let result = unsafe{cvt(...)};let available = match result { ...}

[the8472]

The review items should be addressed now.

[the8472]

Yeah, didn't run tests before pushing. One moment.

[nagisa]

@bors r+

[bors]

📌 Commit 7647d03 has been approved by nagisa

[bors]

⌛ Testing commit 7647d03 with merge e413d89...

[bors]

☀️ Test successful - checks-actions
Approved by: nagisa
Pushing e413d89 to master...