FreeBSD: add pthread_getthreadid_np()

[kostikbel]

What does this PR do

The process::gettid() is missing on FreeBSD.
The PR adds its implementation.

The implementation returns the real tid of the caller's thread, it does
not try to mimic the Linux behavior where initial (main) thread id is
same as the processs id. This might be patched if needed, but pids and
tids are in different namespace on FreeBSD, so better keep it as is
unless there is a pressure.

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API

[SteveLauC]

Thanks for the PR. A changelog entry is needed, BTW, see https://github.com/nix-rust/nix/blob/master/CONTRIBUTING.md#pull-requests

[SteveLauC]

Suggested change

	#[cfg(freebsd)]
	#[cfg(target_os = "freebsd")]

We only use the alias when it contains more than 2 OSes

[SteveLauC]

Suggested change

	#[inline]
	#[inline]
	#[doc(alias = "pthread_getthreadid_np")]

Maybe we should add an alias 👀

[SteveLauC]

If we use its own name, this alias is not needed.

See the below comment

[SteveLauC]

Open question, should we call it gettid(), i.e., follow the Linux convention here, or we should use its own name?

I am slightly inclined to use its original name as Nix, in most cases, only exposes system stuff in Rusty way, we do not wrap them

[kostikbel]

I added it as gettid() because this is what some consumers need, that depend on nix with features = ["process"]. Using pthread_gettreadid_np() there would not allow e.g. perfetto-recorder to compile.

[SteveLauC]

Using pthread_gettreadid_np() there would not allow e.g. perfetto-recorder to compile.

Well, this is unfortunate. But Nix does not provide cross-platform wrappers. Typically, they will be provided by crates that depend on Nix. So I still think we should use the original name: pthread_getthreadid_np :>

[asomers]

I agree. In the past Nix tried to provide a measure of portability. But it didn't work well. It's better when concerns are separated: Nix provides Rusty interfaces to OS APIs, and other crates atop Nix provide cross-platform abstractions.

[kostikbel]

Without this specific name, the patch is not very useful. The delivery path become this, then wait for next nix release, then push the change to that crate, then wait for its release.

[asomers]

Well, I can make a release right now, if you need one.

[kostikbel]

Not that I need one, but it would be nice to have, some day.

[kostikbel]

Thanks for the PR. A changelog entry is needed, BTW, see https://github.com/nix-rust/nix/blob/master/CONTRIBUTING.md#pull-requests

All done, thanks for the review.

[SteveLauC]

Thanks!