feat: introduce macos mount API support

[oowl]

What does this PR do

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

[oowl]

I do not know how to write the MacOS test because MacOS can not support tmpfs and nullfs, Is there any workaround for the MacOS mount test? cc @asomers @SteveLauC

[SteveLauC]

Hi, thanks for your interest in contributing to Nix! I have left some comments.

And:

1. A changelog entry is needed, please take a look at doc on how to add one.
2. There are still some #[cfg(apple_targets)] attributes in bsd.rs, I think we should remove them.

[SteveLauC]

For pointer casting, we prefer .cast() and .cast_mut() over the as keyword:

Suggested change

	d as *mut libc::c_void,
	d.cast().cast_mut(),

BTW, we are casting an immutable reference (&P3) to a muable pinter (*mut c_void), is this safe?

[oowl]

libc:mount() needs to recv libc:c_void type, It's hard to cast type to libc:c_void by the cast function, So I have changed it to d.cast_mut() as *mut libc::c_void,

[SteveLauC]

It's hard to cast type to libc:c_void by the cast function, So I have changed it to d.cast_mut() as *mut libc::c_void,

Yeah, .cast().cast_mut() should not work as the pointee type is unknown, .cast_mut().cast() should work.

BTW, we are casting an immutable reference (&P3) to a muable pinter (*mut c_void), is this safe?

Will mount(2) write to that pointer, if so, then a UB could happen:<

[oowl]

From https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man2/mount.2.html man page

 int
 mount(const char *type, const char *dir, int flags, void *data);

 int
 unmount(const char *dir, int flags);

   Data is a pointer to a structure that contains the type specific argu-ments arguments
   ments to mount.  The format for these argument structures is described in
   the manual page for each filesystem.

I think it's not be written, but nobody knows how Apple doing I think.

[SteveLauC]

I do not know how to write the MacOS test because MacOS can not support tmpfs and nullfs, Is there any workaround for the MacOS mount test?

For tests, I think we can just do something simple to ensure that the binding works, e.g., using the following stupid code:

$ cat src/main.rs
use nix::{errno::Errno, mount::{mount, MntFlags}};

fn main() {
    let res = mount::<str, str, str>("", "", MntFlags::empty(), None);
    assert_eq!(res, Err(Errno::ENOENT));
}

$ cargo r -q
mounting "" to ""

$ echo $?
0

---

I think we should create a directory test_mount under test and move the current test_mount.rs file to linux.rs, then add a dedicated apple.rs file under test_mount.

[oowl]

@SteveLauC I have made some changes depending on your comment, Let's continue.

[SteveLauC]

Thx!

[Xuanwo]

Hello, @SteveLauC, is there a plan to get this PR released?

[SteveLauC]

Hello, @SteveLauC, is there a plan to get this PR released?

Personally, I don't have a plan to do the next release in the near future. Do you guys need to patch? If so, can you use a git dependency?

[Xuanwo]

Personally, I don't have a plan to do the next release in the near future.

Is there a specific reason? I believe it's just a patch update that should have minimal impact.

Do you guys need to patch?

Yes.

If so, can you use a git dependency?

No, we can't release with git dependency.

[SteveLauC]

Is there a specific reason?

Not a strong reason, I simply want to get more work done in a release.

I believe it's just a patch update that should have minimal impact.

Yeah, I agree, Nix always releases on demand, so I guess I will do a release as requested.

[Xuanwo]

Yeah, I agree, Nix always releases on demand, so I guess I will do a release as requested.

That will be very helpful!