Implemented all std::os::fd traits for mqueue::MqdT.

[coderBlitz]

Since linux allows polling message queues ¹, these changes enable that use case. Targeted to linux, since it was the only one I was able to confirm ².

Footnotes

1. mq_overview(7) ↩

2. https://elixir.bootlin.com/linux/latest/source/ipc/mqueue.c#L908 ↩

[asomers]

Why only for Linux? Are you sure these traits shouldn't apply more widely?

[coderBlitz]

I don't have any reason to artificially limit it, as I mentioned originally it was based on my working knowledge of the mqueue POSIX API and the linux implementation of it. Per the POSIX API (see [1] above), there are no guarantees that mqd_t is a file descriptor, or even an identically-sized integer; the linux implementation guarantees a valid file descriptor.

At its broadest, the API could be enabled for all unix per the Rust standard lib (see below for exceptions):

// unix
#[cfg(not(all(
    doc,
    any(
        all(target_arch = "wasm32", not(target_os = "wasi")),
        all(target_vendor = "fortanix", target_env = "sgx")
    )
)))]
#[cfg(target_os = "hermit")]
#[path = "hermit/mod.rs"]
pub mod unix;
#[cfg(not(all(
    doc,
    any(
        all(target_arch = "wasm32", not(target_os = "wasi")),
        all(target_vendor = "fortanix", target_env = "sgx")
    )
)))]
#[cfg(all(not(target_os = "hermit"), any(unix, doc)))]
pub mod unix;

However, there appear to be 2 libc families which would not satisfy the type guarantees¹: freebsdlike and solarish. The rest use the c_int type, so those should all theoretically compile without issue. In my semi-cursory searches, all of the c_int types seem to alias to i32, which matches the RawFd alias, so everything points to those compiling fine.

Footnotes

1. https://github.com/search?q=repo%3Arust-lang%2Flibc+mqd_t&type=code ↩

[asomers]

Man pages suggest that select and poll are supported for message queues on NetBSD and DragonflyBSD. I'm very surprised to see that libc doesn't even define mq_open on Android, so we can't do it there. On FreeBSD select and poll are supported but through a different mechanism: one must first use mq_getfd_np to get the file descriptor. So I think you should enable this for Linux, NetBSD, and DragonflyBSD.

[coderBlitz]

I'm slightly concerned the dragonfly build succeeded, given mqd_t in the freebsdlike files is explicitly a pointer. @asomers Thoughts?

[asomers]

I'm slightly concerned the dragonfly build succeeded, given mqd_t in the freebsdlike files is explicitly a pointer. @asomers Thoughts?

I think you read that wrong. The mqd_t type isn't defined in freebsdlike/mod.rs. It's defined separately in freebsdlike/freebsd/mod.rs and freebsdlike/dragonfly/mod.rs.