Fix Clippy warnings on FreeBSD with the latest nightly

[asomers]

• Better type safety for mqueue
• Suppress clippy::not_unsafe_ptr_arg_deref warnings in ptrace on BSD

[asomers]

Supersedes #1638

[rtzoeller]

bors r+

[bors]

Build succeeded:

• Android aarch64
• Android arm
• Android armv7
• Android i686
• Android x86_64
• DragonFly BSD x86_64
• FreeBSD amd64 & i686
• Fuchsia x86_64
• Illumos
• iOS aarch64
• iOS x86_64
• Linux aarch64
• Linux arm gnueabi
• Linux arm-musleabi
• Linux armv7 gnueabihf
• Linux i686
• Linux i686 musl
• Linux MIPS
• Linux MIPS64
• Linux MIPS64 el
• Linux mipsel
• Linux powerpc
• Linux powerpc64
• Linux powerpc64le
• Linux s390x
• Linux x32
• Linux x86_64
• Linux x86_64 musl
• Minver
• NetBSD x86_64
• OpenBSD x86_64
• OSX x86_64
• Redox x86_64
• Rust Stable

[lucab]

POSIX does explicitly mention that A message queue descriptor may be implemented using a file descriptor, which I think is the case at least on Linux.
Before this, it used to be possible to check an existing open file-descriptor via getattr and then convert it to a message queue descriptor. Would it be possible to get that functionality back, on specific platforms? How should that look like?

[asomers]

@lucab could you give an example in Rust, C, or pseudocode? How would that feature work?

[lucab]

Sure, and thanks for the prompt reply!
The initial context here is systemd, which is a Linux-only service manager.
It supports listening on a message queue (or socket/FIFO/etc.) on behalf of a service and activating it on demand, see https://www.freedesktop.org/software/systemd/man/systemd.socket.html#ListenMessageQueue=.
The activation mechanism works in a way such that systemd performs the mq_open(), then later passes the open file-descriptor to the spawned service/process.
The spawned process here would be a Rust binary, and we have a library method is_mq() to check the inherited FD. It is built on top of the previous design of nix::mqueue::mq_getattr(), see https://docs.rs/libsystemd/latest/libsystemd/activation/trait.IsType.html#method.is_mq.
Overall I'm not particularly attached to the existing code/design, but I feel that we are now missing primitives for fallible conversion between a file-descriptor and a mqueue-descriptor.

[rtzoeller]

I feel that we are now missing primitives for fallible conversion between a file-descriptor and a mqueue-descriptor.

I think it's worth noting that the conversion itself was infallible. You could end up with an invalid mqueue-descriptor, as libsystemd handled, but the conversion still succeeded. I don't think nix can make the conversion fallible without performing a similar syscall, and don't think it needs to.

std::os::unix::io::FromRawFd is surprising unhelpful for you, as it takes ownership and is_mq() takes an immutable reference. Consequently I think we'll need to implement at least std::convert::From.

Implementing std::os::unix::io::AsRawFd and std::os::unix::io::IntoRawFd gets you from an MqdT to a RawFd (which isn't what you need), but IMO is worth doing for symmetry.

We should probably implement all four traits on Linux and any other platforms that use FDs instead of pointers for mqd_t, unless @asomers sees a reason not to.