Use standard library OwnedFd

[ids1024]

Currently dbus defines an OwnedFd type. Since a type with that name and purpose exists in std now, it would be best to use the standard type (the io-lifetimes crate can be used to support the same thing on older compilers, if needed).

This would be a breaking API change since the type is defined differently.

Looking into this, there are a few complications:

• dbus relies on the type implementing PartialOrd, while the standard OwnedFd doesn't. This could be addressed by a manual implementation of PartialOrd in types that contain a OwnedFd, but that's awkward.
• dbus also relies on Clone. While the standard library doesn't offer this and provides OwnedFd::try_clone since the operation may fail. Similar considerations for manual implementations.
• dbus provides a placeholder OwnedFd on non-Unix platforms. Maybe instead the UnixFd enum variant should only exist on Unix? Not sure if there are other complications.

Or without API break, or having to deal with these issue, implementing AsFd and From would offer better interoperability with the standard types, at least.

[diwic]

I think a first step could be to implement possible traits for std::os::fd::OwnedFd just like we do for std::fs::File.

Could you clarify what part of dbus relies on the type implementing PartialOrd and Clone? I checked Arg, RefArg, Append and Get and neither of those required PartialOrd and Clone.

[ids1024]

MessageItem has a UnixFd(OwnedFd) variant, and implements PartialOrd and Clone. Looks like removing PartialOrd from that as well as MessageItemDict and MessageItemArray compiles. But removing Clone leads to errors.

[diwic]

Ok, so MessageItem is really legacy and I don't care that much about it. Is MessageItem the primary place you want it in? Arg/Append/Get can take multiple types. RefArg has a single representation w r t what you get out when you read something, but can also be implemented for many types.

[ids1024]

Ah, I didn't see that was a legacy thing. I was just looking at what code in the repository breaks without the PartialOrd and Clone implementations on OwnedFd.

[ids1024]

So yeah, if implementing those traits for the standard library type is sufficient to use that with non-legacy APIs, that should be doable enough without any breaking API changes.

[diwic]

Hmm, so even if the API changes are not breaking, it does not seem easy to implement this without either bumping rust version or add additional dependencies...right?

[ids1024]

If it's just a a matter of implementing a few traits, io-lifetimes could be added as an optional dependency.

[ids1024]

#407 adds such an optional dependency.

[diwic]

See also #410

[diwic]

Ok, so I decided to remove the io-lifetimes feature and add a stdfd feature instead. I believe this fixes your issue. See #418