*Flags don't implement set-like combinators

[cyphar]

One thing I ran into almost immediately when trying to nicely define a set of flags like you would in C (with something like CLONE_NEWUSER|CLONE_NEWUTS|CLONE_NEWNS or whatever) is that you can't do it with the nix structs without declaring a variable mut and operating on it.

let mut sigmask = signal::SigSet::empty();
sigmask.add(signal::Signal::SIGTTOU);
sigmask.add(signal::Signal::SIGTTIN);
// etc

What if we made it so you could do this instead?

let sigmask = signal::SigSet::empty()
                             .add(signal::Signal::SIGTTOU)
                             .add(signal::Signal::SIGTTIN);

Which allows you to write it (IMO) more concisely but also doesn't mean that you have to make the variable mutable. It also better matches the Iterator combinators.

[Susurrus]

You should be able to do

let sigmask = signal::Signal::SIGTTOU | signal::Signal::SIGTTIN;

Does that not work for you?

[posborne]

Closing as I'm assuming this meets or exceeds @cyphar's requirements.

[cyphar]

Sorry for not responding earlier. No, this code doesn't work:

   Compiling initrs v0.0.0 (file:///home/cyphar/.local/src/initrs)
error[E0369]: binary operation `|` cannot be applied to type `nix::sys::signal::Signal`
   --> src/main.rs:117:19
    |
117 |     let sigmask = signal::Signal::SIGTTOU | signal::Signal::SIGTTIN;
    |                   ^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: an implementation of `std::ops::BitOr` might be missing for `nix::sys::signal::Signal`

error: aborting due to previous error

error: Could not compile `initrs`.

My Cargo.toml:

[dependencies.nix]
version = "~0.8.1"
default-features = true
features = ["signalfd"]

[cyphar]

But the solution would be to implement BitOr for SigSet and also implementing it for Signal such that it produces a SigSet.

[cyphar]

I've implemented this in #615, as well as adding a From impl for SigSet::from(Signal).