tokio can't listen for signals between SIGRTMIN and SIGRTMAX

[weisbrja]

Version
tokio v1.17.0

Platform
Linux thinkpad 5.16.11-arch1-1 #1 SMP PREEMPT Thu, 24 Feb 2022 02:18:20 +0000 x86_64 GNU/Linux

Description
I'm trying to listen to signals between libc::SIGRTMIN() and libc::SIGRTMAX(), which should be possible according to the POSIX standard (and it worked with the signal-hook crate).

I tried this code:

use tokio::signal::unix::{signal, SignalKind};

fn main() {
    let _signal = signal(SignalKind::from_raw(libc::SIGRTMIN() + 3)).unwrap();
}

And it panicked with "signal too large".

I tracked the error down with gdb, and the reason it fails, is because of this check in tokio/src/signal/unix.rs:

fn signal_enable(signal: SignalKind, handle: &Handle) -> io::Result<()> {
    // ...

    let globals = globals();
    let siginfo = match globals.storage().get(signal as EventId) {
        Some(slot) => slot,
        None => return Err(io::Error::new(io::ErrorKind::Other, "signal too large")),
    };

    // ...
}

[weisbrja]

I dug a bit in the source code and I think I found the issue.
The number of unix signals is defined in tokio/src/signal/unix.rs:

// Number of different unix signals
// (FreeBSD has 33)
const SIGNUM: usize = 33;

However, that is simply not true, because the POSIX standard allows for signals up to SIGRTMAX, which is greater than 33.

My proposed solution would be just replacing this:

// Number of different unix signals
// (FreeBSD has 33)
const SIGNUM: usize = 33;

impl Init for OsStorage {
    fn init() -> Self {
        (0..SIGNUM).map(|_| SignalInfo::default()).collect()
    }
}

With this:

impl Init for OsStorage {
    fn init() -> Self {
        (0..=libc::SIGRTMAX()).map(|_| SignalInfo::default()).collect()
    }
}

Any thoughts on this?