signal: use eventfd instead of UnixStream for signal notification

[winter-loo]

Motivation

According to the Linux eventfd doc:

Applications can use an eventfd file descriptor instead of a pipe
(see pipe(2)) in all cases where a pipe is used simply to signal
events. The kernel overhead of an eventfd file descriptor is much
lower than that of a pipe, and only one file descriptor is
required (versus the two required for a pipe).

So, I made an attempt to replace mio::UnixStream with libc::eventfd.

Solution

The benchmark(RUSTFLAGS="" cargo bench -p benches --bench signal) shows some improvements:

1. On arch linux, the performance improvement is about 5%.
2. On ubuntu inside WSL 2, the performance gain is about 15%
3. On centos 7, the performance gain is about 10%

[ipetkov]

Instead of panicking can we return an error here?

[ipetkov]

Same comment, this is a fallible operation, we should return an error here, not panic

[winter-loo]

Thanks for review. It's time to make it better.

[Darksonn]

Comparing with what mio does, this seems to not have a bunch of logic to reset the eventfd and so on... Is it missing here?

[winter-loo]

A read on fd returned by eventfd resets eventfd. Here's libc::eventfd_read in Receiver::read() function.

[vilgotf]

Drive by review: I think you should store Result<Globals, i32> like I did for the per-signal result storage. You can then convert the i32 to an error via Error::from_raw_os_error and avoid allocating.

[martin-g]

This could be merged with the impl above.

[martin-g]

Since it is non-blocking it may return EAGAIN (ErrorKind::WouldBlock) which is not an error.

    let err = std::io::Error::last_os_error();
    // On a non-blocking eventfd, it is expected to get an EAGAIN
    // when the counter is 0.
    if err.kind() == std::io::ErrorKind::WouldBlock {
        Ok(0)
    } else {
        Err(err)
    }

[martin-g]

This impl could be merged with the one above

[martin-g]

There is a small different between the eventfd and unixstream impls here.
The eventfd's Sender::write() returns 0 on success. The UnixStream returns the number of written bytes (1).
AFAIS the successful result is not used.
Maybe change eventfd to return 1 too or change both to return Result<()> ?!