ev3dev-lang-rust 0.16.0

//! Utility functions for cpu efficient `wait` commands.
//! Uses the `libc::epoll_wait` that only works on linux systems.

#[cfg(target_os = "linux")]
use libc;
use std::os::unix::io::RawFd;
use std::time::{Duration, Instant};

/// Wait for until a condition `cond` is `true` or the `timeout` is reached.
/// If the `timeout` is `None` it will wait an infinite time.
/// The condition is checked when the `file` has changed.
///
/// # Arguments
/// * `file` - Listen to changes in this file
/// * `cond` - Condition that should become true
/// * `timeout` - Maximal timeout to wait for the condition or file changes
///
/// # Example
/// ```
/// use std::fs::File;
/// use std::os::unix::io::AsRawFd;
/// use std::time::Duration;
///
/// use ev3dev_lang_rust::wait;
///
/// if let Ok(file) = File::open("...") {
///     let cond = || {
///         // ...
///         true
///     };
///     let timeout = Duration::from_millis(2000);
///
///     wait::wait(file.as_raw_fd(), cond, Some(timeout));
/// }
/// ```
pub fn wait<F>(fd: RawFd, cond: F, timeout: Option<Duration>) -> bool
where
    F: Fn() -> bool,
{
    if cond() {
        return true;
    }

    let start = Instant::now();

    let mut t = timeout;

    loop {
        let wait_timeout = match t {
            Some(duration) => duration.as_millis() as i32,
            None => -1,
        };
        wait_file_changes(fd, wait_timeout);

        if let Some(duration) = timeout {
            let elapsed = start.elapsed();
            if elapsed >= duration {
                return false;
            }
            t = Some(duration - elapsed);
        }

        if cond() {
            return true;
        }
    }
}

/// Wrapper for `libc::epoll_wait`
#[cfg(target_os = "linux")]
fn wait_file_changes(fd: RawFd, timeout: i32) -> bool {
    let epfd = unsafe { libc::epoll_create(1) as i32 };
    let mut ep_event = libc::epoll_event {
        events: 0,
        u64: fd as u64,
    };

    let result = unsafe {
        libc::epoll_ctl(
            epfd,
            libc::EPOLL_CTL_ADD,
            fd,
            &mut ep_event as *mut libc::epoll_event,
        ) as i32
    };

    if result == -1 {
        // cannot register fd as epoll fd
        // just wait for 100ms
        std::thread::sleep(Duration::from_millis(100));
        return false;
    }

    let mut buf: [libc::epoll_event; 1] = [ep_event];

    // number of file descriptors ready for the requested I/O operation
    let result =
        unsafe { libc::epoll_wait(epfd, buf.as_mut_ptr(), buf.len() as i32, timeout) as i32 };

    let err = unsafe { libc::close(epfd) as i32 };
    if err == -1 {
        // epfd cannot be closed, at least we tried
    }

    result > 0
}

/// Stub method for non linux os's
#[cfg(not(target_os = "linux"))]
fn wait_file_changes(_fd: RawFd, _timeout: i32) -> bool {
    std::thread::sleep(Duration::from_millis(100));
    false
}