unix.rs

//! Unix-specific types for signal handling.
//!
//! This module is only defined on Unix platforms and contains the primary
//! `Signal` type for receiving notifications of signals.

#![cfg(unix)]
#![cfg_attr(docsrs, doc(cfg(all(unix, feature = "signal"))))]

use crate::signal::registry::{globals, EventId, EventInfo, Globals, Init, Storage};
use crate::signal::RxFuture;
use crate::sync::watch;

use mio::net::UnixStream;
use std::io::{self, Error, ErrorKind, Write};
use std::pin::Pin;
use std::sync::atomic::{AtomicBool, Ordering};
use std::sync::Once;
use std::task::{Context, Poll};

pub(crate) mod driver;
use self::driver::Handle;

pub(crate) type OsStorage = Vec<SignalInfo>;

impl Init for OsStorage {
    fn init() -> Self {
        // There are reliable signals ranging from 1 to 33 available on every Unix platform.
        #[cfg(not(target_os = "linux"))]
        let possible = 0..=33;

        // On Linux, there are additional real-time signals available.
        #[cfg(target_os = "linux")]
        let possible = 0..=libc::SIGRTMAX();

        possible.map(|_| SignalInfo::default()).collect()
    }
}

impl Storage for OsStorage {
    fn event_info(&self, id: EventId) -> Option<&EventInfo> {
        self.get(id).map(|si| &si.event_info)
    }

    fn for_each<'a, F>(&'a self, f: F)
    where
        F: FnMut(&'a EventInfo),
    {
        self.iter().map(|si| &si.event_info).for_each(f)
    }
}

#[derive(Debug)]
pub(crate) struct OsExtraData {
    sender: UnixStream,
    receiver: UnixStream,
}

impl Init for OsExtraData {
    fn init() -> Self {
        let (receiver, sender) = UnixStream::pair().expect("failed to create UnixStream");

        Self { sender, receiver }
    }
}

/// Represents the specific kind of signal to listen for.
#[derive(Debug, Clone, Copy, Hash, PartialEq, Eq)]
pub struct SignalKind(libc::c_int);

impl SignalKind {
    /// Allows for listening to any valid OS signal.
    ///
    /// For example, this can be used for listening for platform-specific
    /// signals.
    /// ```rust,no_run
    /// # use tokio::signal::unix::SignalKind;
    /// # let signum = -1;
    /// // let signum = libc::OS_SPECIFIC_SIGNAL;
    /// let kind = SignalKind::from_raw(signum);
    /// ```
    // Use `std::os::raw::c_int` on public API to prevent leaking a non-stable
    // type alias from libc.
    // `libc::c_int` and `std::os::raw::c_int` are currently the same type, and are
    // unlikely to change to other types, but technically libc can change this
    // in the future minor version.
    // See https://github.com/tokio-rs/tokio/issues/3767 for more.
    pub const fn from_raw(signum: std::os::raw::c_int) -> Self {
        Self(signum as libc::c_int)
    }

    /// Get the signal's numeric value.
    ///
    /// ```rust
    /// # use tokio::signal::unix::SignalKind;
    /// let kind = SignalKind::interrupt();
    /// assert_eq!(kind.as_raw_value(), libc::SIGINT);
    /// ```
    pub const fn as_raw_value(&self) -> std::os::raw::c_int {
        self.0
    }

    /// Represents the SIGALRM signal.
    ///
    /// On Unix systems this signal is sent when a real-time timer has expired.
    /// By default, the process is terminated by this signal.
    pub const fn alarm() -> Self {
        Self(libc::SIGALRM)
    }

    /// Represents the SIGCHLD signal.
    ///
    /// On Unix systems this signal is sent when the status of a child process
    /// has changed. By default, this signal is ignored.
    pub const fn child() -> Self {
        Self(libc::SIGCHLD)
    }

    /// Represents the SIGHUP signal.
    ///
    /// On Unix systems this signal is sent when the terminal is disconnected.
    /// By default, the process is terminated by this signal.
    pub const fn hangup() -> Self {
        Self(libc::SIGHUP)
    }

    /// Represents the SIGINFO signal.
    ///
    /// On Unix systems this signal is sent to request a status update from the
    /// process. By default, this signal is ignored.
    #[cfg(any(
        target_os = "dragonfly",
        target_os = "freebsd",
        target_os = "macos",
        target_os = "netbsd",
        target_os = "openbsd"
    ))]
    pub const fn info() -> Self {
        Self(libc::SIGINFO)
    }

    /// Represents the SIGINT signal.
    ///
    /// On Unix systems this signal is sent to interrupt a program.
    /// By default, the process is terminated by this signal.
    pub const fn interrupt() -> Self {
        Self(libc::SIGINT)
    }

    /// Represents the SIGIO signal.
    ///
    /// On Unix systems this signal is sent when I/O operations are possible
    /// on some file descriptor. By default, this signal is ignored.
    pub const fn io() -> Self {
        Self(libc::SIGIO)
    }

    /// Represents the SIGPIPE signal.
    ///
    /// On Unix systems this signal is sent when the process attempts to write
    /// to a pipe which has no reader. By default, the process is terminated by
    /// this signal.
    pub const fn pipe() -> Self {
        Self(libc::SIGPIPE)
    }

    /// Represents the SIGQUIT signal.
    ///
    /// On Unix systems this signal is sent to issue a shutdown of the
    /// process, after which the OS will dump the process core.
    /// By default, the process is terminated by this signal.
    pub const fn quit() -> Self {
        Self(libc::SIGQUIT)
    }

    /// Represents the SIGTERM signal.
    ///
    /// On Unix systems this signal is sent to issue a shutdown of the
    /// process. By default, the process is terminated by this signal.
    pub const fn terminate() -> Self {
        Self(libc::SIGTERM)
    }

    /// Represents the SIGUSR1 signal.
    ///
    /// On Unix systems this is a user defined signal.
    /// By default, the process is terminated by this signal.
    pub const fn user_defined1() -> Self {
        Self(libc::SIGUSR1)
    }

    /// Represents the SIGUSR2 signal.
    ///
    /// On Unix systems this is a user defined signal.
    /// By default, the process is terminated by this signal.
    pub const fn user_defined2() -> Self {
        Self(libc::SIGUSR2)
    }

    /// Represents the SIGWINCH signal.
    ///
    /// On Unix systems this signal is sent when the terminal window is resized.
    /// By default, this signal is ignored.
    pub const fn window_change() -> Self {
        Self(libc::SIGWINCH)
    }
}

impl From<std::os::raw::c_int> for SignalKind {
    fn from(signum: std::os::raw::c_int) -> Self {
        Self::from_raw(signum as libc::c_int)
    }
}

impl From<SignalKind> for std::os::raw::c_int {
    fn from(kind: SignalKind) -> Self {
        kind.as_raw_value()
    }
}

pub(crate) struct SignalInfo {
    event_info: EventInfo,
    init: Once,
    initialized: AtomicBool,
}

impl Default for SignalInfo {
    fn default() -> SignalInfo {
        SignalInfo {
            event_info: Default::default(),
            init: Once::new(),
            initialized: AtomicBool::new(false),
        }
    }
}

/// Our global signal handler for all signals registered by this module.
///
/// The purpose of this signal handler is to primarily:
///
/// 1. Flag that our specific signal was received (e.g. store an atomic flag)
/// 2. Wake up the driver by writing a byte to a pipe
///
/// Those two operations should both be async-signal safe.
fn action(globals: Pin<&'static Globals>, signal: libc::c_int) {
    globals.record_event(signal as EventId);

    // Send a wakeup, ignore any errors (anything reasonably possible is
    // full pipe and then it will wake up anyway).
    let mut sender = &globals.sender;
    drop(sender.write(&[1]));
}

/// Enables this module to receive signal notifications for the `signal`
/// provided.
///
/// This will register the signal handler if it hasn't already been registered,
/// returning any error along the way if that fails.
fn signal_enable(signal: SignalKind, handle: &Handle) -> io::Result<()> {
    let signal = signal.0;
    if signal < 0 || signal_hook_registry::FORBIDDEN.contains(&signal) {
        return Err(Error::new(
            ErrorKind::Other,
            format!("Refusing to register signal {}", signal),
        ));
    }

    // Check that we have a signal driver running
    handle.check_inner()?;

    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")),
    };
    let mut registered = Ok(());
    siginfo.init.call_once(|| {
        registered = unsafe {
            signal_hook_registry::register(signal, move || action(globals, signal)).map(|_| ())
        };
        if registered.is_ok() {
            siginfo.initialized.store(true, Ordering::Relaxed);
        }
    });
    registered?;
    // If the call_once failed, it won't be retried on the next attempt to register the signal. In
    // such case it is not run, registered is still `Ok(())`, initialized is still `false`.
    if siginfo.initialized.load(Ordering::Relaxed) {
        Ok(())
    } else {
        Err(Error::new(
            ErrorKind::Other,
            "Failed to register signal handler",
        ))
    }
}

/// A stream of events for receiving a particular type of OS signal.
///
/// In general signal handling on Unix is a pretty tricky topic, and this
/// structure is no exception! There are some important limitations to keep in
/// mind when using `Signal` streams:
///
/// * Signals handling in Unix already necessitates coalescing signals
///   together sometimes. This `Signal` stream is also no exception here in
///   that it will also coalesce signals. That is, even if the signal handler
///   for this process runs multiple times, the `Signal` stream may only return
///   one signal notification. Specifically, before `poll` is called, all
///   signal notifications are coalesced into one item returned from `poll`.
///   Once `poll` has been called, however, a further signal is guaranteed to
///   be yielded as an item.
///
///   Put another way, any element pulled off the returned stream corresponds to
///   *at least one* signal, but possibly more.
///
/// * Signal handling in general is relatively inefficient. Although some
///   improvements are possible in this crate, it's recommended to not plan on
///   having millions of signal channels open.
///
/// If you've got any questions about this feel free to open an issue on the
/// repo! New approaches to alleviate some of these limitations are always
/// appreciated!
///
/// # Caveats
///
/// The first time that a `Signal` instance is registered for a particular
/// signal kind, an OS signal-handler is installed which replaces the default
/// platform behavior when that signal is received, **for the duration of the
/// entire process**.
///
/// For example, Unix systems will terminate a process by default when it
/// receives SIGINT. But, when a `Signal` instance is created to listen for
/// this signal, the next SIGINT that arrives will be translated to a stream
/// event, and the process will continue to execute. **Even if this `Signal`
/// instance is dropped, subsequent SIGINT deliveries will end up captured by
/// Tokio, and the default platform behavior will NOT be reset**.
///
/// Thus, applications should take care to ensure the expected signal behavior
/// occurs as expected after listening for specific signals.
///
/// # Examples
///
/// Wait for SIGHUP
///
/// ```rust,no_run
/// use tokio::signal::unix::{signal, SignalKind};
///
/// #[tokio::main]
/// async fn main() -> Result<(), Box<dyn std::error::Error>> {
///     // An infinite stream of hangup signals.
///     let mut stream = signal(SignalKind::hangup())?;
///
///     // Print whenever a HUP signal is received
///     loop {
///         stream.recv().await;
///         println!("got signal HUP");
///     }
/// }
/// ```
#[must_use = "streams do nothing unless polled"]
#[derive(Debug)]
pub struct Signal {
    inner: RxFuture,
}

/// Creates a new stream which will receive notifications when the current
/// process receives the specified signal `kind`.
///
/// This function will create a new stream which binds to the default reactor.
/// The `Signal` stream is an infinite stream which will receive
/// notifications whenever a signal is received. More documentation can be
/// found on `Signal` itself, but to reiterate:
///
/// * Signals may be coalesced beyond what the kernel already does.
/// * Once a signal handler is registered with the process the underlying
///   libc signal handler is never unregistered.
///
/// A `Signal` stream can be created for a particular signal number
/// multiple times. When a signal is received then all the associated
/// channels will receive the signal notification.
///
/// # Errors
///
/// * If the lower-level C functions fail for some reason.
/// * If the previous initialization of this specific signal failed.
/// * If the signal is one of
///   [`signal_hook::FORBIDDEN`](fn@signal_hook_registry::register#panics)
///
/// # Panics
///
/// This function panics if there is no current reactor set, or if the `rt`
/// feature flag is not enabled.
#[track_caller]
pub fn signal(kind: SignalKind) -> io::Result<Signal> {
    let rx = signal_with_handle(kind, &Handle::current())?;

    Ok(Signal {
        inner: RxFuture::new(rx),
    })
}

pub(crate) fn signal_with_handle(
    kind: SignalKind,
    handle: &Handle,
) -> io::Result<watch::Receiver<()>> {
    // Turn the signal delivery on once we are ready for it
    signal_enable(kind, handle)?;

    Ok(globals().register_listener(kind.0 as EventId))
}

impl Signal {
    /// Receives the next signal notification event.
    ///
    /// `None` is returned if no more events can be received by this stream.
    ///
    /// # Cancel safety
    ///
    /// This method is cancel safe. If you use it as the event in a
    /// [`tokio::select!`](crate::select) statement and some other branch
    /// completes first, then it is guaranteed that no signal is lost.
    ///
    /// # Examples
    ///
    /// Wait for SIGHUP
    ///
    /// ```rust,no_run
    /// use tokio::signal::unix::{signal, SignalKind};
    ///
    /// #[tokio::main]
    /// async fn main() -> Result<(), Box<dyn std::error::Error>> {
    ///     // An infinite stream of hangup signals.
    ///     let mut stream = signal(SignalKind::hangup())?;
    ///
    ///     // Print whenever a HUP signal is received
    ///     loop {
    ///         stream.recv().await;
    ///         println!("got signal HUP");
    ///     }
    /// }
    /// ```
    pub async fn recv(&mut self) -> Option<()> {
        self.inner.recv().await
    }

    /// Polls to receive the next signal notification event, outside of an
    /// `async` context.
    ///
    /// This method returns:
    ///
    ///  * `Poll::Pending` if no signals are available but the channel is not
    ///    closed.
    ///  * `Poll::Ready(Some(()))` if a signal is available.
    ///  * `Poll::Ready(None)` if the channel has been closed and all signals
    ///    sent before it was closed have been received.
    ///
    /// # Examples
    ///
    /// Polling from a manually implemented future
    ///
    /// ```rust,no_run
    /// use std::pin::Pin;
    /// use std::future::Future;
    /// use std::task::{Context, Poll};
    /// use tokio::signal::unix::Signal;
    ///
    /// struct MyFuture {
    ///     signal: Signal,
    /// }
    ///
    /// impl Future for MyFuture {
    ///     type Output = Option<()>;
    ///
    ///     fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
    ///         println!("polling MyFuture");
    ///         self.signal.poll_recv(cx)
    ///     }
    /// }
    /// ```
    pub fn poll_recv(&mut self, cx: &mut Context<'_>) -> Poll<Option<()>> {
        self.inner.poll_recv(cx)
    }
}

// Work around for abstracting streams internally
pub(crate) trait InternalStream {
    fn poll_recv(&mut self, cx: &mut Context<'_>) -> Poll<Option<()>>;
}

impl InternalStream for Signal {
    fn poll_recv(&mut self, cx: &mut Context<'_>) -> Poll<Option<()>> {
        self.poll_recv(cx)
    }
}

pub(crate) fn ctrl_c() -> io::Result<Signal> {
    signal(SignalKind::interrupt())
}

#[cfg(all(test, not(loom)))]
mod tests {
    use super::*;

    #[test]
    fn signal_enable_error_on_invalid_input() {
        signal_enable(SignalKind::from_raw(-1), &Handle::default()).unwrap_err();
    }

    #[test]
    fn signal_enable_error_on_forbidden_input() {
        signal_enable(
            SignalKind::from_raw(signal_hook_registry::FORBIDDEN[0]),
            &Handle::default(),
        )
        .unwrap_err();
    }

    #[test]
    fn from_c_int() {
        assert_eq!(SignalKind::from(2), SignalKind::interrupt());
    }

    #[test]
    fn into_c_int() {
        let value: std::os::raw::c_int = SignalKind::interrupt().into();
        assert_eq!(value, libc::SIGINT as _);
    }
}