base/src/wait_context.rs

// Copyright 2020 The ChromiumOS Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

use std::time::Duration;

pub use base_event_token_derive::*;
use smallvec::SmallVec;

use crate::descriptor::AsRawDescriptor;
use crate::platform::EventContext;
use crate::RawDescriptor;
use crate::Result;

/// Trait that can be used to associate events with arbitrary enums when using
/// `WaitContext`.
///
/// Simple enums that have no or primitive variant data data can use the `#[derive(EventToken)]`
/// custom derive to implement this trait. See
/// [event_token_derive::event_token](../base_event_token_derive/fn.event_token.html) for details.
pub trait EventToken {
    /// Converts this token into a u64 that can be turned back into a token via `from_raw_token`.
    fn as_raw_token(&self) -> u64;

    /// Converts a raw token as returned from `as_raw_token` back into a token.
    ///
    /// It is invalid to give a raw token that was not returned via `as_raw_token` from the same
    /// `Self`. The implementation can expect that this will never happen as a result of its usage
    /// in `WaitContext`.
    fn from_raw_token(data: u64) -> Self;
}

impl EventToken for usize {
    fn as_raw_token(&self) -> u64 {
        *self as u64
    }

    fn from_raw_token(data: u64) -> Self {
        data as Self
    }
}

impl EventToken for u64 {
    fn as_raw_token(&self) -> u64 {
        *self
    }

    fn from_raw_token(data: u64) -> Self {
        data as Self
    }
}

impl EventToken for u32 {
    fn as_raw_token(&self) -> u64 {
        u64::from(*self)
    }

    fn from_raw_token(data: u64) -> Self {
        data as Self
    }
}

impl EventToken for u16 {
    fn as_raw_token(&self) -> u64 {
        u64::from(*self)
    }

    fn from_raw_token(data: u64) -> Self {
        data as Self
    }
}

impl EventToken for u8 {
    fn as_raw_token(&self) -> u64 {
        u64::from(*self)
    }

    fn from_raw_token(data: u64) -> Self {
        data as Self
    }
}

impl EventToken for () {
    fn as_raw_token(&self) -> u64 {
        0
    }

    fn from_raw_token(_data: u64) -> Self {}
}

/// Represents an event that has been signaled and waited for via a wait function.
#[derive(Copy, Clone, Debug)]
pub struct TriggeredEvent<T: EventToken> {
    pub token: T,
    pub is_readable: bool,
    pub is_writable: bool,
    pub is_hungup: bool,
}

/// Represents types of events to watch for.
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub enum EventType {
    // Used to to temporarily stop waiting for events without
    // removing the associated descriptor from the WaitContext.
    // In most cases if a descriptor no longer needs to be
    // waited on, prefer removing it entirely with
    // WaitContext#delete
    None,
    Read,
    Write,
    ReadWrite,
}

/// Used to wait for multiple objects which are eligible for waiting.
///
/// # Example
///
/// ```
/// use base::{Event, EventToken, Result, WaitContext};
///
/// #[derive(EventToken, Copy, Clone, Debug, PartialEq, Eq)]
/// enum ExampleToken {
///    SomeEvent(u32),
///    AnotherEvent,
/// }
///
/// let evt1 = Event::new()?;
/// let evt2 = Event::new()?;
/// let another_evt = Event::new()?;
///
/// let ctx: WaitContext<ExampleToken> = WaitContext::build_with(&[
///     (&evt1, ExampleToken::SomeEvent(1)),
///     (&evt2, ExampleToken::SomeEvent(2)),
///     (&another_evt, ExampleToken::AnotherEvent),
/// ])?;
///
/// // Trigger one of the `SomeEvent` events.
/// evt2.signal()?;
///
/// // Wait for an event to fire. `wait()` will return immediately in this example because `evt2`
/// // has already been triggered, but in normal use, `wait()` will block until at least one event
/// // is signaled by another thread or process.
/// let events = ctx.wait()?;
/// let tokens: Vec<ExampleToken> = events.iter().filter(|e| e.is_readable)
///     .map(|e| e.token).collect();
/// assert_eq!(tokens, [ExampleToken::SomeEvent(2)]);
///
/// // Reset evt2 so it doesn't trigger again in the next `wait()` call.
/// let _ = evt2.reset()?;
///
/// // Trigger a different event.
/// another_evt.signal()?;
///
/// let events = ctx.wait()?;
/// let tokens: Vec<ExampleToken> = events.iter().filter(|e| e.is_readable)
///     .map(|e| e.token).collect();
/// assert_eq!(tokens, [ExampleToken::AnotherEvent]);
///
/// let _ = another_evt.reset()?;
/// # Ok::<(), base::Error>(())
/// ```
pub struct WaitContext<T: EventToken>(pub(crate) EventContext<T>);

impl<T: EventToken> WaitContext<T> {
    /// Creates a new WaitContext.
    pub fn new() -> Result<WaitContext<T>> {
        EventContext::new().map(WaitContext)
    }

    /// Creates a new WaitContext with the the associated triggers.
    pub fn build_with(triggers: &[(&dyn AsRawDescriptor, T)]) -> Result<WaitContext<T>> {
        let ctx = WaitContext::new()?;
        ctx.add_many(triggers)?;
        Ok(ctx)
    }

    /// Adds a trigger to the WaitContext.
    pub fn add(&self, descriptor: &dyn AsRawDescriptor, token: T) -> Result<()> {
        self.add_for_event(descriptor, EventType::Read, token)
    }

    /// Adds a trigger to the WaitContext watching for a specific type of event
    pub fn add_for_event(
        &self,
        descriptor: &dyn AsRawDescriptor,
        event_type: EventType,
        token: T,
    ) -> Result<()> {
        self.0.add_for_event(descriptor, event_type, token)
    }

    /// Adds multiple triggers to the WaitContext.
    pub fn add_many(&self, triggers: &[(&dyn AsRawDescriptor, T)]) -> Result<()> {
        for trigger in triggers {
            self.add(trigger.0, T::from_raw_token(trigger.1.as_raw_token()))?
        }
        Ok(())
    }

    /// Modifies a trigger already added to the WaitContext. If the descriptor is
    /// already registered, its associated token will be updated.
    pub fn modify(
        &self,
        descriptor: &dyn AsRawDescriptor,
        event_type: EventType,
        token: T,
    ) -> Result<()> {
        self.0.modify(descriptor, event_type, token)
    }

    /// Removes the given handle from triggers registered in the WaitContext if
    /// present.
    pub fn delete(&self, descriptor: &dyn AsRawDescriptor) -> Result<()> {
        self.0.delete(descriptor)
    }

    /// Waits for one or more of the registered triggers to become signaled.
    pub fn wait(&self) -> Result<SmallVec<[TriggeredEvent<T>; 16]>> {
        self.wait_timeout(Duration::new(i64::MAX as u64, 0))
    }

    /// Waits for one or more of the registered triggers to become signaled, failing if no triggers
    /// are signaled before the designated timeout has elapsed.
    pub fn wait_timeout(&self, timeout: Duration) -> Result<SmallVec<[TriggeredEvent<T>; 16]>> {
        self.0.wait_timeout(timeout)
    }
}

impl<T: EventToken> AsRawDescriptor for WaitContext<T> {
    fn as_raw_descriptor(&self) -> RawDescriptor {
        self.0.as_raw_descriptor()
    }
}

#[cfg(test)]
mod tests {
    use base_event_token_derive::EventToken;

    use super::*;

    #[test]
    #[allow(dead_code)]
    fn event_token_derive() {
        #[derive(EventToken)]
        enum EmptyToken {}

        #[derive(PartialEq, Debug, EventToken)]
        enum Token {
            Alpha,
            Beta,
            // comments
            Gamma(u32),
            Delta { index: usize },
            Omega,
        }

        assert_eq!(
            Token::from_raw_token(Token::Alpha.as_raw_token()),
            Token::Alpha
        );
        assert_eq!(
            Token::from_raw_token(Token::Beta.as_raw_token()),
            Token::Beta
        );
        assert_eq!(
            Token::from_raw_token(Token::Gamma(55).as_raw_token()),
            Token::Gamma(55)
        );
        assert_eq!(
            Token::from_raw_token(Token::Delta { index: 100 }.as_raw_token()),
            Token::Delta { index: 100 }
        );
        assert_eq!(
            Token::from_raw_token(Token::Omega.as_raw_token()),
            Token::Omega
        );
    }
}