crates/tower/src/buffer/service.rs

use super::{
    future::ResponseFuture,
    message::Message,
    worker::{Handle, Worker},
};

use futures_core::ready;
use std::sync::Arc;
use std::task::{Context, Poll};
use tokio::sync::{mpsc, oneshot, OwnedSemaphorePermit, Semaphore};
use tokio_util::sync::PollSemaphore;
use tower_service::Service;

/// Adds an mpsc buffer in front of an inner service.
///
/// See the module documentation for more details.
#[derive(Debug)]
pub struct Buffer<T, Request>
where
    T: Service<Request>,
{
    // Note: this actually _is_ bounded, but rather than using Tokio's bounded
    // channel, we use Tokio's semaphore separately to implement the bound.
    tx: mpsc::UnboundedSender<Message<Request, T::Future>>,
    // When the buffer's channel is full, we want to exert backpressure in
    // `poll_ready`, so that callers such as load balancers could choose to call
    // another service rather than waiting for buffer capacity.
    //
    // Unfortunately, this can't be done easily using Tokio's bounded MPSC
    // channel, because it doesn't expose a polling-based interface, only an
    // `async fn ready`, which borrows the sender. Therefore, we implement our
    // own bounded MPSC on top of the unbounded channel, using a semaphore to
    // limit how many items are in the channel.
    semaphore: PollSemaphore,
    // The current semaphore permit, if one has been acquired.
    //
    // This is acquired in `poll_ready` and taken in `call`.
    permit: Option<OwnedSemaphorePermit>,
    handle: Handle,
}

impl<T, Request> Buffer<T, Request>
where
    T: Service<Request>,
    T::Error: Into<crate::BoxError>,
{
    /// Creates a new [`Buffer`] wrapping `service`.
    ///
    /// `bound` gives the maximal number of requests that can be queued for the service before
    /// backpressure is applied to callers.
    ///
    /// The default Tokio executor is used to run the given service, which means that this method
    /// must be called while on the Tokio runtime.
    ///
    /// # A note on choosing a `bound`
    ///
    /// When [`Buffer`]'s implementation of [`poll_ready`] returns [`Poll::Ready`], it reserves a
    /// slot in the channel for the forthcoming [`call`]. However, if this call doesn't arrive,
    /// this reserved slot may be held up for a long time. As a result, it's advisable to set
    /// `bound` to be at least the maximum number of concurrent requests the [`Buffer`] will see.
    /// If you do not, all the slots in the buffer may be held up by futures that have just called
    /// [`poll_ready`] but will not issue a [`call`], which prevents other senders from issuing new
    /// requests.
    ///
    /// [`Poll::Ready`]: std::task::Poll::Ready
    /// [`call`]: crate::Service::call
    /// [`poll_ready`]: crate::Service::poll_ready
    pub fn new(service: T, bound: usize) -> Self
    where
        T: Send + 'static,
        T::Future: Send,
        T::Error: Send + Sync,
        Request: Send + 'static,
    {
        let (service, worker) = Self::pair(service, bound);
        tokio::spawn(worker);
        service
    }

    /// Creates a new [`Buffer`] wrapping `service`, but returns the background worker.
    ///
    /// This is useful if you do not want to spawn directly onto the tokio runtime
    /// but instead want to use your own executor. This will return the [`Buffer`] and
    /// the background `Worker` that you can then spawn.
    pub fn pair(service: T, bound: usize) -> (Buffer<T, Request>, Worker<T, Request>)
    where
        T: Send + 'static,
        T::Error: Send + Sync,
        Request: Send + 'static,
    {
        let (tx, rx) = mpsc::unbounded_channel();
        let semaphore = Arc::new(Semaphore::new(bound));
        let (handle, worker) = Worker::new(service, rx, &semaphore);
        let buffer = Buffer {
            tx,
            handle,
            semaphore: PollSemaphore::new(semaphore),
            permit: None,
        };
        (buffer, worker)
    }

    fn get_worker_error(&self) -> crate::BoxError {
        self.handle.get_error_on_closed()
    }
}

impl<T, Request> Service<Request> for Buffer<T, Request>
where
    T: Service<Request>,
    T::Error: Into<crate::BoxError>,
{
    type Response = T::Response;
    type Error = crate::BoxError;
    type Future = ResponseFuture<T::Future>;

    fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
        // First, check if the worker is still alive.
        if self.tx.is_closed() {
            // If the inner service has errored, then we error here.
            return Poll::Ready(Err(self.get_worker_error()));
        }

        // Then, check if we've already acquired a permit.
        if self.permit.is_some() {
            // We've already reserved capacity to send a request. We're ready!
            return Poll::Ready(Ok(()));
        }

        // Finally, if we haven't already acquired a permit, poll the semaphore
        // to acquire one. If we acquire a permit, then there's enough buffer
        // capacity to send a new request. Otherwise, we need to wait for
        // capacity.
        let permit =
            ready!(self.semaphore.poll_acquire(cx)).ok_or_else(|| self.get_worker_error())?;
        self.permit = Some(permit);

        Poll::Ready(Ok(()))
    }

    fn call(&mut self, request: Request) -> Self::Future {
        tracing::trace!("sending request to buffer worker");
        let _permit = self
            .permit
            .take()
            .expect("buffer full; poll_ready must be called first");

        // get the current Span so that we can explicitly propagate it to the worker
        // if we didn't do this, events on the worker related to this span wouldn't be counted
        // towards that span since the worker would have no way of entering it.
        let span = tracing::Span::current();

        // If we've made it here, then a semaphore permit has already been
        // acquired, so we can freely allocate a oneshot.
        let (tx, rx) = oneshot::channel();

        match self.tx.send(Message {
            request,
            span,
            tx,
            _permit,
        }) {
            Err(_) => ResponseFuture::failed(self.get_worker_error()),
            Ok(_) => ResponseFuture::new(rx),
        }
    }
}

impl<T, Request> Clone for Buffer<T, Request>
where
    T: Service<Request>,
{
    fn clone(&self) -> Self {
        Self {
            tx: self.tx.clone(),
            handle: self.handle.clone(),
            semaphore: self.semaphore.clone(),
            // The new clone hasn't acquired a permit yet. It will when it's
            // next polled ready.
            permit: None,
        }
    }
}