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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,
}
}
}