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use super::{
future::ResponseFuture,
message::Message,
worker::{Handle, Worker},
};
use std::{
future::Future,
task::{Context, Poll},
};
use tokio::sync::{mpsc, oneshot};
use tokio_util::sync::PollSender;
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<Req, F> {
tx: PollSender<Message<Req, F>>,
handle: Handle,
}
impl<Req, F> Buffer<Req, F>
where
F: 'static,
{
/// 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<S>(service: S, bound: usize) -> Self
where
S: Service<Req, Future = F> + Send + 'static,
F: Send,
S::Error: Into<crate::BoxError> + Send + Sync,
Req: 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<S>(service: S, bound: usize) -> (Self, Worker<S, Req>)
where
S: Service<Req, Future = F> + Send + 'static,
F: Send,
S::Error: Into<crate::BoxError> + Send + Sync,
Req: Send + 'static,
{
let (tx, rx) = mpsc::channel(bound);
let (handle, worker) = Worker::new(service, rx);
let buffer = Self {
tx: PollSender::new(tx),
handle,
};
(buffer, worker)
}
fn get_worker_error(&self) -> crate::BoxError {
self.handle.get_error_on_closed()
}
}
impl<Req, Rsp, F, E> Service<Req> for Buffer<Req, F>
where
F: Future<Output = Result<Rsp, E>> + Send + 'static,
E: Into<crate::BoxError>,
Req: Send + 'static,
{
type Response = Rsp;
type Error = crate::BoxError;
type Future = ResponseFuture<F>;
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()));
}
// Poll the sender to acquire a permit.
self.tx
.poll_reserve(cx)
.map_err(|_| self.get_worker_error())
}
fn call(&mut self, request: Req) -> Self::Future {
tracing::trace!("sending request to buffer worker");
// 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 channel permit has already been
// acquired, so we can freely allocate a oneshot.
let (tx, rx) = oneshot::channel();
match self.tx.send_item(Message { request, span, tx }) {
Ok(_) => ResponseFuture::new(rx),
// If the channel is closed, propagate the error from the worker.
Err(_) => {
tracing::trace!("buffer channel closed");
ResponseFuture::failed(self.get_worker_error())
}
}
}
}
impl<Req, F> Clone for Buffer<Req, F>
where
Req: Send + 'static,
F: Send + 'static,
{
fn clone(&self) -> Self {
Self {
handle: self.handle.clone(),
tx: self.tx.clone(),
}
}
}