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use crate::{
error::{Error, SpawnError},
future::ResponseFuture,
message::Message,
worker::{Handle, Worker, WorkerExecutor},
};
use futures_core::ready;
use std::task::{Context, Poll};
use tokio_executor::DefaultExecutor;
use tokio_sync::{mpsc, oneshot};
use tower_service::Service;
/// Adds a buffer in front of an inner service.
///
/// See crate level documentation for more details.
pub struct Buffer<T, Request>
where
T: Service<Request>,
{
tx: mpsc::Sender<Message<Request, T::Future>>,
worker: Option<Handle>,
}
impl<T, Request> Buffer<T, Request>
where
T: Service<Request>,
T::Error: Into<Error>,
{
/// 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.
pub fn new(service: T, bound: usize) -> Self
where
T: Send + 'static,
T::Future: Send,
T::Error: Send + Sync,
Request: Send + 'static,
{
Self::with_executor(service, bound, &mut DefaultExecutor::current())
}
/// Creates a new `Buffer` wrapping `service`.
///
/// `executor` is used to spawn a new `Worker` task that is dedicated to
/// draining the buffer and dispatching the requests to the internal
/// service.
///
/// `bound` gives the maximal number of requests that can be queued for the service before
/// backpressure is applied to callers.
pub fn with_executor<E>(service: T, bound: usize, executor: &mut E) -> Self
where
E: WorkerExecutor<T, Request>,
{
let (tx, rx) = mpsc::channel(bound);
let worker = Worker::spawn(service, rx, executor);
Buffer { tx, worker }
}
fn get_worker_error(&self) -> Error {
self.worker
.as_ref()
.map(|w| w.get_error_on_closed())
.unwrap_or_else(|| {
// If there's no worker handle, that's because spawning it
// at the beginning failed.
SpawnError::new().into()
})
}
}
impl<T, Request> Service<Request> for Buffer<T, Request>
where
T: Service<Request>,
T::Error: Into<Error>,
{
type Response = T::Response;
type Error = Error;
type Future = ResponseFuture<T::Future>;
fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
// If the inner service has errored, then we error here.
if let Err(_) = ready!(self.tx.poll_ready(cx)) {
Poll::Ready(Err(self.get_worker_error()))
} else {
Poll::Ready(Ok(()))
}
}
fn call(&mut self, request: Request) -> Self::Future {
// TODO:
// ideally we'd poll_ready again here so we don't allocate the oneshot
// if the try_send is about to fail, but sadly we can't call poll_ready
// outside of task context.
let (tx, rx) = oneshot::channel();
// 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();
tracing::trace!(parent: &span, "sending request to buffer worker");
match self.tx.try_send(Message { request, span, tx }) {
Err(e) => {
if e.is_closed() {
ResponseFuture::failed(self.get_worker_error())
} else {
// When `mpsc::Sender::poll_ready` returns `Ready`, a slot
// in the channel is reserved for the handle. Other `Sender`
// handles may not send a message using that slot. This
// guarantees capacity for `request`.
//
// Given this, the only way to hit this code path is if
// `poll_ready` has not been called & `Ready` returned.
panic!("buffer full; poll_ready must be called first");
}
}
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(),
worker: self.worker.clone(),
}
}
}