buffer: make Buffer::new infallible (#257)

Creating a buffer can internally fail to spawn a worker. Before, that
error was returned immediately from `Buffer::new`. This changes `new` to
always return a `Buffer`, and the spawn error is encountered via
`poll_ready`.

tower-buffer/src/layer.rs

@@ -41,6 +41,10 @@ where
     type Service = Buffer<S, Request>;
 
     fn layer(&self, service: S) -> Result<Self::Service, Self::LayerError> {
-        Buffer::with_executor(service, self.bound, &mut self.executor.clone())
+        Ok(Buffer::with_executor(
+            service,
+            self.bound,
+            &mut self.executor.clone(),
+        ))
     }
 }

tower-buffer/src/service.rs

@@ -1,5 +1,5 @@
 use crate::{
-    error::Error,
+    error::{Error, SpawnError},
     future::ResponseFuture,
     message::Message,
     worker::{Handle, Worker, WorkerExecutor},
@@ -18,7 +18,7 @@ where
     T: Service<Request>,
 {
     tx: mpsc::Sender<Message<Request, T::Future>>,
-    worker: Handle,
+    worker: Option<Handle>,
 }
 
 impl<T, Request> Buffer<T, Request>
@@ -33,7 +33,7 @@ where
     ///
     /// 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) -> Result<Self, Error>
+    pub fn new(service: T, bound: usize) -> Self
     where
         T: Send + 'static,
         T::Future: Send,
@@ -51,13 +51,24 @@ where
     ///
     /// `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) -> Result<Self, Error>
+    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 }
+    }
 
-        Worker::spawn(service, rx, executor).map(|worker| 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()
+            })
     }
 }
 
@@ -72,9 +83,7 @@ where
 
     fn poll_ready(&mut self) -> Poll<(), Self::Error> {
         // If the inner service has errored, then we error here.
-        self.tx
-            .poll_ready()
-            .map_err(|_| self.worker.get_error_on_closed())
+        self.tx.poll_ready().map_err(|_| self.get_worker_error())
     }
 
     fn call(&mut self, request: Request) -> Self::Future {
@@ -87,7 +96,7 @@ where
         match self.tx.try_send(Message { request, tx }) {
             Err(e) => {
                 if e.is_closed() {
-                    ResponseFuture::failed(self.worker.get_error_on_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`

tower-buffer/src/worker.rs

@@ -1,5 +1,5 @@
 use crate::{
-    error::{Closed, Error, ServiceError, SpawnError},
+    error::{Closed, Error, ServiceError},
     message::Message,
 };
 use futures::{try_ready, Async, Future, Poll, Stream};
@@ -58,7 +58,7 @@ where
         service: T,
         rx: mpsc::Receiver<Message<Request, T::Future>>,
         executor: &mut E,
-    ) -> Result<Handle, Error>
+    ) -> Option<Handle>
     where
         E: WorkerExecutor<T, Request>,
     {
@@ -76,8 +76,8 @@ where
         };
 
         match executor.spawn(worker) {
-            Ok(()) => Ok(handle),
-            Err(_) => Err(SpawnError::new().into()),
+            Ok(()) => Some(handle),
+            Err(_) => None,
         }
     }
 

tower-buffer/tests/buffer.rs

@@ -99,20 +99,46 @@ fn when_inner_fails() {
     });
 }
 
+#[test]
+fn when_spawn_fails() {
+    let (service, _handle) = mock::pair::<(), ()>();
+
+    let mut exec = ExecFn(|_| Err(()));
+
+    let mut service = Buffer::with_executor(service, 1, &mut exec);
+
+    let err = with_task(|| {
+        service
+            .poll_ready()
+            .expect_err("buffer poll_ready should error")
+    });
+
+    assert!(
+        err.is::<error::SpawnError>(),
+        "should be a SpawnError: {:?}",
+        err
+    );
+}
+
 #[test]
 fn poll_ready_when_worker_is_dropped_early() {
     let (service, _handle) = mock::pair::<(), ()>();
 
     // drop that worker right on the floor!
-    let mut exec = ExecFn(drop);
+    let mut exec = ExecFn(|fut| {
+        drop(fut);
+        Ok(())
+    });
 
-    let mut service = Buffer::with_executor(service, 1, &mut exec).unwrap();
+    let mut service = Buffer::with_executor(service, 1, &mut exec);
 
-    with_task(|| {
+    let err = with_task(|| {
         service
             .poll_ready()
-            .expect_err("buffer poll_ready should error");
+            .expect_err("buffer poll_ready should error")
     });
+
+    assert!(err.is::<error::Closed>(), "should be a Closed: {:?}", err);
 }
 
 #[test]
@@ -121,9 +147,12 @@ fn response_future_when_worker_is_dropped_early() {
 
     // hold the worker in a cell until we want to drop it later
     let cell = RefCell::new(None);
-    let mut exec = ExecFn(|fut| *cell.borrow_mut() = Some(fut));
+    let mut exec = ExecFn(|fut| {
+        *cell.borrow_mut() = Some(fut);
+        Ok(())
+    });
 
-    let mut service = Buffer::with_executor(service, 1, &mut exec).unwrap();
+    let mut service = Buffer::with_executor(service, 1, &mut exec);
 
     // keep the request in the worker
     handle.allow(0);
@@ -132,7 +161,8 @@ fn response_future_when_worker_is_dropped_early() {
     // drop the worker (like an executor closing up)
     cell.borrow_mut().take();
 
-    response.wait().expect_err("res.wait");
+    let err = response.wait().expect_err("res.wait");
+    assert!(err.is::<error::Closed>(), "should be a Closed: {:?}", err);
 }
 
 type Mock = mock::Mock<&'static str, &'static str>;
@@ -156,23 +186,22 @@ struct ExecFn<Func>(Func);
 
 impl<Func, F> TypedExecutor<F> for ExecFn<Func>
 where
-    Func: Fn(F),
+    Func: Fn(F) -> Result<(), ()>,
     F: Future<Item = (), Error = ()> + Send + 'static,
 {
     fn spawn(&mut self, fut: F) -> Result<(), SpawnError> {
-        (self.0)(fut);
-        Ok(())
+        (self.0)(fut).map_err(|()| SpawnError::shutdown())
     }
 }
 
 fn new_service() -> (Buffer<Mock, &'static str>, Handle) {
     let (service, handle) = mock::pair();
     // bound is >0 here because clears_canceled_requests needs multiple outstanding requests
-    let service = Buffer::with_executor(service, 10, &mut Exec).unwrap();
+    let service = Buffer::with_executor(service, 10, &mut Exec);
     (service, handle)
 }
 
 fn with_task<F: FnOnce() -> U, U>(f: F) -> U {
-    use futures::future::{lazy, Future};
+    use futures::future::lazy;
     lazy(|| Ok::<_, ()>(f())).wait().unwrap()
 }