fix(http1): only send 100 Continue if request body is polled

Before, if a client request included an `Expect: 100-continue` header,
the `100 Continue` response was sent immediately. However, this is
problematic if the service is going to reply with some 4xx status code
and reject the body.

This change delays the automatic sending of the `100 Continue` status
until the service has call `poll_data` on the request body once.

src/body/body.rs

@@ -11,7 +11,7 @@ use futures_util::TryStreamExt;
 use http::HeaderMap;
 use http_body::{Body as HttpBody, SizeHint};
 
-use crate::common::{task, Future, Never, Pin, Poll};
+use crate::common::{task, watch, Future, Never, Pin, Poll};
 use crate::proto::DecodedLength;
 use crate::upgrade::OnUpgrade;
 
@@ -33,7 +33,7 @@ enum Kind {
     Once(Option<Bytes>),
     Chan {
         content_length: DecodedLength,
-        abort_rx: oneshot::Receiver<()>,
+        want_tx: watch::Sender,
         rx: mpsc::Receiver<Result<Bytes, crate::Error>>,
     },
     H2 {
@@ -79,12 +79,14 @@ enum DelayEof {
 /// Useful when wanting to stream chunks from another thread. See
 /// [`Body::channel`](Body::channel) for more.
 #[must_use = "Sender does nothing unless sent on"]
-#[derive(Debug)]
 pub struct Sender {
-    abort_tx: oneshot::Sender<()>,
+    want_rx: watch::Receiver,
     tx: BodySender,
 }
 
+const WANT_PENDING: usize = 1;
+const WANT_READY: usize = 2;
+
 impl Body {
     /// Create an empty `Body` stream.
     ///
@@ -106,17 +108,22 @@ impl Body {
     /// Useful when wanting to stream chunks from another thread.
     #[inline]
     pub fn channel() -> (Sender, Body) {
-        Self::new_channel(DecodedLength::CHUNKED)
+        Self::new_channel(DecodedLength::CHUNKED, /*wanter =*/ false)
     }
 
-    pub(crate) fn new_channel(content_length: DecodedLength) -> (Sender, Body) {
+    pub(crate) fn new_channel(content_length: DecodedLength, wanter: bool) -> (Sender, Body) {
         let (tx, rx) = mpsc::channel(0);
-        let (abort_tx, abort_rx) = oneshot::channel();
 
-        let tx = Sender { abort_tx, tx };
+        // If wanter is true, `Sender::poll_ready()` won't becoming ready
+        // until the `Body` has been polled for data once.
+        let want = if wanter { WANT_PENDING } else { WANT_READY };
+
+        let (want_tx, want_rx) = watch::channel(want);
+
+        let tx = Sender { want_rx, tx };
         let rx = Body::new(Kind::Chan {
             content_length,
-            abort_rx,
+            want_tx,
             rx,
         });
 
@@ -236,11 +243,9 @@ impl Body {
             Kind::Chan {
                 content_length: ref mut len,
                 ref mut rx,
-                ref mut abort_rx,
+                ref mut want_tx,
             } => {
-                if let Poll::Ready(Ok(())) = Pin::new(abort_rx).poll(cx) {
-                    return Poll::Ready(Some(Err(crate::Error::new_body_write_aborted())));
-                }
+                want_tx.send(WANT_READY);
 
                 match ready!(Pin::new(rx).poll_next(cx)?) {
                     Some(chunk) => {
@@ -460,19 +465,29 @@ impl From<Cow<'static, str>> for Body {
 impl Sender {
     /// Check to see if this `Sender` can send more data.
     pub fn poll_ready(&mut self, cx: &mut task::Context<'_>) -> Poll<crate::Result<()>> {
-        match self.abort_tx.poll_canceled(cx) {
-            Poll::Ready(()) => return Poll::Ready(Err(crate::Error::new_closed())),
-            Poll::Pending => (), // fallthrough
-        }
-
+        // Check if the receiver end has tried polling for the body yet
+        ready!(self.poll_want(cx)?);
         self.tx
             .poll_ready(cx)
             .map_err(|_| crate::Error::new_closed())
     }
 
+    fn poll_want(&mut self, cx: &mut task::Context<'_>) -> Poll<crate::Result<()>> {
+        match self.want_rx.load(cx) {
+            WANT_READY => Poll::Ready(Ok(())),
+            WANT_PENDING => Poll::Pending,
+            watch::CLOSED => Poll::Ready(Err(crate::Error::new_closed())),
+            unexpected => unreachable!("want_rx value: {}", unexpected),
+        }
+    }
+
+    async fn ready(&mut self) -> crate::Result<()> {
+        futures_util::future::poll_fn(|cx| self.poll_ready(cx)).await
+    }
+
     /// Send data on this channel when it is ready.
     pub async fn send_data(&mut self, chunk: Bytes) -> crate::Result<()> {
-        futures_util::future::poll_fn(|cx| self.poll_ready(cx)).await?;
+        self.ready().await?;
         self.tx
             .try_send(Ok(chunk))
             .map_err(|_| crate::Error::new_closed())
@@ -498,8 +513,11 @@ impl Sender {
 
     /// Aborts the body in an abnormal fashion.
     pub fn abort(self) {
-        // TODO(sean): this can just be `self.tx.clone().try_send()`
-        let _ = self.abort_tx.send(());
+        let _ = self
+            .tx
+            // clone so the send works even if buffer is full
+            .clone()
+            .try_send(Err(crate::Error::new_body_write_aborted()));
     }
 
     pub(crate) fn send_error(&mut self, err: crate::Error) {
@@ -507,11 +525,29 @@ impl Sender {
     }
 }
 
+impl fmt::Debug for Sender {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        #[derive(Debug)]
+        struct Open;
+        #[derive(Debug)]
+        struct Closed;
+
+        let mut builder = f.debug_tuple("Sender");
+        match self.want_rx.peek() {
+            watch::CLOSED => builder.field(&Closed),
+            _ => builder.field(&Open),
+        };
+
+        builder.finish()
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use std::mem;
+    use std::task::Poll;
 
-    use super::{Body, Sender};
+    use super::{Body, DecodedLength, HttpBody, Sender};
 
     #[test]
     fn test_size_of() {
@@ -541,4 +577,97 @@ mod tests {
             "Option<Sender>"
         );
     }
+
+    #[tokio::test]
+    async fn channel_abort() {
+        let (tx, mut rx) = Body::channel();
+
+        tx.abort();
+
+        let err = rx.data().await.unwrap().unwrap_err();
+        assert!(err.is_body_write_aborted(), "{:?}", err);
+    }
+
+    #[tokio::test]
+    async fn channel_abort_when_buffer_is_full() {
+        let (mut tx, mut rx) = Body::channel();
+
+        tx.try_send_data("chunk 1".into()).expect("send 1");
+        // buffer is full, but can still send abort
+        tx.abort();
+
+        let chunk1 = rx.data().await.expect("item 1").expect("chunk 1");
+        assert_eq!(chunk1, "chunk 1");
+
+        let err = rx.data().await.unwrap().unwrap_err();
+        assert!(err.is_body_write_aborted(), "{:?}", err);
+    }
+
+    #[test]
+    fn channel_buffers_one() {
+        let (mut tx, _rx) = Body::channel();
+
+        tx.try_send_data("chunk 1".into()).expect("send 1");
+
+        // buffer is now full
+        let chunk2 = tx.try_send_data("chunk 2".into()).expect_err("send 2");
+        assert_eq!(chunk2, "chunk 2");
+    }
+
+    #[tokio::test]
+    async fn channel_empty() {
+        let (_, mut rx) = Body::channel();
+
+        assert!(rx.data().await.is_none());
+    }
+
+    #[test]
+    fn channel_ready() {
+        let (mut tx, _rx) = Body::new_channel(DecodedLength::CHUNKED, /*wanter = */ false);
+
+        let mut tx_ready = tokio_test::task::spawn(tx.ready());
+
+        assert!(tx_ready.poll().is_ready(), "tx is ready immediately");
+    }
+
+    #[test]
+    fn channel_wanter() {
+        let (mut tx, mut rx) = Body::new_channel(DecodedLength::CHUNKED, /*wanter = */ true);
+
+        let mut tx_ready = tokio_test::task::spawn(tx.ready());
+        let mut rx_data = tokio_test::task::spawn(rx.data());
+
+        assert!(
+            tx_ready.poll().is_pending(),
+            "tx isn't ready before rx has been polled"
+        );
+
+        assert!(rx_data.poll().is_pending(), "poll rx.data");
+        assert!(tx_ready.is_woken(), "rx poll wakes tx");
+
+        assert!(
+            tx_ready.poll().is_ready(),
+            "tx is ready after rx has been polled"
+        );
+    }
+
+    #[test]
+    fn channel_notices_closure() {
+        let (mut tx, rx) = Body::new_channel(DecodedLength::CHUNKED, /*wanter = */ true);
+
+        let mut tx_ready = tokio_test::task::spawn(tx.ready());
+
+        assert!(
+            tx_ready.poll().is_pending(),
+            "tx isn't ready before rx has been polled"
+        );
+
+        drop(rx);
+        assert!(tx_ready.is_woken(), "dropping rx wakes tx");
+
+        match tx_ready.poll() {
+            Poll::Ready(Err(ref e)) if e.is_closed() => (),
+            unexpected => panic!("tx poll ready unexpected: {:?}", unexpected),
+        }
+    }
 }