Introduce ready_chunks adaptor

futures-util/src/stream/mod.rs

@@ -24,6 +24,9 @@ pub use self::stream::CatchUnwind;
 #[cfg(feature = "alloc")]
 pub use self::stream::Chunks;
 
+#[cfg(feature = "alloc")]
+pub use self::stream::ReadyChunks;
+
 #[cfg(feature = "sink")]
 pub use self::stream::Forward;
 

futures-util/src/stream/stream/mod.rs

@@ -128,6 +128,12 @@ mod chunks;
 #[allow(unreachable_pub)] // https://github.com/rust-lang/rust/issues/57411
 pub use self::chunks::Chunks;
 
+#[cfg(feature = "alloc")]
+mod ready_chunks;
+#[cfg(feature = "alloc")]
+#[allow(unreachable_pub)] // https://github.com/rust-lang/rust/issues/57411
+pub use self::ready_chunks::ReadyChunks;
+
 mod scan;
 #[allow(unreachable_pub)] // https://github.com/rust-lang/rust/issues/57411
 pub use self::scan::Scan;
@@ -1186,6 +1192,32 @@ pub trait StreamExt: Stream {
         Chunks::new(self, capacity)
     }
 
+    /// An adaptor for chunking up ready items of the stream inside a vector.
+    ///
+    /// This combinator will attempt to pull ready items from this stream and
+    /// buffer them into a local vector. At most `capacity` items will get
+    /// buffered before they're yielded from the returned stream. If underlying
+    /// stream returns `Poll::Pending`, and collected chunk is not empty, it will
+    /// be immediately returned.
+    ///
+    /// If the underlying stream ended and only a partial vector was created,
+    /// it'll be returned. Additionally if an error happens from the underlying
+    /// stream then the currently buffered items will be yielded.
+    ///
+    /// This method is only available when the `std` or `alloc` feature of this
+    /// library is activated, and it is activated by default.
+    ///
+    /// # Panics
+    ///
+    /// This method will panic if `capacity` is zero.
+    #[cfg(feature = "alloc")]
+    fn ready_chunks(self, capacity: usize) -> ReadyChunks<Self>
+        where
+            Self: Sized,
+    {
+        ReadyChunks::new(self, capacity)
+    }
+
     /// A future that completes after the given stream has been fully processed
     /// into the sink and the sink has been flushed and closed.
     ///

futures-util/src/stream/stream/ready_chunks.rs

@@ -0,0 +1,146 @@
+use crate::stream::Fuse;
+use futures_core::stream::{Stream, FusedStream};
+use futures_core::task::{Context, Poll};
+#[cfg(feature = "sink")]
+use futures_sink::Sink;
+use pin_utils::{unsafe_pinned, unsafe_unpinned};
+use core::mem;
+use core::pin::Pin;
+use alloc::vec::Vec;
+
+/// Stream for the [`ready_chunks`](super::StreamExt::ready_chunks) method.
+#[derive(Debug)]
+#[must_use = "streams do nothing unless polled"]
+pub struct ReadyChunks<St: Stream> {
+    stream: Fuse<St>,
+    items: Vec<St::Item>,
+    cap: usize, // https://github.com/rust-lang/futures-rs/issues/1475
+}
+
+impl<St: Unpin + Stream> Unpin for ReadyChunks<St> {}
+
+impl<St: Stream> ReadyChunks<St> where St: Stream {
+    unsafe_unpinned!(items:  Vec<St::Item>);
+    unsafe_pinned!(stream: Fuse<St>);
+
+    pub(super) fn new(stream: St, capacity: usize) -> ReadyChunks<St> {
+        assert!(capacity > 0);
+
+        ReadyChunks {
+            stream: super::Fuse::new(stream),
+            items: Vec::with_capacity(capacity),
+            cap: capacity,
+        }
+    }
+
+    fn take(mut self: Pin<&mut Self>) -> Vec<St::Item> {
+        let cap = self.cap;
+        mem::replace(self.as_mut().items(), Vec::with_capacity(cap))
+    }
+
+    /// Acquires a reference to the underlying stream that this combinator is
+    /// pulling from.
+    pub fn get_ref(&self) -> &St {
+        self.stream.get_ref()
+    }
+
+    /// Acquires a mutable reference to the underlying stream that this
+    /// combinator is pulling from.
+    ///
+    /// Note that care must be taken to avoid tampering with the state of the
+    /// stream which may otherwise confuse this combinator.
+    pub fn get_mut(&mut self) -> &mut St {
+        self.stream.get_mut()
+    }
+
+    /// Acquires a pinned mutable reference to the underlying stream that this
+    /// combinator is pulling from.
+    ///
+    /// Note that care must be taken to avoid tampering with the state of the
+    /// stream which may otherwise confuse this combinator.
+    pub fn get_pin_mut(self: Pin<&mut Self>) -> Pin<&mut St> {
+        self.stream().get_pin_mut()
+    }
+
+    /// Consumes this combinator, returning the underlying stream.
+    ///
+    /// Note that this may discard intermediate state of this combinator, so
+    /// care should be taken to avoid losing resources when this is called.
+    pub fn into_inner(self) -> St {
+        self.stream.into_inner()
+    }
+}
+
+impl<St: Stream> Stream for ReadyChunks<St> {
+    type Item = Vec<St::Item>;
+
+    fn poll_next(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+    ) -> Poll<Option<Self::Item>> {
+        loop {
+            match self.as_mut().stream().poll_next(cx) {
+                // Flush all collected data if underlying stream doesn't contain
+                // more ready values
+                Poll::Pending => {
+                    return if self.items.is_empty() {
+                        Poll::Pending
+                    } else {
+                        Poll::Ready(Some(self.as_mut().take()))
+                    }
+                }
+
+                // Push the ready item into the buffer and check whether it is full.
+                // If so, replace our buffer with a new and empty one and return
+                // the full one.
+                Poll::Ready(Some(item)) => {
+                    self.as_mut().items().push(item);
+                    if self.items.len() >= self.cap {
+                        return Poll::Ready(Some(self.as_mut().take()))
+                    }
+                }
+
+                // Since the underlying stream ran out of values, return what we
+                // have buffered, if we have anything.
+                Poll::Ready(None) => {
+                    let last = if self.items.is_empty() {
+                        None
+                    } else {
+                        let full_buf = mem::replace(self.as_mut().items(), Vec::new());
+                        Some(full_buf)
+                    };
+
+                    return Poll::Ready(last);
+                }
+            }
+        }
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let chunk_len = if self.items.is_empty() { 0 } else { 1 };
+        let (lower, upper) = self.stream.size_hint();
+        let lower = lower.saturating_add(chunk_len);
+        let upper = match upper {
+            Some(x) => x.checked_add(chunk_len),
+            None => None,
+        };
+        (lower, upper)
+    }
+}
+
+impl<St: FusedStream> FusedStream for ReadyChunks<St> {
+    fn is_terminated(&self) -> bool {
+        self.stream.is_terminated() && self.items.is_empty()
+    }
+}
+
+// Forwarding impl of Sink from the underlying stream
+#[cfg(feature = "sink")]
+impl<S, Item> Sink<Item> for ReadyChunks<S>
+where
+    S: Stream + Sink<Item>,
+{
+    type Error = S::Error;
+
+    delegate_sink!(stream, Item);
+}

futures/src/lib.rs

@@ -458,7 +458,7 @@ pub mod stream {
     #[cfg(feature = "alloc")]
     pub use futures_util::stream::{
         // For StreamExt:
-        Chunks,
+        Chunks, ReadyChunks,
     };
 
     #[cfg_attr(feature = "cfg-target-has-atomic", cfg(target_has_atomic = "ptr"))]

futures/tests/stream.rs

@@ -124,3 +124,42 @@ fn take_until() {
         assert_eq!(stream.next().await, None);
     });
 }
+
+#[test]
+#[should_panic]
+fn ready_chunks_panic_on_cap_zero() {
+    use futures::channel::mpsc;
+    use futures::stream::StreamExt;
+
+    let (_, rx1) = mpsc::channel::<()>(1);
+
+    let _ = rx1.ready_chunks(0);
+}
+
+#[cfg(feature = "executor")] // executor::
+#[test]
+fn ready_chunks() {
+    use futures::channel::mpsc;
+    use futures::stream::StreamExt;
+    use futures::sink::SinkExt;
+    use futures::FutureExt;
+    use futures_test::task::noop_context;
+
+    let (mut tx, rx1) = mpsc::channel::<i32>(16);
+
+    let mut s = rx1.ready_chunks(2);
+
+    let mut cx = noop_context();
+    assert!(s.next().poll_unpin(&mut cx).is_pending());
+
+    futures::executor::block_on(async {
+        tx.send(1).await.unwrap();
+
+        assert_eq!(s.next().await.unwrap(), vec![1]);
+        tx.send(2).await.unwrap();
+        tx.send(3).await.unwrap();
+        tx.send(4).await.unwrap();
+        assert_eq!(s.next().await.unwrap(), vec![2,3]);
+        assert_eq!(s.next().await.unwrap(), vec![4]);
+    });
+}