Add TryStreamExt::try_ready_chunks as failable version of StreamExt::ready_chunks

futures-util/src/stream/mod.rs

@@ -71,7 +71,7 @@ pub use self::try_stream::{TryBufferUnordered, TryBuffered, TryFlattenUnordered}
 pub use self::try_stream::TryForward;
 
 #[cfg(feature = "alloc")]
-pub use self::try_stream::{TryChunks, TryChunksError};
+pub use self::try_stream::{TryChunks, TryChunksError, TryReadyChunks, TryReadyChunksError};
 
 // Primitive streams
 

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

@@ -121,6 +121,12 @@ mod try_chunks;
 #[allow(unreachable_pub)] // https://github.com/rust-lang/rust/issues/57411
 pub use self::try_chunks::{TryChunks, TryChunksError};
 
+#[cfg(feature = "alloc")]
+mod try_ready_chunks;
+#[cfg(feature = "alloc")]
+#[allow(unreachable_pub)] // https://github.com/rust-lang/rust/issues/57411
+pub use self::try_ready_chunks::{TryReadyChunks, TryReadyChunksError};
+
 mod try_unfold;
 #[allow(unreachable_pub)] // https://github.com/rust-lang/rust/issues/57411
 pub use self::try_unfold::{try_unfold, TryUnfold};
@@ -557,6 +563,55 @@ pub trait TryStreamExt: TryStream {
         )
     }
 
+    /// An adaptor for chunking up successful, ready items of the stream inside a vector.
+    ///
+    /// This combinator will attempt to pull successful 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 the underlying stream
+    /// returns `Poll::Pending`, and the collected chunk is not empty, it will
+    /// be immidiatly returned.
+    ///
+    /// Note that the vectors returned from this iterator may not always have
+    /// `capacity` elements. 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.
+    ///
+    /// This function is similar to
+    /// [`StreamExt::ready_chunks`](crate::stream::StreamExt::ready_chunks) but exits
+    /// early if an error occurs.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # futures::executor::block_on(async {
+    /// use futures::stream::{self, TryReadyChunksError, TryStreamExt};
+    ///
+    /// let stream = stream::iter(vec![Ok::<i32, i32>(1), Ok(2), Ok(3), Err(4), Ok(5), Ok(6)]);
+    /// let mut stream = stream.try_ready_chunks(2);
+    ///
+    /// assert_eq!(stream.try_next().await, Ok(Some(vec![1, 2])));
+    /// assert_eq!(stream.try_next().await, Err(TryReadyChunksError(vec![3], 4)));
+    /// assert_eq!(stream.try_next().await, Ok(Some(vec![5, 6])));
+    /// # })
+    /// ```
+    ///
+    /// # Panics
+    ///
+    /// This method will panic if `capacity` is zero.
+    #[cfg(feature = "alloc")]
+    fn try_ready_chunks(self, capacity: usize) -> TryReadyChunks<Self>
+    where
+        Self: Sized,
+    {
+        assert_stream::<Result<Vec<Self::Ok>, TryReadyChunksError<Self::Ok, Self::Error>>, _>(
+            TryReadyChunks::new(self, capacity),
+        )
+    }
+
     /// Attempt to filter the values produced by this stream according to the
     /// provided asynchronous closure.
     ///

futures-util/src/stream/try_stream/try_ready_chunks.rs

@@ -0,0 +1,126 @@
+use crate::stream::{Fuse, IntoStream, StreamExt};
+
+use alloc::vec::Vec;
+use core::fmt;
+use core::pin::Pin;
+use futures_core::stream::{FusedStream, Stream, TryStream};
+use futures_core::task::{Context, Poll};
+#[cfg(feature = "sink")]
+use futures_sink::Sink;
+use pin_project_lite::pin_project;
+
+pin_project! {
+    /// Stream for the [`try_ready_chunks`](super::TryStreamExt::try_ready_chunks) method.
+    #[derive(Debug)]
+    #[must_use = "streams do nothing unless polled"]
+    pub struct TryReadyChunks<St: TryStream> {
+        #[pin]
+        stream: Fuse<IntoStream<St>>,
+        cap: usize, // https://github.com/rust-lang/futures-rs/issues/1475
+    }
+}
+
+impl<St: TryStream> TryReadyChunks<St> {
+    pub(super) fn new(stream: St, capacity: usize) -> Self {
+        assert!(capacity > 0);
+
+        Self { stream: IntoStream::new(stream).fuse(), cap: capacity }
+    }
+
+    delegate_access_inner!(stream, St, (. .));
+}
+
+type TryReadyChunksStreamError<St> =
+    TryReadyChunksError<<St as TryStream>::Ok, <St as TryStream>::Error>;
+
+impl<St: TryStream> Stream for TryReadyChunks<St> {
+    type Item = Result<Vec<St::Ok>, TryReadyChunksStreamError<St>>;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        let mut this = self.as_mut().project();
+
+        let mut items: Vec<St::Ok> = Vec::new();
+
+        loop {
+            match this.stream.as_mut().poll_next(cx) {
+                // Flush all the collected data if the underlying stream doesn't
+                // contain more ready values
+                Poll::Pending => {
+                    return if items.is_empty() {
+                        Poll::Pending
+                    } else {
+                        Poll::Ready(Some(Ok(items)))
+                    }
+                }
+
+                // Push the ready item into the buffer and check whether it is full.
+                // If so, return the buffer.
+                Poll::Ready(Some(Ok(item))) => {
+                    if items.is_empty() {
+                        items.reserve_exact(*this.cap);
+                    }
+                    items.push(item);
+                    if items.len() >= *this.cap {
+                        return Poll::Ready(Some(Ok(items)));
+                    }
+                }
+
+                // Return the already collected items and the error.
+                Poll::Ready(Some(Err(e))) => {
+                    return Poll::Ready(Some(Err(TryReadyChunksError(items, e))));
+                }
+
+                // Since the underlying stream ran out of values, return what we
+                // have buffered, if we have anything.
+                Poll::Ready(None) => {
+                    let last = if items.is_empty() { None } else { Some(Ok(items)) };
+                    return Poll::Ready(last);
+                }
+            }
+        }
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let (lower, upper) = self.stream.size_hint();
+        let lower = lower / self.cap;
+        (lower, upper)
+    }
+}
+
+impl<St: TryStream + FusedStream> FusedStream for TryReadyChunks<St> {
+    fn is_terminated(&self) -> bool {
+        self.stream.is_terminated()
+    }
+}
+
+// Forwarding impl of Sink from the underlying stream
+#[cfg(feature = "sink")]
+impl<S, Item> Sink<Item> for TryReadyChunks<S>
+where
+    S: TryStream + Sink<Item>,
+{
+    type Error = <S as Sink<Item>>::Error;
+
+    delegate_sink!(stream, Item);
+}
+
+/// Error indicating, that while chunk was collected inner stream produced an error.
+///
+/// Contains all items that were collected before an error occurred, and the stream error itself.
+#[derive(PartialEq, Eq)]
+pub struct TryReadyChunksError<T, E>(pub Vec<T>, pub E);
+
+impl<T, E: fmt::Debug> fmt::Debug for TryReadyChunksError<T, E> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.1.fmt(f)
+    }
+}
+
+impl<T, E: fmt::Display> fmt::Display for TryReadyChunksError<T, E> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.1.fmt(f)
+    }
+}
+
+#[cfg(feature = "std")]
+impl<T, E: fmt::Debug + fmt::Display> std::error::Error for TryReadyChunksError<T, E> {}