Implement try_chunks (#2438)

futures-util/src/stream/mod.rs

@@ -62,6 +62,9 @@ pub use self::try_stream::IntoAsyncRead;
 #[cfg(feature = "alloc")]
 pub use self::try_stream::{TryBufferUnordered, TryBuffered, TryForEachConcurrent};
 
+#[cfg(feature = "alloc")]
+pub use self::try_stream::{TryChunks, TryChunksError};
+
 // Primitive streams
 
 mod iter;

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

@@ -12,6 +12,8 @@ use crate::fns::{
 use crate::future::assert_future;
 use crate::stream::assert_stream;
 use crate::stream::{Inspect, Map};
+#[cfg(feature = "alloc")]
+use alloc::vec::Vec;
 use core::pin::Pin;
 use futures_core::{
     future::{Future, TryFuture},
@@ -94,6 +96,12 @@ mod try_concat;
 #[allow(unreachable_pub)] // https://github.com/rust-lang/rust/issues/57411
 pub use self::try_concat::TryConcat;
 
+#[cfg(feature = "alloc")]
+mod try_chunks;
+#[cfg(feature = "alloc")]
+#[allow(unreachable_pub)] // https://github.com/rust-lang/rust/issues/57411
+pub use self::try_chunks::{TryChunks, TryChunksError};
+
 mod try_fold;
 #[allow(unreachable_pub)] // https://github.com/rust-lang/rust/issues/57411
 pub use self::try_fold::TryFold;
@@ -576,6 +584,53 @@ pub trait TryStreamExt: TryStream {
         assert_future::<Result<C, Self::Error>, _>(TryCollect::new(self))
     }
 
+    /// An adaptor for chunking up successful 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.
+    ///
+    /// 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::chunks`](crate::stream::StreamExt::chunks) but exits
+    /// early if an error occurs.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # futures::executor::block_on(async {
+    /// use futures::stream::{self, TryChunksError, 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_chunks(2);
+    ///
+    /// assert_eq!(stream.try_next().await, Ok(Some(vec![1, 2])));
+    /// assert_eq!(stream.try_next().await, Err(TryChunksError(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_chunks(self, capacity: usize) -> TryChunks<Self>
+    where
+        Self: Sized,
+    {
+        assert_stream::<Result<Vec<Self::Ok>, TryChunksError<Self::Ok, Self::Error>>, _>(
+            TryChunks::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_chunks.rs

@@ -0,0 +1,131 @@
+use crate::stream::{Fuse, IntoStream, StreamExt};
+
+use alloc::vec::Vec;
+use core::pin::Pin;
+use core::{fmt, mem};
+use futures_core::ready;
+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_chunks`](super::TryStreamExt::try_chunks) method.
+    #[derive(Debug)]
+    #[must_use = "streams do nothing unless polled"]
+    pub struct TryChunks<St: TryStream> {
+        #[pin]
+        stream: Fuse<IntoStream<St>>,
+        items: Vec<St::Ok>,
+        cap: usize, // https://github.com/rust-lang/futures-rs/issues/1475
+    }
+}
+
+impl<St: TryStream> TryChunks<St> {
+    pub(super) fn new(stream: St, capacity: usize) -> Self {
+        assert!(capacity > 0);
+
+        Self {
+            stream: IntoStream::new(stream).fuse(),
+            items: Vec::with_capacity(capacity),
+            cap: capacity,
+        }
+    }
+
+    fn take(self: Pin<&mut Self>) -> Vec<St::Ok> {
+        let cap = self.cap;
+        mem::replace(self.project().items, Vec::with_capacity(cap))
+    }
+
+    delegate_access_inner!(stream, St, (. .));
+}
+
+impl<St: TryStream> Stream for TryChunks<St> {
+    #[allow(clippy::type_complexity)]
+    type Item = Result<Vec<St::Ok>, TryChunksError<St::Ok, St::Error>>;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        let mut this = self.as_mut().project();
+        loop {
+            match ready!(this.stream.as_mut().try_poll_next(cx)) {
+                // Push the 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.
+                Some(item) => match item {
+                    Ok(item) => {
+                        this.items.push(item);
+                        if this.items.len() >= *this.cap {
+                            return Poll::Ready(Some(Ok(self.take())));
+                        }
+                    }
+                    Err(e) => {
+                        return Poll::Ready(Some(Err(TryChunksError(self.take(), e))));
+                    }
+                },
+
+                // Since the underlying stream ran out of values, return what we
+                // have buffered, if we have anything.
+                None => {
+                    let last = if this.items.is_empty() {
+                        None
+                    } else {
+                        let full_buf = mem::replace(this.items, Vec::new());
+                        Some(full_buf)
+                    };
+
+                    return Poll::Ready(last.map(Ok));
+                }
+            }
+        }
+    }
+
+    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: TryStream + FusedStream> FusedStream for TryChunks<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 TryChunks<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 occured, and the stream error itself.
+#[derive(PartialEq, Eq)]
+pub struct TryChunksError<T, E>(pub Vec<T>, pub E);
+
+impl<T, E: fmt::Debug> fmt::Debug for TryChunksError<T, E> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.1.fmt(f)
+    }
+}
+
+impl<T, E: fmt::Display> fmt::Display for TryChunksError<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 TryChunksError<T, E> {}