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Fix ResponseDataStream small-buffer reads #457

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Merged
durch merged 1 commit into from
May 4, 2026
Merged

Fix ResponseDataStream small-buffer reads #457

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165 changes: 92 additions & 73 deletions s3/src/request/request_trait.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -21,11 +21,10 @@ use http::header::{
};
use std::fmt::Write as _;

#[cfg(feature = "with-async-std")]
use async_std::stream::Stream;

#[cfg(feature = "with-tokio")]
use tokio_stream::Stream;
#[cfg(any(feature = "with-tokio", feature = "with-async-std"))]
use futures_util::Stream;
#[cfg(any(feature = "with-tokio", feature = "with-async-std"))]
use futures_util::stream::{self, StreamExt};

#[derive(Debug)]

Expand Down Expand Up @@ -126,28 +125,39 @@ impl tokio::io::AsyncRead for ResponseDataStream {
cx: &mut std::task::Context<'_>,
buf: &mut tokio::io::ReadBuf<'_>,
) -> std::task::Poll<std::io::Result<()>> {
// Poll the stream for the next chunk of bytes
match Stream::poll_next(self.bytes.as_mut(), cx) {
std::task::Poll::Ready(Some(Ok(chunk))) => {
// Write as much of the chunk as fits in the buffer
let amt = std::cmp::min(chunk.len(), buf.remaining());
buf.put_slice(&chunk[..amt]);

// AIDEV-NOTE: Bytes that don't fit in the buffer are discarded from this chunk.
// This is expected AsyncRead behavior - consumers should use appropriately sized
// buffers or wrap in BufReader for efficiency with small reads.

std::task::Poll::Ready(Ok(()))
}
std::task::Poll::Ready(Some(Err(error))) => {
// Convert S3Error to io::Error
std::task::Poll::Ready(Err(std::io::Error::other(error)))
}
std::task::Poll::Ready(None) => {
// Stream is exhausted, signal EOF by returning Ok(()) with no bytes written
std::task::Poll::Ready(Ok(()))
if buf.remaining() == 0 {
return std::task::Poll::Ready(Ok(()));
}

loop {
match Stream::poll_next(self.bytes.as_mut(), cx) {
std::task::Poll::Ready(Some(Ok(chunk))) => {
if chunk.is_empty() {
continue;
}

let amt = std::cmp::min(chunk.len(), buf.remaining());
buf.put_slice(&chunk[..amt]);

if amt < chunk.len() {
let remainder = chunk.slice(amt..);
let previous_stream =
std::mem::replace(&mut self.bytes, Box::pin(stream::empty()));
self.bytes = Box::pin(
stream::once(async move { Ok(remainder) }).chain(previous_stream),
);
}

return std::task::Poll::Ready(Ok(()));
}
std::task::Poll::Ready(Some(Err(error))) => {
return std::task::Poll::Ready(Err(std::io::Error::other(error)));
}
std::task::Poll::Ready(None) => {
return std::task::Poll::Ready(Ok(()));
}
std::task::Poll::Pending => return std::task::Poll::Pending,
}
std::task::Poll::Pending => std::task::Poll::Pending,
}
}
}
Expand All @@ -159,28 +169,39 @@ impl async_std::io::Read for ResponseDataStream {
cx: &mut std::task::Context<'_>,
buf: &mut [u8],
) -> std::task::Poll<std::io::Result<usize>> {
// Poll the stream for the next chunk of bytes
match Stream::poll_next(self.bytes.as_mut(), cx) {
std::task::Poll::Ready(Some(Ok(chunk))) => {
// Write as much of the chunk as fits in the buffer
let amt = std::cmp::min(chunk.len(), buf.len());
buf[..amt].copy_from_slice(&chunk[..amt]);

// AIDEV-NOTE: Bytes that don't fit in the buffer are discarded from this chunk.
// This is expected AsyncRead behavior - consumers should use appropriately sized
// buffers or wrap in BufReader for efficiency with small reads.

std::task::Poll::Ready(Ok(amt))
}
std::task::Poll::Ready(Some(Err(error))) => {
// Convert S3Error to io::Error
std::task::Poll::Ready(Err(std::io::Error::other(error)))
}
std::task::Poll::Ready(None) => {
// Stream is exhausted, signal EOF by returning 0 bytes read
std::task::Poll::Ready(Ok(0))
if buf.is_empty() {
return std::task::Poll::Ready(Ok(0));
}

loop {
match Stream::poll_next(self.bytes.as_mut(), cx) {
std::task::Poll::Ready(Some(Ok(chunk))) => {
if chunk.is_empty() {
continue;
}

let amt = std::cmp::min(chunk.len(), buf.len());
buf[..amt].copy_from_slice(&chunk[..amt]);

if amt < chunk.len() {
let remainder = chunk.slice(amt..);
let previous_stream =
std::mem::replace(&mut self.bytes, Box::pin(stream::empty()));
self.bytes = Box::pin(
stream::once(async move { Ok(remainder) }).chain(previous_stream),
);
}

return std::task::Poll::Ready(Ok(amt));
}
std::task::Poll::Ready(Some(Err(error))) => {
return std::task::Poll::Ready(Err(std::io::Error::other(error)));
}
std::task::Poll::Ready(None) => {
return std::task::Poll::Ready(Ok(0));
}
std::task::Poll::Pending => return std::task::Poll::Pending,
}
std::task::Poll::Pending => std::task::Poll::Pending,
}
}
}
Expand Down Expand Up @@ -875,11 +896,9 @@ mod tests {
}

#[tokio::test]
async fn test_async_read_with_small_buffer() {
// Create a stream with a large chunk
let chunks = vec![Ok(Bytes::from(
"This is a much longer string that won't fit in a small buffer",
))];
async fn test_async_read_with_small_buffer_preserves_large_chunk() {
let expected = b"This is a much longer string that won't fit in a small buffer";
let chunks = vec![Ok(Bytes::copy_from_slice(expected))];

let stream = stream::iter(chunks);
let data_stream: DataStream = Box::pin(stream);
Expand All @@ -889,17 +908,18 @@ mod tests {
status_code: 200,
};

// Read with a small buffer - demonstrates that excess bytes are discarded per chunk
let mut output = Vec::new();
let mut buffer = [0u8; 10];
let n = response_stream.read(&mut buffer).await.unwrap();

// We should only get the first 10 bytes
assert_eq!(n, 10);
assert_eq!(&buffer[..n], b"This is a ");
loop {
let n = response_stream.read(&mut buffer).await.unwrap();
if n == 0 {
break;
}
output.extend_from_slice(&buffer[..n]);
}

// Next read should get 0 bytes (EOF) because the chunk was consumed
let n = response_stream.read(&mut buffer).await.unwrap();
assert_eq!(n, 0);
assert_eq!(output, expected);
}

#[tokio::test]
Expand Down Expand Up @@ -992,11 +1012,9 @@ mod async_std_tests {
}

#[async_std::test]
async fn test_async_read_with_small_buffer() {
// Create a stream with a large chunk
let chunks = vec![Ok(Bytes::from(
"This is a much longer string that won't fit in a small buffer",
))];
async fn test_async_read_with_small_buffer_preserves_large_chunk() {
let expected = b"This is a much longer string that won't fit in a small buffer";
let chunks = vec![Ok(Bytes::copy_from_slice(expected))];

let stream = stream::iter(chunks);
let data_stream: DataStream = Box::pin(stream);
Expand All @@ -1006,17 +1024,18 @@ mod async_std_tests {
status_code: 200,
};

// Read with a small buffer - demonstrates that excess bytes are discarded per chunk
let mut output = Vec::new();
let mut buffer = [0u8; 10];
let n = response_stream.read(&mut buffer).await.unwrap();

// We should only get the first 10 bytes
assert_eq!(n, 10);
assert_eq!(&buffer[..n], b"This is a ");
loop {
let n = response_stream.read(&mut buffer).await.unwrap();
if n == 0 {
break;
}
output.extend_from_slice(&buffer[..n]);
}

// Next read should get 0 bytes (EOF) because the chunk was consumed
let n = response_stream.read(&mut buffer).await.unwrap();
assert_eq!(n, 0);
assert_eq!(output, expected);
}

#[async_std::test]
Expand Down
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