src/sys/lib/fidl-fuchsia-pkg-ext/src/serve_fidl_iterator.rs

// Copyright 2021 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

use {
    crate::Measurable,
    anyhow::{Context as _, Result},
    fidl_fuchsia_pkg::{
        BlobIdIteratorNextResponder, BlobIdIteratorRequest, BlobIdIteratorRequestStream,
        BlobInfoIteratorNextResponder, BlobInfoIteratorRequest, BlobInfoIteratorRequestStream,
        PackageIndexEntry, PackageIndexIteratorNextResponder, PackageIndexIteratorRequest,
        PackageIndexIteratorRequestStream,
    },
    fuchsia_zircon_types::ZX_CHANNEL_MAX_MSG_BYTES,
    futures::prelude::*,
};

/// Serves fidl iterators like:
///
/// protocol PayloadIterator {
///    Next() -> (vector<Payload>:MAX payloads);
/// };
///
/// from:
///   `fidl_iterator`: effectively a stream of `PayloadIterator::Next` requests
///   `items`: a slice of `Payload`s.
///
/// Fills each response to `Next()` with as many entries as will fit in a fidl message. The
/// returned future completes after `Next()` yields an empty response or the iterator
/// is interrupted (client closes the channel or the task encounters a FIDL layer error).
///
/// To use with a new protocol (e.g. `PayloadIterator`), in this crate:
///   1. implement `FidlIteratorRequestStream` for `PayloadIteratorRequestStream`
///   2. implement `FidlIteratorNextResponder` for `PayloadIteratorNextResponder`
///   3. implement `Measurable` for `Payload` using functions generated by
///      //tools/fidl/measure-tape
pub async fn serve_fidl_iterator_from_slice<I>(
    mut fidl_iterator: I,
    mut items: impl AsMut<[<I::Responder as FidlIteratorNextResponder>::Item]>,
) -> Result<()>
where
    I: FidlIteratorRequestStream,
{
    let mut items = SliceChunker::new(items.as_mut());

    loop {
        let chunk = items.next();

        let responder =
            match fidl_iterator.try_next().await.context("while waiting for next() request")? {
                None => break,
                Some(request) => I::request_to_responder(request),
            };

        let () = responder.send_chunk(&chunk).context("while responding")?;

        // Yield a single empty chunk, then stop serving the protocol.
        if chunk.is_empty() {
            break;
        }
    }

    Ok(())
}

/// Serves fidl iterators like:
///
/// protocol PayloadIterator {
///    Next() -> (vector<Payload>:MAX payloads);
/// };
///
/// from:
///   `fidl_iterator`: effectively a stream of `PayloadIterator::Next` requests
///   `stream`: a Stream<Vec<Payload>>
///   `max_stream_chunks`: the maximum number of `Vec<Payload>`'s to pull from `stream` at a time.
///     Making this number larger can pack more `Payload`s into the fidl response, decreasing
///     overhead, but the buffer of `Vec<Payload>`s is pre-allocated, so if this number is e.g.
///     `usize::MAX` the program will OOM. This number is the maximum, not the minimum, i.e
///     `serve_fidl_iterator_from_stream` will not block on `stream` if there are available
///     `Payload`s to send. Arguments of `0` will be converted to `1`.
///
///
/// Fills each response to `Next()` with as many available entries as will fit in a fidl message.
/// Only blocks on `stream` if there are no available entries.
/// The returned future completes after `Next()` yields an empty response or the iterator
/// is interrupted (client closes the channel or the task encounters a FIDL layer error).
///
/// To use with a new protocol (e.g. `PayloadIterator`), in this crate:
///   1. implement `FidlIteratorRequestStream` for `PayloadIteratorRequestStream`
///   2. implement `FidlIteratorNextResponder` for `PayloadIteratorNextResponder`
///   3. implement `Measurable` for `Payload` using functions generated by
///      //tools/fidl/measure-tape
pub async fn serve_fidl_iterator_from_stream<I>(
    mut fidl_iterator: I,
    stream: impl futures::stream::Stream<Item = Vec<<I::Responder as FidlIteratorNextResponder>::Item>>
        + Unpin,
    max_stream_chunks: usize,
) -> Result<()>
where
    I: FidlIteratorRequestStream,
{
    let mut chunked_stream = stream.ready_chunks(std::cmp::max(max_stream_chunks, 1));
    let mut fidl_chunker = OwningChunker::new();

    loop {
        let responder =
            match fidl_iterator.try_next().await.context("while waiting for next() request")? {
                None => break,
                Some(request) => I::request_to_responder(request),
            };

        // Get as many new items as possible, to minimize the number of FIDL messages, but don't
        // block if we already have some.
        if fidl_chunker.is_empty() {
            loop {
                if let Some(xss) = chunked_stream.next().await {
                    fidl_chunker.extend(xss.into_iter().flatten());
                    if fidl_chunker.is_empty() {
                        continue;
                    }
                }
                break;
            }
        } else {
            if let Some(Some(xss)) = chunked_stream.next().now_or_never() {
                fidl_chunker.extend(xss.into_iter().flatten());
            }
        }

        let mut chunk = fidl_chunker.next();
        let () = responder.send_chunk(chunk.make_contiguous()).context("while responding")?;
        if chunk.is_empty() {
            break;
        }
    }

    Ok(())
}

/// A FIDL request stream for a FIDL protocol following the iterator pattern.
pub trait FidlIteratorRequestStream:
    fidl::endpoints::RequestStream + TryStream<Error = fidl::Error>
{
    type Responder: FidlIteratorNextResponder;

    fn request_to_responder(request: <Self as TryStream>::Ok) -> Self::Responder;
}

/// A responder to a Next() request for a FIDL iterator.
pub trait FidlIteratorNextResponder {
    type Item: Measurable;

    fn send_chunk(self, chunk: &[Self::Item]) -> Result<(), fidl::Error>;
}

impl FidlIteratorRequestStream for PackageIndexIteratorRequestStream {
    type Responder = PackageIndexIteratorNextResponder;

    fn request_to_responder(request: PackageIndexIteratorRequest) -> Self::Responder {
        let PackageIndexIteratorRequest::Next { responder } = request;
        responder
    }
}

impl FidlIteratorNextResponder for PackageIndexIteratorNextResponder {
    type Item = PackageIndexEntry;

    fn send_chunk(self, chunk: &[Self::Item]) -> Result<(), fidl::Error> {
        self.send(chunk)
    }
}

impl FidlIteratorRequestStream for BlobInfoIteratorRequestStream {
    type Responder = BlobInfoIteratorNextResponder;

    fn request_to_responder(request: BlobInfoIteratorRequest) -> Self::Responder {
        let BlobInfoIteratorRequest::Next { responder } = request;
        responder
    }
}

impl FidlIteratorRequestStream for BlobIdIteratorRequestStream {
    type Responder = BlobIdIteratorNextResponder;

    fn request_to_responder(request: BlobIdIteratorRequest) -> Self::Responder {
        let BlobIdIteratorRequest::Next { responder } = request;
        responder
    }
}

impl FidlIteratorNextResponder for BlobInfoIteratorNextResponder {
    type Item = fidl_fuchsia_pkg::BlobInfo;

    fn send_chunk(self, chunk: &[Self::Item]) -> Result<(), fidl::Error> {
        self.send(chunk)
    }
}

impl FidlIteratorNextResponder for BlobIdIteratorNextResponder {
    type Item = fidl_fuchsia_pkg::BlobId;

    fn send_chunk(self, chunk: &[Self::Item]) -> Result<(), fidl::Error> {
        self.send(chunk)
    }
}

// FIXME(52297) This constant would ideally be exported by the `fidl` crate.
// sizeof(TransactionHeader) + sizeof(VectorHeader)
const FIDL_VEC_RESPONSE_OVERHEAD_BYTES: usize = 32;

/// Assumes the fixed overhead of a single fidl response header and a single vec header per chunk.
/// It must not be used with more complex responses.
fn how_many_items_fit_in_fidl_vec_response<'a>(
    items: impl Iterator<Item = &'a (impl Measurable + 'a)>,
) -> usize {
    let mut bytes_used: usize = FIDL_VEC_RESPONSE_OVERHEAD_BYTES;
    let mut count = 0;

    for item in items {
        bytes_used += item.measure();
        if bytes_used > ZX_CHANNEL_MAX_MSG_BYTES as usize {
            break;
        }
        count += 1;
    }
    count
}

/// Helper to split a slice of items into chunks that will fit in a single FIDL vec response.
///
/// Note, SliceChunker assumes the fixed overhead of a single fidl response header and a single vec
/// header per chunk.  It must not be used with more complex responses.
struct SliceChunker<'a, I> {
    items: &'a mut [I],
}

impl<'a, I> SliceChunker<'a, I>
where
    I: Measurable,
{
    fn new(items: &'a mut [I]) -> Self {
        Self { items }
    }

    /// Produce the next chunk of items to respond with. Iteration stops when this method returns
    /// an empty slice, which occurs when either:
    /// * All items have been returned
    /// * SliceChunker encounters an item so large that it cannot even be stored in a response
    ///   dedicated to just that one item.
    ///
    /// Once next() returns an empty slice, it will continue to do so in future calls.
    fn next(&mut self) -> &'a mut [I] {
        let entry_count = how_many_items_fit_in_fidl_vec_response(self.items.iter());
        // tmp/swap dance to appease the borrow checker.
        let tmp = std::mem::replace(&mut self.items, &mut []);
        let (chunk, rest) = tmp.split_at_mut(entry_count);
        self.items = rest;
        chunk
    }
}

/// Helper to split a collection of items into chunks that will fit in a single FIDL vec response.
///
/// Note, OwningChunker assumes the fixed overhead of a single fidl response header and a single vec
/// header per chunk.  It must not be used with more complex responses.
struct OwningChunker<I> {
    items: std::collections::VecDeque<I>,
}

impl<I> OwningChunker<I>
where
    I: Measurable,
{
    fn new() -> Self {
        Self { items: std::collections::VecDeque::new() }
    }

    /// Produce the next chunk of items to respond with. Iteration stops when this method returns
    /// an empty VecDeque, which occurs when either:
    /// * All items have been returned (and no new items are added)
    /// * OwningChunker encounters an item so large that it cannot even be stored in a response
    ///   dedicated to just that one item.
    fn next(&mut self) -> std::collections::VecDeque<I> {
        let count = how_many_items_fit_in_fidl_vec_response(self.items.iter());
        let mut other = self.items.split_off(count);
        std::mem::swap(&mut self.items, &mut other);
        other
    }

    fn is_empty(&self) -> bool {
        self.items.is_empty()
    }

    fn extend(&mut self, iter: impl IntoIterator<Item = I>) {
        self.items.extend(iter)
    }
}

#[cfg(test)]
mod tests {
    use {
        super::*,
        fidl_fuchsia_pkg::{BlobInfoIteratorMarker, PackageIndexIteratorMarker},
        fuchsia_async::Task,
        fuchsia_hash::HashRangeFull,
        fuchsia_pkg::PackagePath,
        proptest::prelude::*,
    };

    #[test]
    fn zx_channel_max_msg_bytes_fits_in_usize() {
        let _: usize = ZX_CHANNEL_MAX_MSG_BYTES.try_into().unwrap();
    }

    #[derive(Debug, Clone, Copy, PartialEq, Eq)]
    struct Byte(u8);

    impl Measurable for Byte {
        fn measure(&self) -> usize {
            1
        }
    }

    #[test]
    fn slice_chunker_fuses() {
        let items = &mut [Byte(42)];
        let mut chunker = SliceChunker::new(items);

        assert_eq!(chunker.next(), &mut [Byte(42)]);
        assert_eq!(chunker.next(), &mut []);
        assert_eq!(chunker.next(), &mut []);
    }

    #[test]
    fn slice_chunker_chunks_at_expected_boundary() {
        const BYTES_PER_CHUNK: usize =
            ZX_CHANNEL_MAX_MSG_BYTES as usize - FIDL_VEC_RESPONSE_OVERHEAD_BYTES;

        // Expect to fill 2 full chunks with 1 item left over.
        let mut items =
            (0..=(BYTES_PER_CHUNK as u64 * 2)).map(|n| Byte(n as u8)).collect::<Vec<Byte>>();
        let expected = items.clone();
        let mut chunker = SliceChunker::new(&mut items);

        let mut actual: Vec<Byte> = vec![];

        for _ in 0..2 {
            let chunk = chunker.next();
            assert_eq!(chunk.len(), BYTES_PER_CHUNK);

            actual.extend(&*chunk);
        }

        let chunk = chunker.next();
        assert_eq!(chunk.len(), 1);
        actual.extend(&*chunk);

        assert_eq!(actual, expected);
    }

    #[test]
    fn slice_chunker_terminates_at_too_large_item() {
        #[derive(Debug, PartialEq, Eq)]
        struct TooBig;
        impl Measurable for TooBig {
            fn measure(&self) -> usize {
                ZX_CHANNEL_MAX_MSG_BYTES as usize
            }
        }

        let items = &mut [TooBig];
        let mut chunker = SliceChunker::new(items);
        assert_eq!(chunker.next(), &mut []);
    }

    #[test]
    fn owning_chunker_fuses() {
        let items = [Byte(42)];
        let mut chunker = OwningChunker::new();
        chunker.extend(items);

        assert_eq!(chunker.next().make_contiguous(), &[Byte(42)]);
        assert_eq!(chunker.next().make_contiguous(), &[]);
        assert_eq!(chunker.next().make_contiguous(), &[]);
    }

    #[test]
    fn owning_chunker_chunks_at_expected_boundary() {
        const BYTES_PER_CHUNK: usize =
            ZX_CHANNEL_MAX_MSG_BYTES as usize - FIDL_VEC_RESPONSE_OVERHEAD_BYTES;

        // Expect to fill 2 full chunks with 1 item left over.
        let items =
            (0..=(BYTES_PER_CHUNK as u64 * 2)).map(|n| Byte(n as u8)).collect::<Vec<Byte>>();
        let expected = items.clone();
        let mut chunker = OwningChunker::new();
        chunker.extend(items.into_iter());

        let mut actual: Vec<Byte> = vec![];

        for _ in 0..2 {
            let chunk = chunker.next();
            assert_eq!(chunk.len(), BYTES_PER_CHUNK);

            actual.extend(chunk);
        }

        let chunk = chunker.next();
        assert_eq!(chunk.len(), 1);
        actual.extend(chunk);

        assert_eq!(actual, expected);
    }

    #[test]
    fn owning_chunker_terminates_at_too_large_item() {
        #[derive(Debug, PartialEq, Eq)]
        struct TooBig;
        impl Measurable for TooBig {
            fn measure(&self) -> usize {
                ZX_CHANNEL_MAX_MSG_BYTES as usize
            }
        }

        let items = [TooBig];
        let mut chunker = OwningChunker::new();
        chunker.extend(items);
        assert_eq!(chunker.next().make_contiguous(), &mut []);
    }

    #[test]
    fn owning_chunker_extend_after_next() {
        let mut chunker = OwningChunker::new();
        chunker.extend([Byte(0)]);
        chunker.extend([Byte(1)]);

        assert_eq!(chunker.next().make_contiguous(), &[Byte(0), Byte(1)]);
        assert_eq!(chunker.next().make_contiguous(), &[]);

        chunker.extend([Byte(2)]);

        assert_eq!(chunker.next().make_contiguous(), &[Byte(2)]);
    }

    #[test]
    fn verify_fidl_vec_response_overhead() {
        let vec_response_overhead = {
            use fidl::encoding::{
                DynamicFlags, TransactionHeader, TransactionMessage, TransactionMessageType,
                UnboundedVector,
            };

            type Msg = TransactionMessageType<UnboundedVector<u8>>;
            let msg = TransactionMessage {
                header: TransactionHeader::new(0, 0, DynamicFlags::empty()),
                body: &[] as &[u8],
            };
            fidl::encoding::with_tls_encoded::<Msg, _>(msg, |bytes, _handles| Ok(bytes.len()))
                .unwrap()
        };
        assert_eq!(vec_response_overhead, FIDL_VEC_RESPONSE_OVERHEAD_BYTES);
    }

    proptest! {
        #![proptest_config(ProptestConfig{
            // Disable persistence to avoid the warning for not running in the
            // source code directory (since we're running on a Fuchsia target)
            failure_persistence: None,
            .. ProptestConfig::default()
        })]

        #[test]
        fn serve_fidl_iterator_from_slice_yields_expected_entries(items: Vec<crate::BlobInfo>) {
            let mut executor = fuchsia_async::TestExecutor::new();
            executor.run_singlethreaded(async move {
                let (proxy, stream) =
                    fidl::endpoints::create_proxy_and_stream::<BlobInfoIteratorMarker>().unwrap();
                let mut actual_items = vec![];

                let ((), ()) = futures::future::join(
                    async {
                        let items = items
                            .iter()
                            .cloned()
                            .map(fidl_fuchsia_pkg::BlobInfo::from)
                            .collect::<Vec<_>>();
                        serve_fidl_iterator_from_slice(stream, items).await.unwrap()
                    },
                    async {
                        loop {
                            let chunk = proxy.next().await.unwrap();
                            if chunk.is_empty() {
                                break;
                            }
                            let chunk = chunk.into_iter().map(crate::BlobInfo::from);
                            actual_items.extend(chunk);
                        }
                    },
                )
                .await;

                assert_eq!(items, actual_items);
            })
        }

        #[test]
        fn serve_fidl_iterator_from_stream_yields_expected_entries(
            items: Vec<crate::BlobInfo>,
            repetition in 0..4usize,
            max_chunking in 0..4usize,
        ) {
            let mut executor = fuchsia_async::TestExecutor::new();
            executor.run_singlethreaded(async move {
                let (proxy, fidl_stream) =
                    fidl::endpoints::create_proxy_and_stream::<BlobInfoIteratorMarker>().unwrap();
                let (mut item_sender, item_stream) = futures::channel::mpsc::unbounded();
                let mut actual_items = vec![];

                let ((), (), ()) = futures::future::join3(
                    async {
                        for _ in 0..repetition {
                            let () = item_sender.send(items
                                .iter()
                                .cloned()
                                .map(fidl_fuchsia_pkg::BlobInfo::from)
                                .collect::<Vec<_>>()).await.unwrap();
                        }
                        drop(item_sender);
                    },
                    async {
                        let () = serve_fidl_iterator_from_stream(
                            fidl_stream,
                            item_stream,
                            max_chunking
                        )
                        .await
                        .unwrap();
                    },
                    async {
                        loop {
                            let chunk = proxy.next().await.unwrap();
                            if chunk.is_empty() {
                                break;
                            }
                            let chunk = chunk.into_iter().map(crate::BlobInfo::from);
                            actual_items.extend(chunk);
                        }
                    },
                )
                .await;

                let expected_items = {
                    let mut expected_items = vec![];
                    for _ in 0..repetition {
                        expected_items.extend(items.iter().cloned())
                    }
                    expected_items
                };
                assert_eq!(expected_items, actual_items);
            })
        }
    }

    const PACKAGE_INDEX_CHUNK_SIZE_MAX: usize = 818;

    // FIDL message is at most 65,536 bytes because of zx_channel_write [1].
    // `PackageIndexIterator.Next()` return value size, encoded [2], is:
    // 16 bytes FIDL transaction header +
    // 16 bytes vector header +
    // N * (16 bytes string header (from url field of struct PackageUrl) +
    // L bytes string content +
    // 32 bytes array.
    // This totals in 32 + N * (48 + L), where L is 8-byte aligned
    // because secondary objects (e.g. string contents) are 8-byte aligned.
    //
    // The shortest possible package url is 29 bytes "fuchsia-pkg://fuchsia.com/a/0".
    //
    // And the longest is 283 bytes, which is 288 bytes with 8-byte alignment, so
    // PACKAGE_INDEX_CHUNK_SIZE_MIN => 65536 <= 32 + N * (48 + 288) => N = 194
    //
    // [1] https://fuchsia.dev/fuchsia-src/reference/syscalls/channel_write
    // [2] https://fuchsia.dev/fuchsia-src/reference/fidl/language/wire-format
    const PACKAGE_INDEX_CHUNK_SIZE_MIN: usize = 194;

    #[fuchsia_async::run_singlethreaded(test)]
    async fn package_index_iterator_paginates_shortest_entries() {
        let names = ('a'..='z').cycle().map(|c| c.to_string());
        let paths = names.map(|name| {
            PackagePath::from_name_and_variant(name.parse().unwrap(), "0".parse().unwrap())
        });

        verify_package_index_iterator_pagination(paths, PACKAGE_INDEX_CHUNK_SIZE_MAX).await;
    }

    #[fuchsia_async::run_singlethreaded(test)]
    async fn package_index_iterator_paginates_longest_entries() {
        let names = ('a'..='z')
            .map(|c| std::iter::repeat(c).take(PackagePath::MAX_NAME_BYTES).collect::<String>())
            .cycle();
        let paths = names.map(|name| {
            PackagePath::from_name_and_variant(name.parse().unwrap(), "0".parse().unwrap())
        });

        verify_package_index_iterator_pagination(paths, PACKAGE_INDEX_CHUNK_SIZE_MIN).await;
    }

    async fn verify_package_index_iterator_pagination(
        paths: impl Iterator<Item = PackagePath>,
        expected_chunk_size: usize,
    ) {
        let package_entries: Vec<fidl_fuchsia_pkg::PackageIndexEntry> = paths
            .zip(HashRangeFull::default())
            .take(expected_chunk_size * 2)
            .map(|(path, hash)| fidl_fuchsia_pkg::PackageIndexEntry {
                package_url: fidl_fuchsia_pkg::PackageUrl {
                    url: format!("fuchsia-pkg://fuchsia.com/{}", path),
                },
                meta_far_blob_id: crate::BlobId::from(hash).into(),
            })
            .collect();

        let (iter, stream) =
            fidl::endpoints::create_proxy_and_stream::<PackageIndexIteratorMarker>().unwrap();
        let task = Task::local(serve_fidl_iterator_from_slice(stream, package_entries));

        let chunk = iter.next().await.unwrap();
        assert_eq!(chunk.len(), expected_chunk_size);

        let chunk = iter.next().await.unwrap();
        assert_eq!(chunk.len(), expected_chunk_size);

        let chunk = iter.next().await.unwrap();
        assert_eq!(chunk.len(), 0);

        let () = task.await.unwrap();
    }

    // TestExecutor.run_until_stalled is not available on host
    #[cfg(target_os = "fuchsia")]
    use assert_matches::assert_matches;

    // TestExecutor.run_until_stalled is not available on host
    #[cfg(target_os = "fuchsia")]
    #[test]
    fn serve_fidl_iterator_from_stream_ignores_empty_vec() {
        let mut executor = fuchsia_async::TestExecutor::new();
        let (proxy, fidl_stream) =
            fidl::endpoints::create_proxy_and_stream::<BlobInfoIteratorMarker>().unwrap();
        let (item_sender, item_stream) = futures::channel::mpsc::unbounded();
        let mut serve_task = serve_fidl_iterator_from_stream(fidl_stream, item_stream, 10).boxed();

        // serve_fidl_iterator_from_stream should ignore the empty vec of Payloads, so
        // chunk_fut should not complete.
        let () = item_sender.unbounded_send(vec![]).unwrap();
        let mut chunk_fut = proxy.next();
        assert_matches!(executor.run_until_stalled(&mut serve_task), std::task::Poll::Pending);
        assert_matches!(executor.run_until_stalled(&mut chunk_fut), std::task::Poll::Pending);

        // chunk_fut should complete once serve_fidl_iterator_from_stream is given a Payload
        let blob_info = crate::BlobInfo { blob_id: [0; 32].into(), length: 0 };
        let () = item_sender.unbounded_send(vec![blob_info.into()]).unwrap();
        assert_matches!(executor.run_until_stalled(&mut serve_task), std::task::Poll::Pending);
        assert_matches!(
            executor.run_until_stalled(&mut chunk_fut),
            std::task::Poll::Ready(Ok(chunk))
                if chunk == vec![fidl_fuchsia_pkg::BlobInfo::from(blob_info)]
        );
    }

    // TestExecutor.run_until_stalled is not available on host
    #[cfg(target_os = "fuchsia")]
    #[test]
    fn serve_fidl_iterator_from_stream_does_not_block_if_chunker_not_empty() {
        let mut executor = fuchsia_async::TestExecutor::new();
        let (proxy, fidl_stream) =
            fidl::endpoints::create_proxy_and_stream::<BlobInfoIteratorMarker>().unwrap();
        let (item_sender, item_stream) = futures::channel::mpsc::unbounded();
        let mut serve_task = serve_fidl_iterator_from_stream(fidl_stream, item_stream, 10).boxed();

        let blob_info = fidl_fuchsia_pkg::BlobInfo::from(crate::BlobInfo {
            blob_id: [0; 32].into(),
            length: 0,
        });
        let max_payloads_per_fidl_response = (ZX_CHANNEL_MAX_MSG_BYTES as usize
            - FIDL_VEC_RESPONSE_OVERHEAD_BYTES)
            / measure_fuchsia_pkg::Measurable::measure(&blob_info).num_bytes;
        let payloads = vec![blob_info; max_payloads_per_fidl_response + 1];
        assert_eq!(
            how_many_items_fit_in_fidl_vec_response(payloads.iter()),
            max_payloads_per_fidl_response
        );

        // Send all the payloads, the first FIDL response should contain as many as will fit.
        let () = item_sender.unbounded_send(payloads).unwrap();
        let mut chunk_fut = proxy.next();
        assert_matches!(executor.run_until_stalled(&mut serve_task), std::task::Poll::Pending);
        assert_matches!(
            executor.run_until_stalled(&mut chunk_fut),
            std::task::Poll::Ready(Ok(chunk))
                if chunk.len() == max_payloads_per_fidl_response
        );

        // There should be one payload left in the OwningChunker, so we should be able to obtain
        // another FIDL response without sending more payloads.
        let mut chunk_fut = proxy.next();
        assert_matches!(executor.run_until_stalled(&mut serve_task), std::task::Poll::Pending);
        assert_matches!(
            executor.run_until_stalled(&mut chunk_fut),
            std::task::Poll::Ready(Ok(chunk))
                if chunk.len() == 1
        );

        // There should be no payloads left, so the next Next request should block.
        let mut chunk_fut = proxy.next();
        assert_matches!(executor.run_until_stalled(&mut serve_task), std::task::Poll::Pending);
        assert_matches!(executor.run_until_stalled(&mut chunk_fut), std::task::Poll::Pending);

        // The serving task should start providing payloads again when more are provided.
        let () = item_sender.unbounded_send(vec![blob_info; 2]).unwrap();
        assert_matches!(executor.run_until_stalled(&mut serve_task), std::task::Poll::Pending);
        assert_matches!(
            executor.run_until_stalled(&mut chunk_fut),
            std::task::Poll::Ready(Ok(chunk))
                if chunk.len() == 2
        );
    }
}