moq_binary/snapshot/

mod.rs

//! Lossy latest-value binary publishing over [`moq-net`](moq_net) tracks.
//!
//! One opaque value updated over time, for consumers that only care about the current state (a
//! poster image, a serialized state blob). This mode is **lossy** by design: a consumer yields only
//! the most recent value. A late joiner (or a consumer that falls behind) jumps straight to the
//! newest group, and older groups are dropped entirely. For an ordered log where every payload is
//! preserved, use [`stream`](crate::stream) instead.
//!
//! On the wire each value is one group holding one frame, so a group is self-contained and a
//! consumer never needs an older one. With [`Config::compression`] set to
//! [`Compression::Deflate`], that frame is its own raw DEFLATE stream;
//! there is no window to share across a single-frame group.

use crate::Compression;

pub mod consumer;
pub mod producer;

pub use consumer::Consumer;
pub use producer::Producer;

/// Codec options for a snapshot track.
///
/// Build from [`Default`] and override fields (the struct is `#[non_exhaustive]`, so new options
/// stay additive).
#[derive(Debug, Clone, Default)]
#[non_exhaustive]
pub struct Config {
	/// Compress each value as its own raw DEFLATE stream.
	///
	/// A snapshot group holds a single self-contained value, so there is no window to share: each
	/// value is compressed alone. [`Compression::None`] (the default) writes the bytes through
	/// untouched. A [`Consumer`] must set the same [`compression`](Self::compression).
	pub compression: Compression,
}

#[cfg(test)]
mod test {
	use std::task::Poll;

	use bytes::Bytes;

	use super::*;

	fn cfg(compression: bool) -> Config {
		Config {
			compression: if compression {
				Compression::Deflate
			} else {
				Compression::None
			},
		}
	}

	fn producer(compression: bool) -> (Producer, moq_net::track::Subscriber) {
		let track = moq_net::broadcast::Info::new()
			.produce()
			.create_track("test", None)
			.unwrap();
		let consumer = track.subscribe(None);
		(Producer::new(track, cfg(compression)), consumer)
	}

	fn consume(track: moq_net::track::Subscriber, compression: bool) -> Consumer {
		Consumer::new(track, cfg(compression))
	}

	/// Drain every value currently available without blocking.
	fn drain(mut consumer: Consumer) -> Vec<Bytes> {
		let waiter = kio::Waiter::noop();
		let mut out = Vec::new();
		while let Poll::Ready(Ok(Some(payload))) = consumer.poll_next(&waiter) {
			out.push(payload);
		}
		out
	}

	/// A snapshot group the transport can no longer serve -- `Old` when the relay reclaims a
	/// superseded group, `Evicted` under memory pressure, `Lagged` past the drift budget -- is not
	/// fatal. A snapshot reader only wants the newest value, so it drops the group and takes the
	/// replacement rather than ending the reader.
	#[test]
	fn a_lost_group_waits_for_its_replacement() {
		let track = moq_net::broadcast::Info::new()
			.produce()
			.create_track("test", None)
			.unwrap();
		let mut consumer = consume(track.subscribe(None), false);
		let waiter = kio::Waiter::noop();

		// Group 0 delivers a value, then stays open with the reader parked on its next frame.
		let mut group = track.create_group(moq_net::group::Info { sequence: 0 }).unwrap();
		group
			.write_frame(moq_net::Timestamp::now(), Bytes::from_static(b"one"))
			.unwrap();
		assert!(matches!(consumer.poll_next(&waiter), Poll::Ready(Ok(Some(v))) if v == "one"));
		assert!(consumer.poll_next(&waiter).is_pending());

		// The relay reclaims the group out from under the reader.
		group.abort(moq_net::Error::Old).unwrap();
		assert!(
			consumer.poll_next(&waiter).is_pending(),
			"a lost group must not end the reader"
		);

		// The replacement arrives and the reader picks up where the value now lives.
		let mut group = track.create_group(moq_net::group::Info { sequence: 1 }).unwrap();
		group
			.write_frame(moq_net::Timestamp::now(), Bytes::from_static(b"two"))
			.unwrap();
		assert!(matches!(consumer.poll_next(&waiter), Poll::Ready(Ok(Some(v))) if v == "two"));
	}

	#[test]
	fn one_group_per_update() {
		let (mut producer, track) = producer(false);
		producer.update(&b"first"[..]).unwrap();
		producer.update(&b"second"[..]).unwrap();
		producer.finish().unwrap();

		// Two updates => two groups. A consumer that joins after both only sees the latest.
		assert_eq!(track.latest(), Some(1));
		assert_eq!(drain(consume(track, false)), vec![Bytes::from_static(b"second")]);
	}

	#[test]
	fn live_consumer_sees_each_update() {
		let (mut producer, track) = producer(false);
		let mut consumer = consume(track, false);
		let waiter = kio::Waiter::noop();

		for n in 0..3u8 {
			producer.update(vec![n]).unwrap();
			match consumer.poll_next(&waiter) {
				Poll::Ready(Ok(Some(payload))) => assert_eq!(&payload[..], &[n]),
				other => panic!("expected value, got {other:?}"),
			}
		}
	}

	#[test]
	fn compressed_roundtrip() {
		let (mut producer, track) = producer(true);
		let payload = Bytes::from(b"the quick brown fox".repeat(64));
		producer.update(payload.clone()).unwrap();
		producer.finish().unwrap();

		assert_eq!(drain(consume(track, true)), vec![payload]);
	}

	/// The compressed frame on the wire is much smaller than the value it carries, so a consumer
	/// that ignored the flag would read garbage rather than the payload.
	#[test]
	fn compression_shrinks_the_frame() {
		let (mut producer, track) = producer(true);
		let payload = Bytes::from(b"the quick brown fox".repeat(64));
		producer.update(payload.clone()).unwrap();
		producer.finish().unwrap();

		// Read the raw frame, bypassing the consumer's decompression.
		let waiter = kio::Waiter::noop();
		let Poll::Ready(Ok(Some(mut group))) = track.ordered().poll_next_group(&waiter) else {
			panic!("expected a group");
		};
		let Poll::Ready(Ok(Some(frame))) = group.poll_read_frame(&waiter) else {
			panic!("expected a frame");
		};
		assert!(
			frame.payload.len() < payload.len() / 4,
			"compressed frame {} should be far below the raw {}",
			frame.payload.len(),
			payload.len()
		);
	}

	/// `append_group` publishes immediately, so rejecting the frame inside `write_frame` would leave
	/// an empty newest group behind. A snapshot consumer jumps to the newest, so the previous value
	/// would vanish even though the update reported an error.
	#[test]
	fn a_rejected_update_leaves_the_previous_value_readable() {
		let (mut producer, track) = producer(false);
		producer.update(&b"keep"[..]).unwrap();

		let oversized = Bytes::from(vec![0u8; moq_net::group::MAX_CACHE_BYTES as usize + 1]);
		assert!(producer.update(oversized).is_err());
		producer.finish().unwrap();

		// A reader arriving now still finds the last good value, not an empty superseding group.
		assert_eq!(drain(consume(track, false)), vec![Bytes::from_static(b"keep")]);
	}

	/// `finish` closes the underlying track, so a later update fails rather than being silently
	/// accepted, and that holds for every clone since they share one track.
	#[test]
	fn updating_after_finish_fails_on_every_clone() {
		let (mut producer, _track) = producer(false);
		let mut clone = producer.clone();

		producer.update(&b"first"[..]).unwrap();
		producer.finish().unwrap();

		assert!(producer.update(&b"late"[..]).is_err());
		assert!(clone.update(&b"late"[..]).is_err());
	}

	#[test]
	fn a_finished_track_ends_the_consumer() {
		let (mut producer, track) = producer(false);
		producer.update(&b"only"[..]).unwrap();
		producer.finish().unwrap();

		let mut consumer = consume(track, false);
		let waiter = kio::Waiter::noop();
		assert!(matches!(consumer.poll_next(&waiter), Poll::Ready(Ok(Some(_)))));
		assert!(matches!(consumer.poll_next(&waiter), Poll::Ready(Ok(None))));
	}
}