moq-net 0.3.2

use crate::runtime::Timers as _;
use crate::{frame, group, origin, track};
use std::{
	collections::HashMap,
	sync::{Arc, atomic},
	task::{Poll, ready},
	time::Duration,
};

use crate::{
	AsPath, Error, Path, PathOwned, Timescale, Timestamp, bandwidth,
	coding::{Decode, Reader, Stream},
	lite,
	track::{Position, Subscription},
};

use super::Version;

use kio::Lock;

pub(super) struct SubscriberConfig<S: crate::transport::poll::Session> {
	pub runtime: crate::time::Clock,
	pub session: S,
	/// The origin into which remote broadcasts are inserted. Traffic stats are
	/// attributed through this handle: tag it with [`origin::Producer::with_stats`]
	/// first.
	pub origin: origin::Producer,
	/// Receiver-side bandwidth producer for PROBE feedback. None disables the
	/// feature (used by versions that don't carry probe streams).
	pub recv_bandwidth: Option<bandwidth::Producer>,
	pub version: Version,
	/// Shared slot for the peer's SETUP (lite-05+). Written when the peer's Setup
	/// stream is read; the probe stream waits on it before opening.
	pub peer_setup: super::PeerSetup,
	/// The origin (hop) id assigned to the peer, used whenever the peer doesn't
	/// declare one itself. See `Client::with_peer_hop`.
	pub peer_hop: Option<crate::Hop>,
	/// Local policy for what pulling from this peer costs, overriding whatever it
	/// declared in its SETUP. `None` charges the peer's declared price.
	pub cost: Option<u64>,
	/// Set once the peer sends a GOAWAY; new request streams are then rejected
	/// with [`Error::GoingAway`] (the peer told us to stop asking).
	pub going_away: crate::goaway::GoingAway,
}

#[derive(Clone)]
pub(super) struct Subscriber<S: crate::transport::poll::Session> {
	runtime: crate::time::Clock,
	session: S,

	origin: origin::Producer,
	recv_bandwidth: Option<bandwidth::Producer>,
	// Session-level origin id shared with the Publisher. Used to drop reflected
	// announces: any incoming announce whose hop chain already passed through us
	// has looped, so it is neither used as a route nor forwarded. On lite-04/05
	// we also ask the peer to filter them out (AnnounceRequest.exclude_hop) so
	// they never hit the wire, but this check is what makes it correct.
	self_origin: crate::Hop,
	// The origin stored as `Route.via` for broadcasts from versions that don't
	// carry real hop ids on the wire (Lite01/02/03), and for a peer that reports
	// 0 in AnnounceOk. Lite03 placeholders stay 0 and count as anonymous.
	//
	// This is the peer's assigned identity (`peer_hop`) when the caller gave
	// it one. Otherwise it is `Hop::UNKNOWN` (0), the reserved "no identity" value.
	//
	// Assigning one is the caller's call, not this layer's: a server gives every
	// accepted session a fresh id so its routes are at least distinguishable from
	// another session's, while a client only assigns one it knows out of band. The
	// assigned id stays local and is never written into a hop chain.
	session_origin: crate::Hop,
	subscribes: Lock<HashMap<u64, TrackEntry>>,
	next_id: Arc<atomic::AtomicU64>,
	version: Version,
	/// The peer's advertised SETUP (lite-05+), set when its Setup stream is read.
	peer_setup: super::PeerSetup,
	/// Local policy overriding the peer's declared egress price. See `poll_link_cost`.
	cost: Option<u64>,
	/// Sources created by the announce half, drained by the driver into
	/// [`SourceServe`] machines.
	sources: kio::Queue<(PathOwned, crate::broadcast::Dynamic)>,
	going_away: crate::goaway::GoingAway,
}

#[derive(Clone)]
struct TrackEntry {
	producer: track::Producer,
	/// Timestamp scale from this track's TRACK_INFO, known before the SUBSCRIBE is
	/// even opened, so group streams decode frames without blocking.
	timescale: Option<Timescale>,
}

impl<S: crate::transport::poll::Session> Subscriber<S> {
	pub fn new(config: SubscriberConfig<S>) -> Self {
		// Identity for incoming-hop loop detection. Derived from the local
		// origin we publish into so it matches the relay identity across
		// every session sharing that origin, required for cross-session
		// loop detection.
		let self_origin = config.origin.hop();
		Self {
			session: config.session,
			runtime: config.runtime,
			origin: config.origin,
			recv_bandwidth: config.recv_bandwidth,
			self_origin,
			session_origin: config.peer_hop.unwrap_or(crate::Hop::UNKNOWN),
			subscribes: Default::default(),
			next_id: Default::default(),
			version: config.version,
			peer_setup: config.peer_setup,
			cost: config.cost,
			sources: kio::Queue::new(),
			going_away: config.going_away,
		}
	}

	/// Reject a new request once the peer has sent a GOAWAY: it told us to stop
	/// opening streams on this session (existing subscriptions keep flowing).
	fn check_going_away(&self) -> Result<(), Error> {
		if self.going_away.is_set() {
			return Err(Error::GoingAway);
		}
		Ok(())
	}

	/// What pulling content across this session's link costs, added to the route cost
	/// of every announcement received over it.
	///
	/// A locally configured price wins, since what we charge our own routing is local
	/// policy. Otherwise we charge what the peer declared, which is how a server prices
	/// a link at all: it cannot tell a sibling from a stranger, so the dialer that chose
	/// the peer declares the price for both of them. Falls back to
	/// [`super::DEFAULT_COST`] when neither priced it, and to `0` on a version that
	/// carries no cost at all, whose routes rank on hop count alone.
	///
	/// Our own price short-circuits the peer's, so a session that configured one never
	/// blocks on a SETUP to start routing.
	fn poll_link_cost(&self, waiter: &kio::Waiter) -> Poll<u64> {
		// Older versions carry no cost on the wire, so nothing is charged and their
		// routes rank on hop count alone. Returning early also avoids blocking on a
		// SETUP that versions without a Setup Stream never send.
		if !self.version.has_route_cost() {
			return Poll::Ready(0);
		}
		match self.cost {
			Some(cost) => Poll::Ready(cost),
			None => self
				.peer_setup
				.poll_cost(waiter)
				.map(|cost| cost.unwrap_or(super::DEFAULT_COST)),
		}
	}

	/// Apply one received announce message to the origin and the per-stream
	/// bookkeeping in `run`.
	fn handle_announce(
		&mut self,
		prefix: &PathOwned,
		announce: lite::AnnounceBroadcast<'_>,
		run: &mut PrefixRun,
	) -> Result<(), Error> {
		match announce {
			lite::AnnounceBroadcast::Active { suffix, hops, cost } => {
				let path = prefix.join(&suffix);
				if self.version.has_announce_id() {
					// Every `active` assigns the next ordinal, even ones we drop locally.
					run.announced_by_id.insert(run.next_announce_id, path.clone());
					run.next_announce_id += 1;
				}
				if lite::restart_supported(self.version)
					&& !self.version.has_announce_id()
					&& run.announced.contains(&path)
				{
					// lite-05 only: a duplicate ANNOUNCE for an already-announced path is a RESTART;
					// atomically replace the broadcast. Lite06+ restarts by announce id, and older
					// versions never defined restarts, so both fall through to start_announce, which
					// rejects the duplicate (Error::ProtocolViolation).
					self.restart_announce(
						path,
						hops,
						cost,
						run.link_cost,
						run.responder_origin,
						&mut run.announced,
					)?;
				} else {
					self.start_announce(
						path,
						hops,
						cost,
						run.link_cost,
						run.responder_origin,
						&mut run.announced,
					)?;
				}
			}
			lite::AnnounceBroadcast::Ended { suffix, .. } => {
				let path = prefix.join(&suffix);
				tracing::debug!(broadcast = %self.log_path(&path), "unannounced");
				run.announced.retire(&path);
			}
			lite::AnnounceBroadcast::EndedId { id } => {
				// Resolve and retire the id; an unknown or already-retired id is a
				// protocol violation.
				let Some(path) = run.announced_by_id.remove(&id) else {
					return Err(Error::ProtocolViolation);
				};
				tracing::debug!(broadcast = %self.log_path(&path), "unannounced");
				run.announced.retire(&path);
			}
			lite::AnnounceBroadcast::Restart { id, hops, cost } => {
				// Resolve the id; it stays live (the replacement reuses it). An unknown
				// or retired id is a protocol violation.
				let Some(path) = run.announced_by_id.get(&id).cloned() else {
					return Err(Error::ProtocolViolation);
				};
				self.restart_announce(
					path,
					hops,
					cost,
					run.link_cost,
					run.responder_origin,
					&mut run.announced,
				)?;
			}
			lite::AnnounceBroadcast::Skipped => {}
		}
		Ok(())
	}

	/// Records the advertisement either way. Returns `Ok(true)` if it was accepted
	/// and attached a route, or `Ok(false)` if it was declined locally.
	fn start_announce(
		&mut self,
		path: PathOwned,
		mut hops: crate::Hops,
		// The route cost off the wire, i.e. as the peer advertised it.
		// [`Cost::UNKNOWN`] before lite-06, leaving the hop chain as the only
		// routing input as before.
		cost: crate::origin::Cost,
		// This link's price, added to the wire cost.
		link_cost: u64,
		// Lite05+: the announce sender's origin id (from AnnounceOk). The sender no
		// longer stamps itself onto the chain, so we append it here to reconstruct
		// the full `[src...sender]` chain Lite04 stored. None for older versions,
		// where the sender already appended itself.
		responder_origin: Option<crate::Hop>,
		announced: &mut Announced,
	) -> Result<bool, Error> {
		// One current advertisement per prefix per stream. Test what the peer
		// advertised, not only what we accepted locally.
		if announced.contains(&path) {
			return Err(Error::ProtocolViolation);
		}

		// The peer holds this prefix now. Everything below either accepts the announcement,
		// replacing this, or declines it and leaves it exactly as reserved.
		announced.reserve(path.clone());

		if let Some(responder) = responder_origin {
			// A chain already naming the sender came back through it: a reflection, and
			// appending the sender again would name it twice. That is legal in a lite
			// chain but a PROTOCOL_VIOLATION for an IETF peer we forward it to, so it
			// must not enter the model at all. Zero names nobody, so it may repeat.
			if responder != crate::Hop::UNKNOWN && hops.contains(&responder) {
				tracing::debug!(route = %self.log_path(&path), "dropping announce reflected by its sender");
				return Ok(false);
			}
			// If the chain is already full, drop the announce. This is the same decision
			// the Lite04 sender makes at its push site.
			if hops.push(responder).is_err() {
				tracing::warn!(
					route = %self.log_path(&path),
					"dropping announce; hop chain at MAX_HOPS (possible loop)",
				);
				return Ok(false);
			}
		}

		// Drop announces that already passed through us. This connection is
		// a reflection, not a new path. Lite04/05 peers filter these out for us
		// via AnnounceRequest.exclude_hop, but that is only an optimization:
		// this is the authoritative cluster-loop check, and the only one on
		// every other version.
		if hops.contains(&self.self_origin) {
			tracing::debug!(route = %self.log_path(&path), "dropping reflected announce");
			return Ok(false);
		}

		// Lite03 carries its hop count as UNKNOWN placeholders rather than real
		// ids; they stay 0 and count as anonymous. Lite01/02 send no list at all.
		// Either way the chain must have at least the anonymous mark so a
		// downstream hop can see that this path passed through an unidentified hop.
		if hops.is_empty() {
			hops.push(crate::Hop::UNKNOWN)
				.expect("an empty hop chain always has room for one entry, and repeats nothing");
		}

		tracing::debug!(route = %self.log_path(&path), hops = hops.len(), "announce");

		// Announce this session's route into the origin: paths under the prefix
		// resolve through this session on demand. An error means the prefix is
		// outside our scope, so don't serve it. Reflections are already
		// filtered above.
		let route = self.announced_route(hops, cost, link_cost, responder_origin);
		let Ok(dynamic) = self.origin.dynamic(&path, route.clone()) else {
			return Ok(false);
		};

		announced.attach(path, AnnouncedRoute::new(route, dynamic));

		Ok(true)
	}

	/// The route to announce for a prefix this peer advertised, charging our
	/// link's price on top of the cost it advertised.
	///
	/// Once the peer has sent a GOAWAY every route it announces starts out draining,
	/// including a restart of one already attached: a connection on its way out must
	/// not win selection, however good the path it advertises looks.
	fn announced_route(
		&self,
		hops: crate::Hops,
		cost: crate::origin::Cost,
		link_cost: u64,
		responder: Option<crate::Hop>,
	) -> crate::origin::Route {
		let mut route = crate::origin::Route::default()
			.with_hops(hops)
			.with_cost(cost.charged(link_cost))
			.with_via(self.via(responder));

		if self.going_away.is_set() {
			route.cost = crate::origin::Cost::DRAIN;
		}

		route
	}

	/// The announcing session's declared or assigned identity, for split-horizon.
	///
	/// A non-zero AnnounceOk origin is the declared id. Otherwise the caller-assigned
	/// identity stands in, locally: it is never written into the hop chain.
	fn via(&self, responder: Option<crate::Hop>) -> crate::Hop {
		responder
			.filter(|hop| *hop != crate::Hop::UNKNOWN)
			.unwrap_or(self.session_origin)
	}

	/// Handle a RESTART (an explicit restart status, or a duplicate ANNOUNCE on lite-05).
	///
	/// The first hop of the chain identifies the original publisher. When it matches
	/// the prior advertisement and is a real identity, the broadcast is the same
	/// content on a new path: this session's route metadata updates in place,
	/// in-flight tracks keep flowing, and the origin only hands over if the winner
	/// changed. Consumers observe nothing. When the first hop differs, or is
	/// [`Hop::UNKNOWN`](crate::Hop::UNKNOWN), the old route detaches gracefully
	/// and a fresh one attaches, so downstream sees a real Ended + Active.
	/// The advertisement is already live, so this can attach a route even when the
	/// original advertisement was declined locally.
	///
	/// Returns `Ok(false)` if the new hop chain is a reflected loop (this session's
	/// route is now gone), `Ok(true)` otherwise.
	fn restart_announce(
		&mut self,
		path: PathOwned,
		mut hops: crate::Hops,
		// The route cost off the wire and this link's price. See `start_announce`.
		cost: crate::origin::Cost,
		link_cost: u64,
		// Lite05+: the announce sender's origin id (from AnnounceOk), appended here to
		// rebuild the full chain since the sender no longer stamps itself. None for older
		// versions. See `start_announce`.
		responder_origin: Option<crate::Hop>,
		announced: &mut Announced,
	) -> Result<bool, Error> {
		// Reflected loop (or a full chain): detach its route but keep the advertisement live.
		let reflected = match responder_origin {
			// A chain already naming the sender came back through it; see `start_announce`.
			Some(responder) => {
				(responder != crate::Hop::UNKNOWN && hops.contains(&responder))
					|| hops.push(responder).is_err()
					|| hops.contains(&self.self_origin)
			}
			None => hops.contains(&self.self_origin),
		};
		if reflected {
			tracing::debug!(route = %self.log_path(&path), "dropping reflected restart");
			announced.declined(path);
			return Ok(false);
		}

		if hops.is_empty() {
			hops.push(crate::Hop::UNKNOWN)
				.expect("an empty hop chain always has room for one entry, and repeats nothing");
		}

		tracing::debug!(route = %self.log_path(&path), hops = hops.len(), "restart");
		let metadata = self.announced_route(hops, cost, link_cost, responder_origin);

		// A restart is a metadata update: the route keeps its prefix (and its
		// served paths) and re-prices in place. In-flight tracks keep flowing.
		if let Some(entry) = announced.attached(&path) {
			entry.update(metadata);
			return Ok(true);
		}

		let Ok(dynamic) = self.origin.dynamic(&path, metadata.clone()) else {
			announced.declined(path);
			return Ok(false);
		};
		announced.attach(path, AnnouncedRoute::new(metadata, dynamic));

		Ok(true)
	}

	/// Remove a subscription, releasing the session's handle on its producer.
	fn remove_subscribe(&self, id: u64) {
		self.subscribes.lock().remove(&id);
	}

	/// Decode one datagram body and hand it to the matching subscription's producer.
	fn route_datagram(&self, payload: bytes::Bytes) -> Result<(), Error> {
		let mut buf = payload;
		let dg = lite::Datagram::decode(&mut buf, self.version)?;

		// Write through the map rather than cloning the entry out: a `TrackEntry` clone
		// is a handful of atomic bumps on every datagram, and a producer held past its
		// removal would keep the track (its cached groups, its stats subscription) alive.
		// The group path already writes to a producer under this lock.
		let mut subscribes = self.subscribes.lock();
		let Some(entry) = subscribes.get_mut(&dg.subscribe) else {
			// Unknown or already-closed subscription: drop the datagram.
			return Ok(());
		};

		// Datagrams are lite-05+, which always negotiates a timescale; default defensively.
		let scale = entry.timescale.unwrap_or_default();
		let timestamp =
			Timestamp::new(dg.timestamp, scale).map_err(|_| Error::BoundsExceeded(crate::coding::BoundsExceeded))?;

		entry.producer.insert_datagram(dg.sequence, timestamp, dg.payload)?;
		Ok(())
	}

	fn log_path(&self, path: impl AsPath) -> Path<'_> {
		self.origin.root().join(path)
	}
}

/// The subscriber half's driver: the announce prefixes, the uni-stream accept
/// loop, PROBE feedback, datagrams, and the per-source serve machines. Only an
/// error ends it.
// Owns the active subscriptions for exactly as long as the driver. A dropped
// driver is how a cancelled session unwinds, so cleanup cannot depend on any
// poll returning Ready.
struct SubscriptionCleanup(Lock<HashMap<u64, TrackEntry>>);

impl Drop for SubscriptionCleanup {
	fn drop(&mut self) {
		// Group machines own their cancellation cleanup independently. This records
		// session cancellation as the track's terminal state.
		for (_, entry) in self.0.lock().drain() {
			let _ = entry.producer.abort(Error::Cancel);
		}
	}
}

pub(super) struct SubscriberDriver<S: crate::transport::poll::Session> {
	subscriber: Subscriber<S>,
	/// Aborts whatever is still subscribed when the driver is dropped.
	_cleanup: SubscriptionCleanup,
	/// One machine per permitted prefix. Only an error ends the session; a
	/// prefix finishing cleanly (publisher FIN) just retires.
	prefixes: Vec<AnnouncePrefix<S>>,
	uni: UniAccept<S>,
	/// PROBE feedback; finishes quietly when unsupported or given up on.
	bandwidth: Option<RecvBandwidth<S>>,
	/// Datagram receive; inert on a version or transport without datagrams.
	datagrams: Option<DatagramRecv<S>>,
	/// One machine per announced source, serving the origin's track requests.
	sources: kio::Tasks<SourceServe<S>>,
}

impl<S: crate::transport::poll::Session> SubscriberDriver<S> {
	pub fn new(subscriber: Subscriber<S>) -> Self {
		// The wire speaks announce interest by prefix: ask for each granted
		// pattern's literal head and let the origin's scope filter what arrives.
		let prefixes = crate::model::interest_prefixes(&subscriber.origin.allowed())
			.into_iter()
			.map(|prefix| AnnouncePrefix::new(subscriber.clone(), prefix))
			.collect();

		Self {
			prefixes,
			_cleanup: SubscriptionCleanup(subscriber.subscribes.clone()),
			uni: UniAccept::new(subscriber.clone()),
			bandwidth: Some(RecvBandwidth::new(subscriber.clone())),
			datagrams: Some(DatagramRecv::new(subscriber.clone())),
			sources: kio::Tasks::new(),
			subscriber,
		}
	}

	pub fn poll(&mut self, waiter: &kio::Waiter) -> Poll<Result<(), Error>> {
		let mut i = 0;
		while i < self.prefixes.len() {
			match self.prefixes[i].poll(waiter) {
				Poll::Ready(Ok(())) => {
					self.prefixes.swap_remove(i);
				}
				Poll::Ready(Err(err)) => return Poll::Ready(Err(err)),
				Poll::Pending => i += 1,
			}
		}
		if let Poll::Ready(res) = self.uni.poll(waiter) {
			return Poll::Ready(res);
		}
		if let Some(bandwidth) = &mut self.bandwidth {
			match bandwidth.poll(waiter) {
				Poll::Ready(Err(err)) => return Poll::Ready(Err(err)),
				Poll::Ready(Ok(())) => self.bandwidth = None,
				Poll::Pending => {}
			}
		}
		if let Some(datagrams) = &mut self.datagrams {
			match datagrams.poll(waiter) {
				Poll::Ready(Err(err)) => return Poll::Ready(Err(err)),
				Poll::Ready(Ok(())) => self.datagrams = None,
				Poll::Pending => {}
			}
		}

		// Sources created by the announce half; their completion never ends the
		// session (the origin delivers the unannounce itself).
		while let Poll::Ready(Ok((path, dynamic))) = self.subscriber.sources.poll_pop(waiter) {
			self.sources
				.push(SourceServe::new(self.subscriber.clone(), path, dynamic));
		}
		let _ = self.sources.poll(waiter);

		Poll::Pending
	}
}

/// Accepts incoming uni streams (GROUP data plus the peer's SETUP) and drives
/// each as a child machine. Resolves only on a transport error.
struct UniAccept<S: crate::transport::poll::Session> {
	subscriber: Subscriber<S>,
	// A dedicated accept handle: the poll interface takes `&mut self`.
	accept: S,
	children: kio::Tasks<UniServe<S>>,
}

impl<S: crate::transport::poll::Session> UniAccept<S> {
	fn new(subscriber: Subscriber<S>) -> Self {
		let accept = subscriber.session.clone();
		Self {
			subscriber,
			accept,
			children: kio::Tasks::new(),
		}
	}

	fn poll(&mut self, waiter: &kio::Waiter) -> Poll<Result<(), Error>> {
		let _ = self.children.poll(waiter);

		let mut cx = std::task::Context::from_waker(waiter.waker());
		loop {
			match self.accept.poll_accept_uni(&mut cx) {
				Poll::Ready(Ok(stream)) => {
					self.children.push(UniServe {
						subscriber: self.subscriber.clone(),
						state: UniState::Start {
							reader: Reader::new(stream, self.subscriber.version),
						},
					});
				}
				Poll::Ready(Err(err)) => return Poll::Ready(Err(Error::from_transport(err))),
				Poll::Pending => break,
			}
		}

		// Newly accepted children start now rather than on the next wake.
		let _ = self.children.poll(waiter);
		Poll::Pending
	}
}

/// One accepted uni stream, dispatched on its first varint.
struct UniServe<S: crate::transport::poll::Session> {
	subscriber: Subscriber<S>,
	state: UniState<S>,
}

// A state machine's enum is its storage: one transient instance per stream, so the
// big variant is the working state, not padding held in bulk.
#[allow(clippy::large_enum_variant)]
enum UniState<S: crate::transport::poll::Session> {
	/// Reading the stream's type.
	Start {
		reader: Reader<S::RecvStream, Version>,
	},
	/// Reading the peer's single SETUP message, recorded so capability-gated
	/// streams (PROBE) can consult it. lite-05+ only.
	Setup {
		reader: Reader<S::RecvStream, Version>,
	},
	Group(GroupRecv<S>),
	Done,
}

impl<S: crate::transport::poll::Session> kio::Task for UniServe<S> {
	fn poll(&mut self, waiter: &kio::Waiter) -> Poll<()> {
		if let Err(err) = ready!(self.poll_serve(waiter)) {
			tracing::debug!(%err, "error running uni stream");
		}
		Poll::Ready(())
	}
}

impl<S: crate::transport::poll::Session> UniServe<S> {
	/// Abort the stream with the given error, wherever the reader currently lives.
	fn abort(&mut self, err: &Error) {
		match &mut self.state {
			UniState::Start { reader } | UniState::Setup { reader } => reader.abort(err),
			UniState::Group(recv) => recv.reader.abort(err),
			UniState::Done => {}
		}
	}

	fn poll_serve(&mut self, waiter: &kio::Waiter) -> Poll<Result<(), Error>> {
		loop {
			match &mut self.state {
				UniState::Start { reader } => {
					let mut cx = std::task::Context::from_waker(waiter.waker());
					// A decode error here is only logged; the peer hung up or spoke garbage
					// before the stream had a type.
					let kind = ready!(reader.poll_decode::<lite::DataType>(&mut cx))?;
					let UniState::Start { reader } = std::mem::replace(&mut self.state, UniState::Done) else {
						unreachable!()
					};
					self.state = match kind {
						lite::DataType::Group => UniState::Group(GroupRecv::new(self.subscriber.clone(), reader)),
						lite::DataType::Setup => UniState::Setup { reader },
					};
				}
				UniState::Setup { reader } => {
					if !self.subscriber.version.has_setup_stream() {
						let err = Error::UnexpectedStream;
						self.abort(&err);
						return Poll::Ready(Ok(()));
					}
					let mut cx = std::task::Context::from_waker(waiter.waker());
					let res = ready!(reader.poll_decode::<lite::Setup>(&mut cx));
					match res {
						Ok(setup) => {
							tracing::debug!(?setup, "received peer setup");
							self.subscriber.peer_setup.set(setup);
							return Poll::Ready(Ok(()));
						}
						Err(err) => {
							self.abort(&err);
							return Poll::Ready(Ok(()));
						}
					}
				}
				UniState::Group(recv) => {
					let res = ready!(recv.poll_serve(waiter));
					if let Err(err) = res {
						self.abort(&err);
					}
					return Poll::Ready(Ok(()));
				}
				UniState::Done => return Poll::Ready(Ok(())),
			}
		}
	}
}

/// Receives one GROUP stream into its subscription's track producer.
struct GroupRecv<S: crate::transport::poll::Session> {
	subscriber: Subscriber<S>,
	reader: Reader<S::RecvStream, Version>,
	state: GroupRecvState,
}

// A state machine's enum is its storage: one transient instance per stream, so the
// big variant is the working state, not padding held in bulk.
#[allow(clippy::large_enum_variant)]
enum GroupRecvState {
	/// Reading the GROUP header.
	Header,
	/// Filling the group, bailing if the track or group dies first.
	Serve {
		/// Guarded: dropping this machine mid-group is a cancellation, not a clean end.
		group: crate::recv::Group,
		track: track::Producer,
		ingest: FrameIngest,
	},
	Done,
}

impl<S: crate::transport::poll::Session> GroupRecv<S> {
	fn new(subscriber: Subscriber<S>, reader: Reader<S::RecvStream, Version>) -> Self {
		Self {
			subscriber,
			reader,
			state: GroupRecvState::Header,
		}
	}

	fn poll_serve(&mut self, waiter: &kio::Waiter) -> Poll<Result<(), Error>> {
		loop {
			match &mut self.state {
				GroupRecvState::Header => {
					let mut cx = std::task::Context::from_waker(waiter.waker());
					let hdr = ready!(self.reader.poll_decode::<lite::Group>(&mut cx))?;

					let (group, track, timescale) = {
						let mut subs = self.subscriber.subscribes.lock();
						let entry = subs.get_mut(&hdr.subscribe).ok_or(Error::Cancel)?;

						let group_info = group::Info { sequence: hdr.sequence };
						// Stats (groups/frames/bytes) are counted in the model as the group
						// is written, through the tagged `track::Producer`.
						let mut group = entry.producer.create_group(group_info)?;
						// The stream may carry only the tail of the group; number the frames
						// from where the publisher said they start so a reader splicing
						// across routes lines them up.
						group.start_at(hdr.frame_start)?;
						(group, entry.producer.clone(), entry.timescale)
					};

					// The timescale came from TRACK_INFO (read before this subscription was
					// even registered), so frames decode immediately. No SUBSCRIBE_OK to
					// wait on.
					self.state = GroupRecvState::Serve {
						group: crate::recv::Group::new(group),
						track,
						ingest: FrameIngest::new(self.subscriber.runtime.clone(), timescale),
					};
				}
				GroupRecvState::Serve { group, track, ingest } => {
					// The track or group dying cancels the stream; the peer's own close
					// arrives through the ingest's reads.
					let res = 'serve: {
						if let Poll::Ready(err) = track.poll_closed(waiter) {
							break 'serve Err(err);
						}
						if let Poll::Ready(err) = group.poll_closed(waiter) {
							break 'serve Err(err);
						}
						match ingest.poll(&mut self.reader, group, waiter) {
							Poll::Ready(res) => break 'serve res,
							Poll::Pending => return Poll::Pending,
						}
					};

					let GroupRecvState::Serve { group, .. } = std::mem::replace(&mut self.state, GroupRecvState::Done)
					else {
						unreachable!()
					};
					match res {
						Ok(()) => {
							let _ = group.finish();
						}
						Err(err @ (Error::Cancel | Error::Stream(crate::StreamError::Cancel))) => {
							let _ = group.abort(err);
						}
						Err(err) => {
							tracing::debug!(%err, group = %group.sequence, "group error");
							let _ = group.abort(err.clone());
							return Poll::Ready(Err(err));
						}
					}
					return Poll::Ready(Ok(()));
				}
				GroupRecvState::Done => return Poll::Ready(Ok(())),
			}
		}
	}
}

/// Pumps bare FRAME messages from a reader into a group producer: the wire
/// format shared by GROUP streams and FETCH responses.
struct FrameIngest {
	runtime: crate::time::Clock,
	/// `Some` decodes the lite-05 zigzag-delta timestamp prefix; `None` stamps
	/// local receive time (pre-lite-05).
	timescale: Option<Timescale>,
	/// Previous frame's raw timestamp value (in `timescale` units), for the
	/// zigzag-delta decode. The first frame's delta is absolute (prev = 0).
	prev_ts: u64,
	phase: IngestPhase,
}

enum IngestPhase {
	/// Reading the timestamp delta (skipped without a timescale). Stream end here
	/// means the group has no more frames.
	Timing,
	/// Reading the frame size. Stream end here also ends the group (pre-lite-05,
	/// where there is no timing prefix to act as the sentinel).
	Size { timestamp: Option<Timestamp> },
	/// Streaming the frame payload.
	Payload { frame: frame::ProducerOwned },
}

impl FrameIngest {
	fn new(runtime: crate::time::Clock, timescale: Option<Timescale>) -> Self {
		Self {
			timescale,
			prev_ts: 0,
			phase: IngestPhase::Timing,
			runtime,
		}
	}

	/// `Ready(Ok(()))` once the stream FINs on a frame boundary. The caller
	/// finishes or aborts the group; a frame cut short mid-payload was already
	/// aborted here with the reason.
	fn poll<R: crate::transport::poll::RecvStream>(
		&mut self,
		reader: &mut Reader<R, Version>,
		group: &mut group::Producer,
		waiter: &kio::Waiter,
	) -> Poll<Result<(), Error>> {
		let mut cx = std::task::Context::from_waker(waiter.waker());
		loop {
			match &mut self.phase {
				IngestPhase::Timing => {
					let Some(scale) = self.timescale else {
						self.phase = IngestPhase::Size { timestamp: None };
						continue;
					};
					// The timestamp delta doubles as the per-frame sentinel.
					let Some(zz) = ready!(reader.poll_decode_maybe::<crate::coding::VarInt>(&mut cx))? else {
						return Poll::Ready(Ok(()));
					};
					let next: u64 = (self.prev_ts as i128 + zz.to_zigzag() as i128)
						.try_into()
						.map_err(|_| Error::BoundsExceeded(crate::coding::BoundsExceeded))?;
					self.prev_ts = next;
					let timestamp = Timestamp::new(next, scale)
						.map_err(|_| Error::BoundsExceeded(crate::coding::BoundsExceeded))?;
					self.phase = IngestPhase::Size {
						timestamp: Some(timestamp),
					};
				}
				IngestPhase::Size { timestamp } => {
					let Some(size) = ready!(reader.poll_decode_maybe::<u64>(&mut cx))? else {
						return Poll::Ready(Ok(()));
					};
					// `create_frame_owned` is the allocation chokepoint and rejects an
					// oversized `size` before allocating, so no pre-check is needed. No
					// wire timestamp (pre-lite-05) means local receive time.
					let timestamp = timestamp.unwrap_or_else(|| Timestamp::from(self.runtime.now()));
					let frame = group.create_frame_owned(frame::Info { size, timestamp })?;
					self.phase = IngestPhase::Payload { frame };
				}
				IngestPhase::Payload { frame } => {
					let failed = ready!(reader.poll_read_frame(&mut cx, frame)).err();

					let IngestPhase::Payload { frame } = std::mem::replace(&mut self.phase, IngestPhase::Timing) else {
						unreachable!()
					};
					match failed {
						None => frame.finish()?,
						Some(err) => {
							// Fail the group with the reason, not the Drop fallback's
							// generic `Dropped`.
							let _ = frame.abort(err.clone());
							return Poll::Ready(Err(err));
						}
					}
				}
			}
		}
	}
}

/// Receives QUIC datagrams and routes each to its subscription's track producer
/// (lite-05 §6.4).
///
/// A decode error or an unknown subscribe id drops that datagram without tearing
/// down the session (best-effort); only a transport-level failure ends the loop.
struct DatagramRecv<S: crate::transport::poll::Session> {
	subscriber: Subscriber<S>,
	// A dedicated receive handle: the poll interface takes `&mut self`.
	recv: S,
	enabled: bool,
}

impl<S: crate::transport::poll::Session> DatagramRecv<S> {
	fn new(subscriber: Subscriber<S>) -> Self {
		let recv = subscriber.session.clone();
		let enabled = subscriber.version.has_datagrams() && recv.max_datagram_size() > 0;
		Self {
			subscriber,
			recv,
			enabled,
		}
	}

	fn poll(&mut self, waiter: &kio::Waiter) -> Poll<Result<(), Error>> {
		if !self.enabled {
			return Poll::Ready(Ok(()));
		}
		let mut cx = std::task::Context::from_waker(waiter.waker());
		loop {
			let payload = ready!(self.recv.poll_recv_datagram(&mut cx)).map_err(Error::from_transport)?;
			if let Err(err) = self.subscriber.route_datagram(payload) {
				tracing::debug!(%err, "dropping datagram");
			}
		}
	}
}

/// Opens a PROBE stream on demand while a consumer is interested.
///
/// Loops forever: wait for a consumer, race the probe stream against the
/// consumer leaving, then loop back. Probe is best-effort, so stream errors are
/// logged but never tear down the session.
struct RecvBandwidth<S: crate::transport::poll::Session> {
	subscriber: Subscriber<S>,
	state: BandwidthState<S>,
}

// A state machine's enum is its storage: one transient instance per stream, so the
// big variant is the working state, not padding held in bulk.
#[allow(clippy::large_enum_variant)]
enum BandwidthState<S: crate::transport::poll::Session> {
	/// lite-05+ negotiates probing: only open a PROBE stream if the peer
	/// advertised it (Report or higher) in its SETUP. Older versions have no
	/// SETUP, so probe is always available there.
	Gate,
	/// Wait until at least one consumer is interested in the estimate.
	WaitUsed,
	/// Race the last consumer leaving against the probe stream ending.
	Probing(ProbeStream<S>),
}

impl<S: crate::transport::poll::Session> RecvBandwidth<S> {
	fn new(subscriber: Subscriber<S>) -> Self {
		Self {
			subscriber,
			state: BandwidthState::Gate,
		}
	}

	fn poll(&mut self, waiter: &kio::Waiter) -> Poll<Result<(), Error>> {
		loop {
			match &mut self.state {
				BandwidthState::Gate => {
					if self.subscriber.recv_bandwidth.is_none() {
						return Poll::Ready(Ok(()));
					}
					if self.subscriber.version.has_setup_stream()
						&& ready!(self.subscriber.peer_setup.poll_probe_level(waiter)) < lite::ProbeLevel::Report
					{
						tracing::debug!("peer does not support probing; skipping probe stream");
						return Poll::Ready(Ok(()));
					}
					self.state = BandwidthState::WaitUsed;
				}
				BandwidthState::WaitUsed => {
					let bandwidth = self.subscriber.recv_bandwidth.as_ref().expect("gated above");
					match ready!(bandwidth.poll_used(waiter)) {
						Ok(()) => self.state = BandwidthState::Probing(ProbeStream::new(&self.subscriber)),
						Err(_) => return Poll::Ready(Ok(())),
					}
				}
				BandwidthState::Probing(probe) => {
					let bandwidth = self.subscriber.recv_bandwidth.as_ref().expect("gated above");
					match bandwidth.poll_unused(waiter) {
						// Loop back: a new consumer may arrive later. Dropping the probe
						// machine resets its stream.
						Poll::Ready(Ok(())) => {
							self.state = BandwidthState::WaitUsed;
							continue;
						}
						// The channel closed: give up for the rest of the session.
						Poll::Ready(Err(_)) => return Poll::Ready(Ok(())),
						Poll::Pending => {}
					}
					match ready!(probe.poll(waiter)) {
						Ok(()) => tracing::debug!("probe stream closed"),
						Err(err) => tracing::warn!(%err, "probe stream error"),
					}
					// The stream ended (peer FIN'd or errored). Don't hammer an
					// uncooperative peer; give up for the rest of the session.
					return Poll::Ready(Ok(()));
				}
			}
		}
	}
}

/// One PROBE stream: send the type, then feed the peer's estimates into the
/// bandwidth producer until it FINs.
struct ProbeStream<S: crate::transport::poll::Session> {
	subscriber: Subscriber<S>,
	session: S,
	state: ProbeState<S>,
}

enum ProbeState<S: crate::transport::poll::Session> {
	Open,
	Send { stream: Stream<S, Version> },
	Read { stream: Stream<S, Version> },
}

impl<S: crate::transport::poll::Session> ProbeStream<S> {
	fn new(subscriber: &Subscriber<S>) -> Self {
		Self {
			subscriber: subscriber.clone(),
			session: subscriber.session.clone(),
			state: ProbeState::Open,
		}
	}

	fn poll(&mut self, waiter: &kio::Waiter) -> Poll<Result<(), Error>> {
		let mut cx = std::task::Context::from_waker(waiter.waker());
		loop {
			match &mut self.state {
				ProbeState::Open => {
					// After a GOAWAY the peer must not see new streams. Probe is
					// best-effort; skip it rather than erroring.
					if self.subscriber.going_away.is_set() {
						return Poll::Ready(Ok(()));
					}
					let mut stream = ready!(Stream::poll_open(&mut self.session, self.subscriber.version, &mut cx))?;
					stream.writer.buffer(&lite::ControlType::Probe)?;
					self.state = ProbeState::Send { stream };
				}
				ProbeState::Send { stream } => {
					ready!(stream.writer.poll_flush(&mut cx))?;
					let ProbeState::Send { stream } = std::mem::replace(&mut self.state, ProbeState::Open) else {
						unreachable!()
					};
					self.state = ProbeState::Read { stream };
				}
				ProbeState::Read { stream } => {
					let bandwidth = self.subscriber.recv_bandwidth.as_ref().expect("gated by RecvBandwidth");
					loop {
						let Some(probe) = ready!(stream.reader.poll_decode_maybe::<lite::Probe>(&mut cx))? else {
							return Poll::Ready(Ok(()));
						};
						bandwidth.set(probe.bitrate.map(bandwidth::Rate::from_bps))?;
					}
				}
			}
		}
	}
}

/// One announce-interest stream: sends the ANNOUNCE_REQUEST for a prefix, then
/// feeds every received announce into the origin. Only its *error* ends the
/// session; a publisher FIN is a clean end for the prefix alone.
struct AnnouncePrefix<S: crate::transport::poll::Session> {
	subscriber: Subscriber<S>,
	prefix: PathOwned,
	state: PrefixState<S>,
}

enum PrefixState<S: crate::transport::poll::Session> {
	/// Opening the control stream (after the GOAWAY gate).
	Open,
	/// Flushing the buffered request.
	Send { stream: Stream<S, Version> },
	/// Lite05+: reading the publisher's ANNOUNCE_OK.
	ReadOk { stream: Stream<S, Version> },
	/// Waiting for the link cost (may block on the peer's SETUP).
	Cost {
		stream: Stream<S, Version>,
		responder_origin: Option<crate::Hop>,
	},
	/// Lite01/02: reading the ANNOUNCE_INIT set.
	ReadInit { stream: Stream<S, Version>, run: PrefixRun },
	/// Streaming announce updates.
	Run { stream: Stream<S, Version>, run: PrefixRun },
}

/// The announce-decode loop's state, split out so the states above can share it.
struct PrefixRun {
	responder_origin: Option<crate::Hop>,
	/// What we charge every announcement arriving on this stream. Resolved once:
	/// it comes from the connect config or the peer's SETUP, neither of which
	/// changes for the life of the session.
	link_cost: u64,
	announced: Announced,
	// Lite06+: announce ids. Each received `active` implicitly assigns the next
	// per-stream ordinal; `ended`/`restart` reference it instead of repeating the
	// path. Tracked even for announces we drop locally (reflected loops), since
	// the sender doesn't know we dropped them. We never send a restart ourselves,
	// but a peer may.
	next_announce_id: u64,
	announced_by_id: HashMap<u64, PathOwned>,
}

impl<S: crate::transport::poll::Session> AnnouncePrefix<S> {
	fn new(subscriber: Subscriber<S>, prefix: PathOwned) -> Self {
		Self {
			subscriber,
			prefix,
			state: PrefixState::Open,
		}
	}

	fn poll(&mut self, waiter: &kio::Waiter) -> Poll<Result<(), Error>> {
		let mut cx = std::task::Context::from_waker(waiter.waker());
		loop {
			match &mut self.state {
				PrefixState::Open => {
					// A peer that sent GOAWAY told us to stop opening streams on this session.
					self.subscriber.check_going_away()?;
					let mut stream = ready!(Stream::poll_open(
						&mut self.subscriber.session,
						self.subscriber.version,
						&mut cx
					))?;

					stream.writer.buffer(&lite::ControlType::Announce)?;
					// Lite04/05: ask the peer to filter out announces that already passed
					// through us, so the reflected ones never hit the wire. Encoding drops
					// this on every other version, where start_announce below is the only
					// filter.
					// Hidden routes are requested too: the session mirrors the peer into
					// the origin, and each local reader opts in on its own
					// (`origin::Consumer::with_hidden`).
					stream.writer.buffer(&lite::AnnounceRequest {
						prefix: self.prefix.as_path(),
						exclude_hop: self.subscriber.self_origin.id(),
						hidden: true,
					})?;
					self.state = PrefixState::Send { stream };
				}
				PrefixState::Send { stream } => {
					ready!(stream.writer.poll_flush(&mut cx))?;
					let PrefixState::Send { stream } = std::mem::replace(&mut self.state, PrefixState::Open) else {
						unreachable!()
					};
					self.state = match self.subscriber.version.has_announce_ok() {
						true => PrefixState::ReadOk { stream },
						false => PrefixState::Cost {
							stream,
							responder_origin: None,
						},
					};
				}
				PrefixState::ReadOk { stream } => {
					// Lite05+: the publisher reports its own origin id, which we stamp onto
					// every received Announce's hop chain since it no longer does so itself.
					// Its `active` count marks where the initial set ends; nothing here needs
					// that boundary, so it is read and dropped. Callers that must not race an
					// announcement use `origin::Consumer::announced_broadcast`, which waits
					// for the path itself.
					let ok = ready!(stream.reader.poll_decode::<lite::AnnounceOk>(&mut cx))?;
					// A peer may legally report id 0 (no identity). Keep it: the assigned
					// identity stays on `via` and is never forwarded as a hop.
					let origin = ok.origin;
					let PrefixState::ReadOk { stream } = std::mem::replace(&mut self.state, PrefixState::Open) else {
						unreachable!()
					};
					self.state = PrefixState::Cost {
						stream,
						responder_origin: Some(origin),
					};
				}
				PrefixState::Cost { .. } => {
					let link_cost = ready!(self.subscriber.poll_link_cost(waiter));
					let PrefixState::Cost {
						stream,
						responder_origin,
					} = std::mem::replace(&mut self.state, PrefixState::Open)
					else {
						unreachable!()
					};

					let run = PrefixRun {
						responder_origin,
						link_cost,
						announced: Announced::default(),
						next_announce_id: 0,
						announced_by_id: HashMap::new(),
					};

					// Lite01/02 send the initial set as one ANNOUNCE_INIT message, so they
					// read that before the update stream. Every other version streams it as
					// ordinary announces.
					self.state = match self.subscriber.version {
						Version::Lite01 | Version::Lite02 => PrefixState::ReadInit { stream, run },
						_ => PrefixState::Run { stream, run },
					};
				}
				PrefixState::ReadInit { stream, run } => {
					let msg = ready!(stream.reader.poll_decode::<lite::AnnounceInit>(&mut cx))?;
					for suffix in msg.suffixes {
						let path = self.prefix.join(&suffix);
						// Lite01/02 don't carry hop information; the broadcast starts with
						// an empty chain and an unpriced link. Stats are attributed in the
						// model when this enters the origin via `create_broadcast`.
						self.subscriber.start_announce(
							path,
							crate::Hops::new(),
							crate::origin::Cost::UNKNOWN,
							0,
							run.responder_origin,
							&mut run.announced,
						)?;
					}
					let PrefixState::ReadInit { stream, run } = std::mem::replace(&mut self.state, PrefixState::Open)
					else {
						unreachable!()
					};
					self.state = PrefixState::Run { stream, run };
				}
				PrefixState::Run { stream, run } => {
					// A draining peer usually stops announcing, so react to the
					// GOAWAY itself; waiting for another message would leave the
					// route primary until the session finally closed. Idempotent,
					// since the signal stays set.
					if self.subscriber.going_away.poll(waiter).is_ready() {
						run.announced.drain();
					}
					// Drain every buffered announce BEFORE serving requests: a route
					// this pass attaches must have its request queue polled (and this
					// machine's waiter registered on it) below, in the same pass.
					// Serving first and then parking on the decode would strand a
					// request that arrives in between: its wake finds no waiter, and
					// nothing else re-polls this machine.
					loop {
						match stream.reader.poll_decode_maybe::<lite::AnnounceBroadcast>(&mut cx) {
							Poll::Ready(Ok(Some(announce))) => {
								self.subscriber.handle_announce(&self.prefix, announce, run)?;
							}
							Poll::Ready(Ok(None)) => {
								// The publisher FINed: it has nothing (more) to announce for this
								// prefix (e.g. a publish-only peer). That's a clean completion of
								// this announce stream, not a session error, so finish our side
								// and return Ok. Tearing down only the announce stream is correct
								// since no further progress can be made, but we must not
								// propagate an error that would kill the whole connection.
								stream.writer.finish().ok();
								return Poll::Ready(Ok(()));
							}
							Poll::Ready(Err(err)) => return Poll::Ready(Err(err)),
							Poll::Pending => break,
						}
					}
					// Materialize requested paths under the attached routes; leaves the
					// waiter registered on every route's request queue.
					run.announced.poll_serve(&self.subscriber, waiter);
					return Poll::Pending;
				}
			}
		}
	}
}

/// Serves the origin's track requests for one announced source until the peer
/// unannounces it (the source is finished) or the session dies. An unannounce
/// takes no new tracks but lets those in flight run to their own end (moq-lite:
/// retraction does not disturb subscriptions already in flight); the session
/// dying drops them.
struct SourceServe<S: crate::transport::poll::Session> {
	subscriber: Subscriber<S>,
	path: PathOwned,
	dynamic: crate::broadcast::Dynamic,
	// A dedicated close-watch handle, since each pending operation needs its own.
	closed: S,
	tracks: kio::Tasks<TrackServeRun<S>>,
	// The source ended: no more track requests will arrive.
	ended: bool,
}

impl<S: crate::transport::poll::Session> SourceServe<S> {
	fn new(subscriber: Subscriber<S>, path: PathOwned, dynamic: crate::broadcast::Dynamic) -> Self {
		let closed = subscriber.session.clone();
		Self {
			subscriber,
			path,
			dynamic,
			closed,
			tracks: kio::Tasks::new(),
			ended: false,
		}
	}
}

impl<S: crate::transport::poll::Session> kio::Task for SourceServe<S> {
	fn poll(&mut self, waiter: &kio::Waiter) -> Poll<()> {
		let _ = self.tracks.poll(waiter);

		let mut cx = std::task::Context::from_waker(waiter.waker());
		loop {
			if self.closed.poll_closed(&mut cx).is_ready() {
				// Session gone.
				return Poll::Ready(());
			}
			if self.ended {
				// Done once the tracks in flight are.
				return self.tracks.poll(waiter);
			}
			match self.dynamic.poll_requested_track(waiter) {
				Poll::Ready(Ok(request)) => {
					let serve = TrackServe {
						subscriber: self.subscriber.clone(),
						path: self.path.clone(),
						name: request.name().to_string(),
					};
					// One machine per track serves its lone subscription and any number
					// of fetches concurrently.
					self.tracks.push(TrackServeRun::new(serve, request));
				}
				// The source was finished (unannounced) or aborted.
				Poll::Ready(Err(err)) => {
					tracing::debug!(%err, "source closed");
					self.ended = true;
				}
				Poll::Pending => break,
			}
		}

		// Newly requested tracks start now rather than on the next wake.
		let _ = self.tracks.poll(waiter);
		Poll::Pending
	}
}

#[cfg(test)]
mod tests {
	use super::*;
	use crate::coding::{Decode, Encode};
	use crate::lite::test_transport::SinkSession;
	use crate::model::ProduceTest;
	use futures::FutureExt;

	const VERSION: Version = Version::Lite05;

	/// Removing a subscription both stops delivery and releases the session's handle
	/// on the producer, so the track (its cached groups, its stats subscription) ends
	/// rather than outliving the subscription it belonged to.
	#[test]
	fn unsubscribe_drops_the_datagram_and_releases_the_producer() {
		let origin = origin::Config::new(crate::Hop::new(1).unwrap()).produce();
		let subscriber = Subscriber::new(SubscriberConfig {
			runtime: crate::time::Clock::tokio(),
			session: SinkSession::default(),
			origin,
			recv_bandwidth: None,
			version: VERSION,
			peer_setup: Default::default(),
			peer_hop: None,
			cost: None,
			going_away: Default::default(),
		});

		let broadcast = crate::broadcast::Info::new().produce();
		// The broadcast keeps only a weak handle, so the map below owns the only strong
		// `track::Producer`: dropping it is what ends the track.
		let producer = broadcast.create_track("datagrams", None).unwrap();
		let mut received = producer.subscribe(None);
		subscriber.subscribes.lock().insert(
			7,
			TrackEntry {
				producer,
				timescale: Some(Timescale::default()),
			},
		);

		let payload = |sequence| {
			lite::Datagram {
				subscribe: 7,
				sequence,
				timestamp: sequence,
				payload: bytes::Bytes::from_static(b"x"),
			}
			.encode_bytes(VERSION)
			.unwrap()
		};
		subscriber.route_datagram(payload(1)).unwrap();
		assert_eq!(
			received
				.recv_datagram()
				.now_or_never()
				.unwrap()
				.unwrap()
				.unwrap()
				.sequence,
			1
		);

		subscriber.remove_subscribe(7);
		subscriber.route_datagram(payload(2)).unwrap();
		// Dropping the last producer is an abrupt teardown, so the track resolves with
		// `Dropped` rather than parking. A route that outlived the removal would keep the
		// producer alive and leave this pending forever.
		assert!(
			matches!(received.recv_datagram().now_or_never(), Some(Err(Error::Dropped))),
			"the track outlived its subscription"
		);
	}

	/// `establish` puts exactly one SUBSCRIBE on the wire, and the id is registered
	/// before any of it reaches the transport.
	///
	/// Both halves matter: a second stream re-requests the same id, which the peer is
	/// free to serve twice, and a late insert loses the race with a publisher that
	/// serves its first group the instant it reads the request (`recv_group` drops a
	/// group whose id isn't in the map yet).
	#[tokio::test]
	async fn establish_sends_one_registered_subscribe() {
		// Writes park until this opens, so the assertions below run at the exact moment
		// the request would hit the wire.
		let gate = kio::Producer::new(false);
		let session = SinkSession::gated_bi(gate.consume());

		let origin = origin::Config::new(crate::Hop::new(1).unwrap()).produce();
		let subscriber = Subscriber::new(SubscriberConfig {
			runtime: crate::time::Clock::tokio(),
			session: session.clone(),
			origin,
			recv_bandwidth: None,
			version: VERSION,
			peer_setup: Default::default(),
			peer_hop: None,
			cost: None,
			going_away: Default::default(),
		});
		let subscribes = subscriber.subscribes.clone();
		let serve = TrackServe {
			subscriber,
			path: Path::new("room/host").to_owned(),
			name: "catalog.json".to_string(),
		};

		let broadcast = crate::broadcast::Info::new().produce();
		let mut producer = broadcast.create_track("catalog.json", None).unwrap();
		let mut sub = Sub::None;
		let mut establish = std::pin::pin!(serve.establish(
			&mut producer,
			&mut sub,
			Subscription::default(),
			Some(Timescale::default()),
		));

		// Parked on the first write: the stream is open and nothing has been sent yet.
		assert!(futures::poll!(establish.as_mut()).is_pending());
		assert_eq!(session.log.bi_opens(), 1);
		assert!(subscribes.lock().contains_key(&0), "registered before the wire");

		let Ok(mut open) = gate.write() else {
			panic!("gate closed")
		};
		*open = true;
		drop(open);

		establish.await.unwrap();

		// One request, on one stream, and nothing else behind it.
		assert_eq!(session.log.bi_opens(), 1);

		let writes = session.log.writes.lock().unwrap().clone();
		let mut wire = writes.as_slice();
		assert_eq!(
			lite::ControlType::decode(&mut wire, VERSION).unwrap(),
			lite::ControlType::Subscribe
		);
		let msg = lite::Subscribe::decode(&mut wire, VERSION).unwrap();
		assert_eq!(msg.id, 0);
		assert_eq!(msg.track, "catalog.json");
		assert!(wire.is_empty(), "a second SUBSCRIBE trailed the first");
	}

	/// Everything a `handle_subscription` test needs to stay alive for the call.
	struct Harness {
		serve: TrackServe<SinkSession>,
		session: SinkSession,
		producer: track::Producer,
		_broadcast: crate::broadcast::Producer,
		_gate: kio::Producer<bool>,
	}

	impl Harness {
		/// A `TrackServe` writing straight to a sink session at `version`.
		fn new(version: Version) -> Self {
			// Open the gate up front: these tests assert what reached the wire, not
			// what was true at the instant it would.
			let gate = kio::Producer::new(true);
			let session = SinkSession::gated_bi(gate.consume());
			let origin = origin::Config::new(crate::Hop::new(1).unwrap()).produce();
			let subscriber = Subscriber::new(SubscriberConfig {
				runtime: crate::time::Clock::tokio(),
				session: session.clone(),
				origin,
				recv_bandwidth: None,
				version,
				peer_setup: Default::default(),
				peer_hop: None,
				cost: None,
				going_away: Default::default(),
			});
			let broadcast = crate::broadcast::Info::new().produce();
			let producer = broadcast.create_track("catalog.json", None).unwrap();

			Self {
				serve: TrackServe {
					subscriber,
					path: Path::new("room/host").to_owned(),
					name: "catalog.json".to_string(),
				},
				session,
				producer,
				_broadcast: broadcast,
				_gate: gate,
			}
		}

		/// Everything written to the session so far.
		fn wire(&self) -> Vec<u8> {
			self.session.log.writes.lock().unwrap().clone()
		}
	}

	/// The `(group, frame)` bounds `resume::slice` produces after a mid-group takeover: a
	/// subscriber resuming at frame 3 of group 5, capped by a later boundary in the same
	/// group.
	fn mid_group_demand() -> Subscription {
		Subscription::default()
			.with_start(Position { group: 5, frame: 3 })
			.with_end(Position::after(5, 7))
	}

	/// A mid-group resume boundary handed to a peer that predates lite-06 is widened to
	/// the whole group rather than refused.
	///
	/// The codec rejects a frame bound such a peer cannot carry, so passing the demand
	/// through unchanged fails the SUBSCRIBE and hands the track back. The origin then
	/// re-splices the same route indefinitely, since the splice itself keeps succeeding
	/// and the retry budget never trips.
	#[tokio::test]
	async fn frame_bounds_widen_for_an_older_peer() {
		let mut h = Harness::new(Version::Lite05);
		let mut sub = Sub::None;

		h.serve
			.handle_subscription(
				&mut h.producer,
				&mut sub,
				Some(mid_group_demand()),
				true,
				Some(Timescale::default()),
			)
			.await
			.expect("an older peer must not fail the subscribe");

		let wire = h.wire();
		let mut wire = wire.as_slice();
		assert_eq!(
			lite::ControlType::decode(&mut wire, Version::Lite05).unwrap(),
			lite::ControlType::Subscribe
		);
		let msg = lite::Subscribe::decode(&mut wire, Version::Lite05).unwrap();
		// The group bounds survive; only the frame offsets are widened away.
		assert_eq!((msg.start_group, msg.end_group), (Some(5), Some(5)));
		assert_eq!((msg.start_frame, msg.end_frame), (0, None));
	}

	/// The same demand on a lite-06 peer keeps its frame offsets, so the widening is
	/// version-gated rather than unconditional.
	#[tokio::test]
	async fn frame_bounds_survive_on_a_lite06_peer() {
		let mut h = Harness::new(Version::Lite06);
		let mut sub = Sub::None;

		h.serve
			.handle_subscription(
				&mut h.producer,
				&mut sub,
				Some(mid_group_demand()),
				true,
				Some(Timescale::default()),
			)
			.await
			.unwrap();

		let wire = h.wire();
		let mut wire = wire.as_slice();
		assert_eq!(
			lite::ControlType::decode(&mut wire, Version::Lite06).unwrap(),
			lite::ControlType::Subscribe
		);
		let msg = lite::Subscribe::decode(&mut wire, Version::Lite06).unwrap();
		assert_eq!((msg.start_frame, msg.end_frame), (3, Some(7)));
	}

	/// The model's exclusive end maps back to the wire's inclusive pair.
	///
	/// The two disagree deliberately (see [`Subscription::end`]), so this pins the seam:
	/// an end at the head of a group means the group below it, served whole, while one
	/// mid-group caps the frame below it. Off by one here would silently drop or
	/// duplicate a frame at every relay hop.
	#[test]
	fn wire_bounds_convert_the_exclusive_end() {
		// The whole of group 5 is the head of group 6.
		let bounds = WireBounds::new(None, Some(Position::group(6)));
		assert_eq!((bounds.end_group, bounds.end_frame), (Some(5), None));

		// Group 5 through frame 2 is the head of frame 3.
		let bounds = WireBounds::new(None, Some(Position { group: 5, frame: 3 }));
		assert_eq!((bounds.end_group, bounds.end_frame), (Some(5), Some(2)));

		// Unbounded stays unbounded.
		let bounds = WireBounds::new(None, None);
		assert_eq!((bounds.end_group, bounds.end_frame), (None, None));

		// Starts are inclusive on both sides, so they pass straight through.
		let bounds = WireBounds::new(Some(Position { group: 5, frame: 3 }), None);
		assert_eq!((bounds.start_group, bounds.start_frame), (Some(5), 3));
	}

	/// The builders produce exactly what the wire conversion expects, so an inclusive
	/// bound survives the trip out to a peer unchanged.
	#[test]
	fn wire_bounds_match_the_builders() {
		let whole = Subscription::default().with_end(Position::after_group(5));
		let bounds = WireBounds::new(whole.start, whole.end);
		assert_eq!((bounds.end_group, bounds.end_frame), (Some(5), None));

		let capped = Subscription::default().with_end(Position::after(5, 2));
		let bounds = WireBounds::new(capped.start, capped.end);
		assert_eq!((bounds.end_group, bounds.end_frame), (Some(5), Some(2)));

		let started = Subscription::default().with_start(Position { group: 5, frame: 3 });
		let bounds = WireBounds::new(started.start, started.end);
		assert_eq!((bounds.start_group, bounds.start_frame), (Some(5), 3));
	}

	/// A subscription that asks for nothing opens nothing.
	///
	/// `Position::group(0)` is the empty range: nothing sorts below it. The wire cannot
	/// say that, and the nearest thing it can say is "through group 0", which would
	/// deliver the single group the caller excluded.
	#[tokio::test]
	async fn an_empty_range_opens_no_subscription() {
		let mut h = Harness::new(Version::Lite06);
		let mut sub = Sub::None;

		let empty = Subscription::default().with_end(Position::group(0));
		h.serve
			.handle_subscription(&mut h.producer, &mut sub, Some(empty), true, Some(Timescale::default()))
			.await
			.unwrap();

		assert!(matches!(sub, Sub::None), "must not open a subscription");
		assert!(h.wire().is_empty(), "nothing reached the wire");
	}

	/// Demand collapsing to nothing cancels the upstream rather than sending a bound
	/// that means the opposite.
	#[tokio::test]
	async fn an_empty_range_cancels_a_live_subscription() {
		let mut h = Harness::new(Version::Lite06);
		let mut sub = Sub::None;

		h.serve
			.handle_subscription(
				&mut h.producer,
				&mut sub,
				Some(Subscription::default()),
				true,
				Some(Timescale::default()),
			)
			.await
			.unwrap();
		assert!(matches!(sub, Sub::Active(_)), "the first subscriber opens one");
		let established = h.wire().len();

		let empty = Subscription::default().with_end(Position::group(0));
		h.serve
			.handle_subscription(&mut h.producer, &mut sub, Some(empty), true, Some(Timescale::default()))
			.await
			.unwrap();

		assert!(matches!(sub, Sub::None), "the upstream must be canceled");
		assert_eq!(h.wire().len(), established, "no SUBSCRIBE_UPDATE claiming group 0");
	}

	/// Bounds that meet anywhere in the track are just as empty as ones that meet at the
	/// first position.
	///
	/// The wire has no encoding for either: an exclusive end at a group head floors to
	/// the group below it, so group 5 through group 5 would go out as `start_group = 5`,
	/// `end_group = 4`, an inverted range the publisher happily parks on.
	#[tokio::test]
	async fn a_nonzero_empty_range_opens_no_subscription() {
		let mut h = Harness::new(Version::Lite06);
		let mut sub = Sub::None;

		let empty = Subscription::default()
			.with_start(Position::group(5))
			.with_end(Position::group(5));
		h.serve
			.handle_subscription(&mut h.producer, &mut sub, Some(empty), true, Some(Timescale::default()))
			.await
			.unwrap();

		assert!(matches!(sub, Sub::None), "must not open a subscription");
		assert!(h.wire().is_empty(), "nothing reached the wire");
	}

	/// Demand collapsing to an empty range mid-track cancels the upstream, the same way
	/// it does at the first position.
	#[tokio::test]
	async fn a_nonzero_empty_range_cancels_a_live_subscription() {
		let mut h = Harness::new(Version::Lite06);
		let mut sub = Sub::None;

		h.serve
			.handle_subscription(
				&mut h.producer,
				&mut sub,
				Some(Subscription::default()),
				true,
				Some(Timescale::default()),
			)
			.await
			.unwrap();
		assert!(matches!(sub, Sub::Active(_)), "the first subscriber opens one");
		let established = h.wire().len();

		let empty = Subscription::default()
			.with_start(Position::group(5))
			.with_end(Position::group(5));
		h.serve
			.handle_subscription(&mut h.producer, &mut sub, Some(empty), true, Some(Timescale::default()))
			.await
			.unwrap();

		assert!(matches!(sub, Sub::None), "the upstream must be canceled");
		assert_eq!(h.wire().len(), established, "no SUBSCRIBE_UPDATE inverting the range");
	}

	/// Widening rounds the end outward at the last group too, keeping the range at least
	/// as wide as the request.
	///
	/// Rounding inward there would empty a range the caller asked for, and the filter in
	/// `handle_subscription` runs before the widening, so nothing downstream would catch
	/// it.
	#[tokio::test]
	async fn frame_bounds_widen_outward_at_the_last_group() {
		let h = Harness::new(Version::Lite05);

		let mut subscription = Subscription::default()
			.with_start(Position {
				group: u64::MAX,
				frame: 1,
			})
			.with_end(Position::after(u64::MAX, 5));
		h.serve.widen_frame_bounds(&mut subscription);

		assert_eq!(subscription.start, Some(Position::group(u64::MAX)));
		// Past the last group there is no position to round up to, and unbounded is the
		// wider request.
		assert_eq!(subscription.end, None);
	}

	/// The widening covers SUBSCRIBE_UPDATE too: a downstream peer asking for a frame
	/// offset must not tear down an older upstream that is already serving.
	#[tokio::test]
	async fn frame_bounds_widen_on_update() {
		let mut h = Harness::new(Version::Lite05);
		let mut sub = Sub::None;

		h.serve
			.handle_subscription(
				&mut h.producer,
				&mut sub,
				Some(Subscription::default()),
				true,
				Some(Timescale::default()),
			)
			.await
			.unwrap();
		let established = h.wire().len();

		// A lite-06 subscriber downstream now wants to resume mid-group.
		h.serve
			.handle_subscription(
				&mut h.producer,
				&mut sub,
				Some(mid_group_demand()),
				true,
				Some(Timescale::default()),
			)
			.await
			.expect("a downstream frame offset must not tear down an older upstream");

		// SUBSCRIBE_UPDATE rides the subscribe stream with no control type ahead of it.
		let wire = h.wire();
		let mut wire = &wire[established..];
		let msg = lite::SubscribeUpdate::decode(&mut wire, Version::Lite05).unwrap();
		assert_eq!((msg.start_group, msg.start_frame), (Some(5), 0));
		assert_eq!((msg.end_group, msg.end_frame), (Some(5), None));
	}

	/// A buffered SUBSCRIBE_START describes the demand its SUBSCRIBE carried, so
	/// it applies exactly while the current start matches that demand: an update
	/// that moves the start makes it stale (applying it could reopen a range the
	/// publisher no longer serves, or clamp one it still does), and an update
	/// that moves back restores it (the publisher declared that range gone and
	/// sends no replacement START).
	#[tokio::test]
	async fn buffered_start_applies_iff_demand_matches() {
		let mut h = Harness::new(Version::Lite05);
		let mut sub = Sub::None;

		let demand = |group: u64| Some(Subscription::default().with_start(Position::group(group)));
		let applies = |sub: &Sub<SinkSession>| matches!(sub, Sub::Active(active) if active.start == active.requested);

		// Establish from group 3; the peer's START is considered in flight.
		h.serve
			.handle_subscription(&mut h.producer, &mut sub, demand(3), true, Some(Timescale::default()))
			.await
			.unwrap();
		assert!(applies(&sub), "a fresh subscription accepts its START");

		// An end-only update leaves the start intact: the START stays valid.
		h.serve
			.handle_subscription(
				&mut h.producer,
				&mut sub,
				Some(
					Subscription::default()
						.with_start(Position::group(3))
						.with_end(Position::group(9)),
				),
				true,
				Some(Timescale::default()),
			)
			.await
			.unwrap();
		assert!(applies(&sub), "an unmoved start keeps the START applicable");

		// The start moves: a buffered START is stale while it sits elsewhere.
		h.serve
			.handle_subscription(&mut h.producer, &mut sub, demand(8), true, Some(Timescale::default()))
			.await
			.unwrap();
		assert!(!applies(&sub), "a moved start must invalidate a buffered START");

		// The start returns: the declaration matches the demand again, so the
		// publisher's skip (it sends no replacement START) must land.
		h.serve
			.handle_subscription(&mut h.producer, &mut sub, demand(3), true, Some(Timescale::default()))
			.await
			.unwrap();
		assert!(applies(&sub), "demand returning restores the START");
	}

	/// The permanent-miss floor follows the demand in every direction: an update
	/// that moves the start forward retires the skipped range (the publisher
	/// stops serving it and no fresh START says so), one that moves it backward
	/// reopens it, and dropping to the live edge clears it entirely.
	#[tokio::test]
	async fn updates_move_the_declared_floor_both_ways() {
		let mut h = Harness::new(Version::Lite05);
		let mut sub = Sub::None;

		let demand = |group: u64| Some(Subscription::default().with_start(Position::group(group)));

		// Establish from group 5: the floor tracks the request until START lands.
		h.serve
			.handle_subscription(&mut h.producer, &mut sub, demand(5), true, Some(Timescale::default()))
			.await
			.unwrap();
		assert_eq!(h.producer.start_sequence(), Some(5));

		// Forward: a reader waiting in [5, 8) must fail over, not stall.
		h.serve
			.handle_subscription(&mut h.producer, &mut sub, demand(8), true, Some(Timescale::default()))
			.await
			.unwrap();
		assert_eq!(h.producer.start_sequence(), Some(8));

		// Backward: the reopened range must stop being a permanent miss.
		h.serve
			.handle_subscription(&mut h.producer, &mut sub, demand(3), true, Some(Timescale::default()))
			.await
			.unwrap();
		assert_eq!(h.producer.start_sequence(), Some(3));

		// Live edge: the floor is unknown until the next declaration, so a
		// group below the stale one must not be a permanent miss.
		h.serve
			.handle_subscription(
				&mut h.producer,
				&mut sub,
				Some(Subscription::default()),
				true,
				Some(Timescale::default()),
			)
			.await
			.unwrap();
		assert_eq!(h.producer.start_sequence(), None);
	}

	/// A second announce for a live path is a protocol error whatever its hops say. It
	/// must be caught before the reflection drops, because the caller has already bound
	/// an announce id to it: dropping it silently leaves that id pointing at a path
	/// owned by the earlier announce, and the `ANNOUNCE_END` that follows retires the
	/// wrong route.
	#[tokio::test]
	async fn a_double_announce_is_an_error_even_when_reflected() {
		let assigned = crate::Hop::new(777).unwrap();
		let origin = origin::Config::new(crate::Hop::new(1).unwrap()).produce();
		let mut subscriber = Subscriber::new(SubscriberConfig {
			runtime: crate::time::Clock::tokio(),
			session: SinkSession::new(Default::default()),
			origin,
			recv_bandwidth: None,
			version: VERSION,
			peer_setup: Default::default(),
			cost: None,
			peer_hop: Some(assigned),
			going_away: Default::default(),
		});

		let path = Path::new("room/host").to_owned();
		let mut announced = Announced::default();
		assert!(
			subscriber
				.start_announce(
					path.clone(),
					crate::Hops::new(),
					crate::origin::Cost::default(),
					0,
					Some(assigned),
					&mut announced,
				)
				.unwrap()
		);

		// The same path again, this time with a chain that names the sender.
		let mut reflected = crate::Hops::new();
		reflected.push(assigned).unwrap();
		assert!(
			matches!(
				subscriber.start_announce(
					path.clone(),
					reflected,
					crate::origin::Cost::default(),
					0,
					Some(assigned),
					&mut announced,
				),
				Err(Error::ProtocolViolation)
			),
			"the double announce must be reported, not silently dropped",
		);
	}

	/// Every path out of `start_announce` that declines an announce records it first.
	///
	/// The decline paths are a list one edit can fall off the end of, and a miss is silent:
	/// the path reads as free, a later announce takes it, and the declined one's
	/// `ANNOUNCE_END` retires that route instead. This walks a reflection: the chain
	/// already names this session.
	#[tokio::test]
	async fn every_declined_announce_is_recorded() {
		let assigned = crate::Hop::new(777).unwrap();
		let origin = origin::Config::new(crate::Hop::new(1).unwrap()).produce();
		let mut subscriber = Subscriber::new(SubscriberConfig {
			runtime: crate::time::Clock::tokio(),
			session: SinkSession::new(Default::default()),
			origin,
			recv_bandwidth: None,
			version: Version::Lite03,
			peer_setup: Default::default(),
			cost: None,
			peer_hop: Some(assigned),
			going_away: Default::default(),
		});

		let path = Path::new("room/host").to_owned();
		let mut announced = Announced::default();

		// A chain that already names us is a reflection, declined but still recorded.
		let hops = crate::Hops::try_from(vec![crate::Hop::new(1).unwrap()]).unwrap();
		assert!(
			!subscriber
				.start_announce(
					path.clone(),
					hops,
					crate::origin::Cost::default(),
					0,
					None,
					&mut announced
				)
				.unwrap(),
			"a chain that already names this session must be declined",
		);

		// Declined, but still the peer's advertisement at that path.
		let mut fresh = crate::Hops::new();
		fresh.push(crate::Hop::new(7).unwrap()).unwrap();
		assert!(
			matches!(
				subscriber.start_announce(
					path.clone(),
					fresh,
					crate::origin::Cost::default(),
					0,
					None,
					&mut announced
				),
				Err(Error::ProtocolViolation)
			),
			"a declined announce still holds its path, so a second start for it is a violation",
		);
	}

	/// A dropped announce still holds its path until the peer retracts it.
	#[tokio::test]
	async fn a_dropped_announce_still_holds_its_path() {
		let assigned = crate::Hop::new(777).unwrap();
		let origin = origin::Config::new(crate::Hop::new(1).unwrap()).produce();
		let consumer = origin.consume();
		let mut subscriber = Subscriber::new(SubscriberConfig {
			runtime: crate::time::Clock::tokio(),
			session: SinkSession::new(Default::default()),
			origin,
			recv_bandwidth: None,
			version: VERSION,
			peer_setup: Default::default(),
			cost: None,
			peer_hop: Some(assigned),
			going_away: Default::default(),
		});

		let path = Path::new("room/host").to_owned();
		let mut announced = Announced::default();
		let mut reflected = crate::Hops::new();
		reflected.push(assigned).unwrap();
		assert!(
			!subscriber
				.start_announce(
					path.clone(),
					reflected,
					crate::origin::Cost::default(),
					0,
					Some(assigned),
					&mut announced,
				)
				.unwrap(),
			"a chain naming its own sender must be dropped",
		);
		assert!(consumer.get_broadcast("room/host").is_none());

		let mut hops = crate::Hops::new();
		hops.push(crate::Hop::new(7).unwrap()).unwrap();
		let err = subscriber
			.start_announce(
				path.clone(),
				hops,
				crate::origin::Cost::default(),
				0,
				Some(assigned),
				&mut announced,
			)
			.expect_err("a second start for the peer-owned path must be rejected");
		assert!(matches!(err, Error::ProtocolViolation));
	}

	/// An announce whose chain already names the sender came back through the sender.
	/// Appending it again would name one identity twice, which is tolerated in a lite
	/// chain but is a PROTOCOL_VIOLATION for an IETF peer the route is later forwarded
	/// to, so the route must never enter the model carrying it.
	#[tokio::test]
	async fn an_announce_reflected_by_its_sender_is_dropped() {
		let assigned = crate::Hop::new(777).unwrap();

		let origin = origin::Config::new(crate::Hop::new(1).unwrap()).produce();
		let consumer = origin.consume();
		let mut subscriber = Subscriber::new(SubscriberConfig {
			runtime: crate::time::Clock::tokio(),
			session: SinkSession::new(Default::default()),
			origin,
			recv_bandwidth: None,
			version: VERSION,
			peer_setup: Default::default(),
			cost: None,
			peer_hop: Some(assigned),
			going_away: Default::default(),
		});

		// The sender's identity is already in the chain: the route came back through it.
		let mut hops = crate::Hops::new();
		hops.push(assigned).unwrap();

		let mut announced = Announced::default();
		let accepted = subscriber
			.start_announce(
				Path::new("room/host").to_owned(),
				hops,
				crate::origin::Cost::default(),
				0,
				Some(assigned),
				&mut announced,
			)
			.unwrap();
		assert!(!accepted, "a chain naming its own sender must not become a route");

		tokio::time::sleep(std::time::Duration::from_millis(1)).await;
		assert!(consumer.get_broadcast("room/host").is_none());
	}

	/// A peer that was advertised a local path and announces it back with this
	/// origin's hop in the chain is a reflection, not a new path. The announce
	/// is dropped, the local front keeps serving, and the peer's own
	/// subscription (split-horizon excluded) still reads from it.
	#[tokio::test]
	async fn a_reflected_announce_does_not_displace_the_local_front() {
		let relay = crate::Hop::new(1).unwrap();
		let origin = origin::Config::new(relay).produce();
		let assigned = crate::Hop::new(777).unwrap();

		let local = origin.publish("room/host", origin::Route::default()).unwrap();
		let track = local.create_track("video", None).unwrap();
		let mut group = track.append_group().unwrap();
		group.write_frame(crate::Timestamp::ZERO, b"local".as_ref()).unwrap();
		group.finish().unwrap();

		// The peer's own subscription, excluding the hop the server minted for
		// it, is served from the local front before anything is announced back.
		let peer = origin.consume().excluding(assigned);
		let resolved = peer.request_broadcast("room/host").await.expect("resolves");
		let mut sub = resolved
			.track("video")
			.unwrap()
			.subscribe(None)
			.await
			.expect("subscribe");
		let mut group = sub.recv_group().await.expect("recv group").expect("track ended early");
		assert_eq!(
			&group.read_frame().await.expect("read frame").expect("frame").payload[..],
			b"local"
		);

		let mut subscriber = Subscriber::new(SubscriberConfig {
			runtime: crate::time::Clock::tokio(),
			session: SinkSession::new(Default::default()),
			origin: origin.clone(),
			recv_bandwidth: None,
			version: VERSION,
			peer_setup: Default::default(),
			cost: None,
			peer_hop: Some(assigned),
			going_away: Default::default(),
		});

		// The path as advertised to the peer: our hop is already in the chain.
		let mut hops = crate::Hops::new();
		hops.push(relay).unwrap();
		let mut announced = Announced::default();
		let accepted = subscriber
			.start_announce(
				Path::new("room/host").to_owned(),
				hops,
				crate::origin::Cost::default(),
				0,
				Some(assigned),
				&mut announced,
			)
			.unwrap();
		assert!(!accepted, "an announce that already names this origin must be dropped");

		// The local front is still the one at the path, and still serving: the
		// peer's next request joins it rather than minting another.
		let still = peer
			.request_broadcast("room/host")
			.await
			.expect("the local front keeps serving");
		assert!(
			still.is_clone(&resolved),
			"the reflected announce must not replace the local front"
		);

		let mut group = track.append_group().unwrap();
		group.write_frame(crate::Timestamp::ZERO, b"still".as_ref()).unwrap();
		group.finish().unwrap();
		let mut group = sub.recv_group().await.expect("recv group").expect("track ended early");
		assert_eq!(
			&group.read_frame().await.expect("read frame").expect("frame").payload[..],
			b"still"
		);
	}

	/// A peer that declares no identity is marked anonymous (hop 0). The assigned
	/// identity stays on `via` for split-horizon and is never written into the chain.
	#[tokio::test]
	async fn assigned_peer_hop_attributes_announces() {
		let session = SinkSession::new(Default::default());
		let assigned = crate::Hop::new(777).unwrap();

		let origin = origin::Config::new(crate::Hop::new(1).unwrap()).produce();
		let consumer = origin.consume();
		let mut subscriber = Subscriber::new(SubscriberConfig {
			runtime: crate::time::Clock::tokio(),
			session,
			origin,
			recv_bandwidth: None,
			version: VERSION,
			peer_setup: Default::default(),
			peer_hop: Some(assigned),
			cost: None,
			going_away: Default::default(),
		});

		// An announce with an empty chain and no responder id: the versions that
		// carry no hop information on the wire.
		let mut announced = Announced::default();
		let accepted = subscriber
			.start_announce(
				Path::new("room/host").to_owned(),
				crate::Hops::new(),
				crate::origin::Cost::UNKNOWN,
				0,
				None,
				&mut announced,
			)
			.unwrap();
		assert!(accepted);

		// The route is announced synchronously: hop 0 on the wire, assigned id local.
		let mut cursor = consumer.announced();
		let route = cursor.assert_next_active("room/host");
		let hops: Vec<_> = route.hops.iter().copied().collect();
		assert_eq!(hops, vec![crate::Hop::UNKNOWN]);
		assert!(route.is_anonymous());

		let mut hidden = consumer.excluding(assigned).announced();
		hidden.assert_next_wait();
	}

	/// Lite03 hop-count placeholders stay 0 and count as anonymous; they are not
	/// rewritten with the assigned identity.
	#[tokio::test]
	async fn lite03_placeholders_stay_anonymous() {
		let assigned = crate::Hop::new(777).unwrap();
		let origin = origin::Config::new(crate::Hop::new(1).unwrap()).produce();
		let consumer = origin.consume();
		let mut subscriber = Subscriber::new(SubscriberConfig {
			runtime: crate::time::Clock::tokio(),
			session: SinkSession::new(Default::default()),
			origin,
			recv_bandwidth: None,
			version: Version::Lite03,
			peer_setup: Default::default(),
			cost: None,
			peer_hop: Some(assigned),
			going_away: Default::default(),
		});

		let hops = crate::Hops::try_from(vec![crate::Hop::UNKNOWN, crate::Hop::UNKNOWN]).unwrap();
		let mut announced = Announced::default();
		assert!(
			subscriber
				.start_announce(
					Path::new("room/host").to_owned(),
					hops,
					crate::origin::Cost::UNKNOWN,
					0,
					None,
					&mut announced,
				)
				.unwrap()
		);

		let mut cursor = consumer.announced();
		let route = cursor.assert_next_active("room/host");
		let hops: Vec<_> = route.hops.iter().copied().collect();
		assert_eq!(hops, vec![crate::Hop::UNKNOWN, crate::Hop::UNKNOWN]);
		assert!(route.is_anonymous());
	}

	/// A peer with no assigned identity is attributed the reserved origin 0
	/// (UNKNOWN). This layer never mints one of its own: whether an anonymous peer
	/// gets an identity, and whether two of its sessions share it, is the caller's
	/// policy, and a minted id here would be indistinguishable from a declared one.
	#[tokio::test]
	async fn absent_peer_hop_stamps_unknown() {
		let (mut subscriber, consumer) = restart_subscriber(SinkSession::new(Default::default()));

		let mut announced = Announced::default();
		subscriber
			.start_announce(
				Path::new("room/host").to_owned(),
				crate::Hops::new(),
				crate::origin::Cost::UNKNOWN,
				0,
				None,
				&mut announced,
			)
			.unwrap();

		let mut cursor = consumer.announced();
		let route = cursor.assert_next_active("room/host");
		let hops: Vec<_> = route.hops.iter().copied().collect();
		assert_eq!(hops, vec![crate::Hop::UNKNOWN]);
	}

	fn restart_subscriber(session: SinkSession) -> (Subscriber<SinkSession>, crate::origin::Consumer) {
		let origin = origin::Config::new(crate::Hop::new(1).unwrap()).produce();
		let consumer = origin.consume();
		let subscriber = Subscriber::new(SubscriberConfig {
			runtime: crate::time::Clock::tokio(),
			session,
			origin,
			recv_bandwidth: None,
			version: VERSION,
			peer_setup: Default::default(),
			cost: None,
			peer_hop: None,
			going_away: Default::default(),
		});
		(subscriber, consumer)
	}

	/// A restart re-prices the announced route in place: consumers observe another
	/// active update with the new metadata rather than a retract-and-announce.
	#[tokio::test]
	async fn restart_updates_the_route_in_place() {
		let (mut subscriber, consumer) = restart_subscriber(SinkSession::new(Default::default()));

		let mut announced = Announced::default();
		let path = Path::new("room/host").to_owned();
		subscriber
			.start_announce(
				path.clone(),
				crate::Hops::new(),
				crate::origin::Cost::UNKNOWN,
				0,
				Some(crate::Hop::new(7).unwrap()),
				&mut announced,
			)
			.unwrap();

		let mut cursor = consumer.announced();
		cursor.assert_next_active("room/host");

		subscriber
			.restart_announce(
				path.clone(),
				crate::Hops::new(),
				crate::origin::Cost::new(5),
				0,
				Some(crate::Hop::new(7).unwrap()),
				&mut announced,
			)
			.unwrap();

		let route = cursor.assert_next_active("room/host");
		assert_eq!(route.cost, crate::origin::Cost::new(5).charged(0));
	}

	/// An announce stream that dies without an explicit `ended` retracts the route
	/// as promptly as an explicit retraction: a route into a dead session must not
	/// stay announced.
	///
	/// This falls out of `Announced` being a local whose announcements drop,
	/// which is exactly what makes it worth pinning: a refactor that hoisted the map
	/// to the session (outliving the stream) would leak the announcement instead.
	#[tokio::test(start_paused = true)]
	async fn a_lost_announce_stream_retracts_the_route() {
		let origin = crate::origin::Config::new(crate::Hop::new(1).unwrap()).produce();
		let consumer = origin.consume();
		let mut subscriber = Subscriber::new(SubscriberConfig {
			runtime: crate::time::Clock::tokio(),
			session: SinkSession::new(Default::default()),
			origin,
			recv_bandwidth: None,
			version: VERSION,
			peer_setup: Default::default(),
			cost: None,
			peer_hop: None,
			going_away: Default::default(),
		});

		let path = Path::new("room/host").to_owned();
		let hops = crate::Hops::try_from(vec![crate::Hop::new(7).unwrap()]).unwrap();
		let mut announced = Announced::default();
		subscriber
			.start_announce(
				path.clone(),
				hops,
				crate::origin::Cost::default(),
				1,
				None,
				&mut announced,
			)
			.unwrap();
		let mut cursor = consumer.announced();
		cursor.assert_next_active("room/host");

		// The stream ends without retracting anything: the map dies with it and the
		// route retracts.
		drop(announced);
		cursor.assert_next_ended("room/host");

		// An explicit retraction retracts it the same way.
		let hops = crate::Hops::try_from(vec![crate::Hop::new(7).unwrap()]).unwrap();
		let mut announced = Announced::default();
		subscriber
			.start_announce(
				path.clone(),
				hops,
				crate::origin::Cost::default(),
				1,
				None,
				&mut announced,
			)
			.unwrap();
		cursor.assert_next_active("room/host");
		assert!(announced.contains(&path.clone()), "the announce was not recorded");
		announced.retire(&path.clone());
		cursor.assert_next_ended("room/host");
	}
}

/// The four wire fields a subscription's half-open range encodes to.
///
/// The inverse of the publisher's `Bounds::positions`: the model carries whole
/// positions with an exclusive end, while the wire splits each bound into a group and a
/// frame and states both ends inclusive. The range must be non-empty, since an inclusive
/// end has nothing to say below the first position it excludes.
struct WireBounds {
	start_group: Option<u64>,
	start_frame: u64,
	end_group: Option<u64>,
	end_frame: Option<u64>,
}

impl WireBounds {
	fn new(start: Option<Position>, end: Option<Position>) -> Self {
		// An empty range has no wire encoding: flooring its end below asks for the
		// position it excludes, or inverts the range. `handle_subscription` drops such a
		// subscription instead. An absent start is the live edge, so it only makes the
		// range empty when the end sits at the very first position.
		debug_assert!(
			end.is_none_or(|end| end > start.unwrap_or_default()),
			"an empty range cannot be encoded; it should have been dropped as no demand"
		);

		let (end_group, end_frame) = match end {
			// An exclusive end at the head of a group means the group below it is the
			// last one, and it is served whole.
			Some(end) if end.frame == 0 => (Some(end.group.saturating_sub(1)), None),
			Some(end) => (Some(end.group), Some(end.frame - 1)),
			None => (None, None),
		};

		Self {
			start_group: start.map(|start| start.group),
			start_frame: start.map_or(0, |start| start.frame),
			end_group,
			end_frame,
		}
	}
}

/// The at-most-one live upstream subscription: its control stream plus the params
/// echoed in every SUBSCRIBE_UPDATE.
struct SubStream<S: crate::transport::poll::Session> {
	stream: Stream<S, Version>,
	id: u64,
	/// Original SUBSCRIBE params, echoed in every SUBSCRIBE_UPDATE; refreshed as the
	/// downstream aggregate changes.
	max_age: Duration,
	start: Option<Position>,
	priority: u8,
	/// The start the SUBSCRIBE itself carried, fixed for the stream's life. A
	/// SUBSCRIBE_START describes this demand and no fresh one follows an update,
	/// so a buffered START only applies while `start` still equals it: while the
	/// start sits elsewhere the request-tracked floor stands instead, and demand
	/// returning here makes the declaration valid again.
	requested: Option<Position>,
}

enum Sub<S: crate::transport::poll::Session> {
	None,
	Active(SubStream<S>),
}

/// Every advertisement the peer currently has live on one announce stream.
///
/// A declined advertisement remains present with no route because the peer still
/// owns its path and announce id until it retracts or restarts it.
#[derive(Default)]
struct Announced(HashMap<PathOwned, Option<AnnouncedRoute>>);

impl Announced {
	fn contains(&self, path: &PathOwned) -> bool {
		self.0.contains_key(path)
	}

	fn attach(&mut self, path: PathOwned, route: AnnouncedRoute) {
		self.0.insert(path, Some(route));
	}

	fn declined(&mut self, path: PathOwned) {
		if let Some(Some(route)) = self.0.insert(path, None) {
			route.finish();
		}
	}

	/// Record an advertisement before deciding what to do with it.
	///
	/// The peer owns the prefix from the moment it announces, whatever the receiver makes
	/// of it, so the record is taken up front and every way out of the decision leaves it
	/// standing. Accepting replaces it via [`Self::attach`]. Doing it this way rather than
	/// at each rejection is what stops the next early return from silently freeing a path
	/// the peer still holds.
	/// Only valid on a prefix the peer does not already hold, which the caller establishes
	/// with [`Self::contains`]. Overwriting an attached route here would drop its source
	/// without finishing it, which is [`Self::declined`]'s job.
	fn reserve(&mut self, path: PathOwned) {
		debug_assert!(!self.0.contains_key(&path), "reserved a prefix already advertised");
		self.0.insert(path, None);
	}

	fn attached(&mut self, path: &PathOwned) -> Option<&mut AnnouncedRoute> {
		self.0.get_mut(path)?.as_mut()
	}

	fn retire(&mut self, path: &PathOwned) {
		if let Some(Some(route)) = self.0.remove(path) {
			route.finish();
		}
	}

	/// Serve queued requests on every attached route: mint a source per requested
	/// path, hand its dynamic to the driver's serve machines, and answer the
	/// requester with its consumer.
	fn poll_serve<S: crate::transport::poll::Session>(&mut self, subscriber: &Subscriber<S>, waiter: &kio::Waiter) {
		let root = subscriber.origin.root().to_owned();
		for entry in self.0.values_mut().flatten() {
			while let Poll::Ready(Ok(request)) = entry.dynamic.poll_requested_broadcast(waiter) {
				// The request path is absolute; the wire (and our origin handle)
				// speak paths relative to the session's root.
				let Some(path) = request.path().strip_prefix(&root) else {
					// Outside our root: nothing we could name on the wire.
					continue;
				};
				let path = path.to_owned();
				let source = subscriber.origin.create_source(&path);
				let _ = subscriber.sources.try_push((path.clone(), source.dynamic()));
				request.accept(&source);
				entry
					.sources
					.insert(path, crate::model::broadcast::SourceGuard::new(source));
			}
		}
	}

	/// Re-price every attached route to a draining cost (the peer sent a GOAWAY).
	fn drain(&mut self) {
		for entry in self.0.values_mut().flatten() {
			entry.drain();
		}
	}
}

/// One received announce: the served route announced into the origin (the
/// advertisement plus its request queue), and the sources minted to serve
/// requested paths beneath it.
struct AnnouncedRoute {
	/// The route as last announced (post-charge), so a drain can re-price it
	/// without recomputing the chain.
	route: crate::origin::Route,
	/// Dropping it retracts the route and rejects its queued requests.
	dynamic: crate::origin::Dynamic,
	/// One minted source per requested path, finished on a clean retraction and
	/// aborted (via drop) when the session dies.
	sources: HashMap<PathOwned, crate::model::broadcast::SourceGuard>,
	/// Whether the GOAWAY drain already re-priced this route.
	drained: bool,
}

impl AnnouncedRoute {
	fn new(route: crate::origin::Route, dynamic: crate::origin::Dynamic) -> Self {
		Self {
			route,
			dynamic,
			sources: HashMap::new(),
			drained: false,
		}
	}

	/// The peer deliberately retracted the route: finish the minted sources so
	/// their consumers observe a clean end, and retract the announcement.
	fn finish(self) {
		for (_, source) in self.sources {
			source.finish();
		}
	}

	/// Update the announced route in place (a restart).
	fn update(&mut self, route: crate::origin::Route) {
		self.route = route.clone();
		self.drained = false;
		let _ = self.dynamic.update(route);
	}

	/// Re-price the route to [`crate::origin::Cost::DRAIN`] (the peer sent a
	/// GOAWAY): every other candidate outranks it while it stays selectable as
	/// the last path. Idempotent, since the signal stays set.
	fn drain(&mut self) {
		if self.drained {
			return;
		}
		self.drained = true;
		let mut route = self.route.clone();
		route.cost = crate::origin::Cost::DRAIN;
		let _ = self.dynamic.update(route);
	}
}

/// How a [`TrackServe`] run ends.
enum ServeEnd {
	/// The upstream FIN'd: the track is over for good.
	Finished,
	/// The route or session failed: abort the track so the origin re-splices it
	/// from another source.
	GiveBack(Error),
	/// No consumers or in-flight fetches remain; the owner must commit the idle abort.
	Idle,
}

/// Serves one requested track for a relay: owns this session's copy of the
/// track (spliced into the origin's logical track), driving the single upstream
/// subscription (opened lazily on the first downstream subscriber, canceled when
/// the last one leaves) concurrently with any number of one-shot fetches.
#[derive(Clone)]
struct TrackServe<S: crate::transport::poll::Session> {
	subscriber: Subscriber<S>,
	path: PathOwned,
	name: String,
}

impl<S: crate::transport::poll::Session> TrackServe<S> {
	fn widen_frame_bounds(&self, subscription: &mut Subscription) {
		if self.subscriber.version.has_frame_bounds() {
			return;
		}

		// Round both bounds outward to the enclosing group, so the peer sends at least
		// what was asked for and never less. Rounding the end down instead would be able
		// to empty a non-empty range, which has no wire encoding at all.
		let start = subscription.start.map(|start| Position::group(start.group));
		let end = subscription.end.and_then(|end| match end.frame {
			0 => Some(end),
			// Past the last group there is no position to round up to, and unbounded is
			// the wider request.
			_ => Position::after_group(end.group),
		});

		if (start, end) != (subscription.start, subscription.end) {
			tracing::debug!(
				track = %self.name,
				version = ?self.subscriber.version,
				"widening frame bounds to whole groups for an older peer"
			);
		}
		subscription.start = start;
		subscription.end = end;
	}

	/// Apply a subscription-demand change: hand back an [`Establish`] to open the
	/// upstream SUBSCRIBE on the first subscriber, buffer a SUBSCRIBE_UPDATE while
	/// live (the caller flushes), or cancel outright when the last one leaves.
	fn begin_subscription(
		&self,
		producer: &mut track::Producer,
		sub: &mut Sub<S>,
		pref: Option<Subscription>,
		supports_update: bool,
		timescale: Option<Timescale>,
	) -> Result<Begin<S>, Error> {
		// An empty half-open range asks for nothing, and the wire cannot say that: its
		// bounds are inclusive, so the nearest encoding either hands back the position
		// the caller excluded or inverts the range outright once the two bounds meet. No
		// demand at all is the faithful translation. `resume::slice` reaches this on its
		// own: a subscriber resuming exactly at a segment's cap owes that segment nothing.
		// An absent start is the live edge, wherever that lands, so the only end that is
		// certainly empty is the very first position, which is what `Position::default()`
		// stands in for.
		let pref = pref.filter(|sub| sub.end.is_none_or(|end| end > sub.start.unwrap_or_default()));

		match pref {
			Some(mut subscription) => {
				self.widen_frame_bounds(&mut subscription);
				match sub {
					Sub::None => {
						// Open an upstream SUBSCRIBE for the first subscriber.
						Ok(Begin::Establish(self.prepare_establish(
							producer,
							subscription,
							timescale,
						)))
					}
					Sub::Active(active) => {
						// Downstream preferences changed: forward them upstream as a
						// SUBSCRIBE_UPDATE (Lite03+ only; older peers can't carry one).
						let start_moved = active.start != subscription.start;
						active.priority = subscription.priority;
						active.max_age = subscription.max_age;
						active.start = subscription.start;
						if supports_update {
							// The floor follows the requested start, in both directions:
							// moving below a declared SUBSCRIBE_START reopens those groups
							// (the peer may serve them now), moving forward retires the
							// skipped range (the peer stops serving it, and no fresh START
							// will say so), and dropping to the live edge clears it until
							// the next declaration. A buffered START re-applies only if the
							// start returns to the demand that produced it (see
							// `SubStream::requested`).
							if start_moved {
								let _ = producer.start_at(active.start.map(|start| start.group));
							}
							buffer_update(active, subscription.end)?;
						}
						Ok(Begin::None)
					}
				}
			}
			None => {
				// Last subscriber left: cancel the upstream subscription outright. An
				// idle subscription still streams every group into a cache nobody
				// reads, and the upstream counts it as a live viewer of the broadcast.
				// A returning subscriber re-establishes from the current demand.
				if let Sub::Active(active) = sub {
					self.subscriber.remove_subscribe(active.id);
					let _ = active.stream.writer.finish();
					tracing::info!(track = %self.name, "subscribe canceled (idle)");
					*sub = Sub::None;
				}
				Ok(Begin::None)
			}
		}
	}

	/// Allocate the id, set the demand floor, and register the subscription, so the
	/// returned [`Establish`] can put the SUBSCRIBE on the wire.
	///
	/// Registration happens here, before any of it reaches the transport: `id` is
	/// live the moment the peer reads it, and a publisher may serve its first group
	/// immediately, so a late insert races the group stream (a group whose id isn't
	/// in the map yet is dropped, stalling the track forever). The caller
	/// deregisters `id` if the establish fails.
	///
	/// The subscription's bounds come straight from the demand aggregate. After a
	/// route change a takeover boundary caps the *previous* segment's `end_group`;
	/// the segment resuming the track keeps whatever start the subscribers asked for,
	/// so a live-edge subscriber gets the live edge upstream rather than a replay of
	/// the outage.
	fn prepare_establish(
		&self,
		producer: &mut track::Producer,
		subscription: Subscription,
		timescale: Option<Timescale>,
	) -> Establish<S> {
		let id = self.subscriber.next_id.fetch_add(1, atomic::Ordering::Relaxed);

		// Both halves of each bound come from the same position, so a frame can never
		// reach the wire without the group it counts from (which the peer would reject).
		// The floor tracks the requested start until this subscription's own
		// SUBSCRIBE_START refines it: the peer never serves below the request, a
		// previous subscription's declaration must not outlive its demand, and
		// live-edge demand (None) starts with no floor at all.
		let _ = producer.start_at(subscription.start.map(|start| start.group));

		tracing::info!(id, broadcast = %self.subscriber.log_path(&self.path), track = %self.name, "subscribe started");

		self.subscriber.subscribes.lock().insert(
			id,
			TrackEntry {
				producer: producer.clone(),
				timescale,
			},
		);

		let session = self.subscriber.session.clone();
		Establish {
			serve: self.clone(),
			closed: session.clone(),
			session,
			id,
			subscription,
			state: EstablishState::Open,
		}
	}

	/// Test shim: drive the upstream SUBSCRIBE open like the old `establish`.
	#[cfg(test)]
	async fn establish(
		&self,
		producer: &mut track::Producer,
		sub: &mut Sub<S>,
		subscription: Subscription,
		timescale: Option<Timescale>,
	) -> Result<(), Error> {
		let mut est = Box::new(self.prepare_establish(producer, subscription, timescale));
		let id = est.id;
		match kio::wait(move |waiter| est.poll(waiter)).await {
			Ok(active) => {
				*sub = Sub::Active(active);
				Ok(())
			}
			Err(err) => {
				self.subscriber.remove_subscribe(id);
				Err(err)
			}
		}
	}

	/// Test shim: apply one demand change like the old `handle_subscription`,
	/// driving the establish (or the update flush) to completion inline.
	#[cfg(test)]
	async fn handle_subscription(
		&self,
		producer: &mut track::Producer,
		sub: &mut Sub<S>,
		pref: Option<Subscription>,
		supports_update: bool,
		timescale: Option<Timescale>,
	) -> Result<(), Error> {
		match self.begin_subscription(producer, sub, pref, supports_update, timescale)? {
			Begin::Establish(est) => {
				let mut est = Box::new(est);
				let id = est.id;
				match kio::wait(move |waiter| est.poll(waiter)).await {
					Ok(active) => *sub = Sub::Active(active),
					Err(err) => {
						self.subscriber.remove_subscribe(id);
						return Err(err);
					}
				}
			}
			Begin::None => {
				if let Sub::Active(active) = sub {
					std::future::poll_fn(|cx| active.stream.writer.poll_flush(cx)).await?;
				}
			}
		}
		Ok(())
	}
}

/// What a demand change asks the serve loop to do next.
// A state machine's enum is its storage: one transient instance per stream, so the
// big variant is the working state, not padding held in bulk.
#[allow(clippy::large_enum_variant)]
enum Begin<S: crate::transport::poll::Session> {
	/// Nothing further: the update (if any) sits in the active stream's write
	/// buffer, flushed by the loop.
	None,
	/// Open an upstream SUBSCRIBE for the first subscriber.
	Establish(Establish<S>),
}

/// Buffer a SUBSCRIBE_UPDATE echoing the current params, varying only the end
/// bound. The caller flushes.
fn buffer_update<S: crate::transport::poll::Session>(
	active: &mut SubStream<S>,
	end: Option<Position>,
) -> Result<(), Error> {
	let bounds = WireBounds::new(active.start, end);
	active.stream.writer.buffer(&lite::SubscribeUpdate {
		priority: active.priority,
		max_age: active.max_age,
		start_group: bounds.start_group,
		end_group: bounds.end_group,
		start_frame: bounds.start_frame,
		end_frame: bounds.end_frame,
	})
}

/// Opens the upstream SUBSCRIBE control stream: send the request, then (pre
/// lite-05) wait for the SUBSCRIBE_OK. Resolves with the live [`SubStream`];
/// the caller deregisters the id on failure.
struct Establish<S: crate::transport::poll::Session> {
	serve: TrackServe<S>,
	session: S,
	// A dedicated close-watch handle for the SUBSCRIBE_OK wait.
	closed: S,
	id: u64,
	subscription: Subscription,
	state: EstablishState<S>,
}

enum EstablishState<S: crate::transport::poll::Session> {
	Open,
	Send {
		stream: Stream<S, Version>,
	},
	/// Older drafts: the first SUBSCRIBE_OK confirms it. Bail if the session
	/// dies meanwhile; a dying route hands the assignment back through the
	/// serve loop's teardown instead.
	WaitOk {
		stream: Stream<S, Version>,
	},
}

impl<S: crate::transport::poll::Session> Establish<S> {
	fn poll(&mut self, waiter: &kio::Waiter) -> Poll<Result<SubStream<S>, Error>> {
		let mut cx = std::task::Context::from_waker(waiter.waker());
		loop {
			match &mut self.state {
				EstablishState::Open => {
					// A peer that sent GOAWAY told us to stop opening streams.
					self.serve.subscriber.check_going_away()?;
					let mut stream = ready!(Stream::poll_open(
						&mut self.session,
						self.serve.subscriber.version,
						&mut cx
					))?;

					let bounds = WireBounds::new(self.subscription.start, self.subscription.end);
					let msg = lite::Subscribe {
						id: self.id,
						broadcast: self.serve.path.as_path(),
						track: self.serve.name.as_str().into(),
						priority: self.subscription.priority,
						max_age: self.subscription.max_age,
						start_group: bounds.start_group,
						end_group: bounds.end_group,
						start_frame: bounds.start_frame,
						end_frame: bounds.end_frame,
					};
					stream.writer.buffer(&lite::ControlType::Subscribe)?;
					stream.writer.buffer(&msg)?;
					self.state = EstablishState::Send { stream };
				}
				EstablishState::Send { stream } => {
					ready!(stream.writer.poll_flush(&mut cx))?;
					let EstablishState::Send { stream } = std::mem::replace(&mut self.state, EstablishState::Open)
					else {
						unreachable!()
					};
					if !self.serve.subscriber.version.has_track_stream() {
						self.state = EstablishState::WaitOk { stream };
						continue;
					}
					return Poll::Ready(Ok(self.activate(stream)));
				}
				EstablishState::WaitOk { stream } => {
					if self.closed.poll_closed(&mut cx).is_ready() {
						return Poll::Ready(Err(Error::Dropped));
					}
					let resp = ready!(stream.reader.poll_decode::<lite::SubscribeResponse>(&mut cx))?;
					if !matches!(resp, lite::SubscribeResponse::Ok(_)) {
						return Poll::Ready(Err(Error::ProtocolViolation));
					}
					let EstablishState::WaitOk { stream } = std::mem::replace(&mut self.state, EstablishState::Open)
					else {
						unreachable!()
					};
					return Poll::Ready(Ok(self.activate(stream)));
				}
			}
		}
	}

	fn activate(&self, stream: Stream<S, Version>) -> SubStream<S> {
		SubStream {
			stream,
			id: self.id,
			max_age: self.subscription.max_age,
			start: self.subscription.start,
			priority: self.subscription.priority,
			requested: self.subscription.start,
		}
	}
}

/// Drives one [`TrackServe`]: the TRACK_INFO fetch, then the serve loop, then
/// the teardown that decides how the origin sees this copy end.
struct TrackServeRun<S: crate::transport::poll::Session> {
	serve: TrackServe<S>,
	state: TrackRunState<S>,
}

// A state machine's enum is its storage: one transient instance per stream, so the
// big variant is the working state, not padding held in bulk.
#[allow(clippy::large_enum_variant)]
enum TrackRunState<S: crate::transport::poll::Session> {
	/// Lite05+ learns the track's immutable properties once, up front, via a
	/// TRACK stream. The timescale then flows into every SUBSCRIBE and FETCH
	/// without a per-response header.
	Info {
		request: Option<track::Request>,
		info: TrackInfoFetch<S>,
	},
	Serve(ServeLoop<S>),
	Done,
}

impl<S: crate::transport::poll::Session> TrackServeRun<S> {
	fn new(serve: TrackServe<S>, request: track::Request) -> Self {
		let state = if serve.subscriber.version.has_track_stream() {
			TrackRunState::Info {
				request: Some(request),
				info: TrackInfoFetch::new(&serve),
			}
		} else {
			// No TRACK stream, so the publisher's retention window never reaches us:
			// the accepting side picks it (see `origin::Config::default_max_age`).
			let info = track::Info::default().with_max_age(serve.subscriber.origin.default_max_age());
			TrackRunState::Serve(ServeLoop::new(&serve, request, info, None))
		};
		Self { serve, state }
	}
}

impl<S: crate::transport::poll::Session> kio::Task for TrackServeRun<S> {
	fn poll(&mut self, waiter: &kio::Waiter) -> Poll<()> {
		loop {
			match &mut self.state {
				TrackRunState::Info { request, info } => {
					let res = ready!(info.poll_fetch(&self.serve, waiter));
					let request = request.take().expect("request pending");
					match res {
						Ok(info) => {
							// Lite05 carries per-frame timestamps on the wire at this scale;
							// `Some` tells the ingest to decode them instead of stamping
							// local receive time.
							let timescale = Some(info.timescale);
							self.state = TrackRunState::Serve(ServeLoop::new(&self.serve, request, info, timescale));
						}
						Err(err) => {
							tracing::warn!(broadcast = %self.serve.subscriber.log_path(&self.serve.path), track = %self.serve.name, %err, "track info failed");
							// Rejecting the request lets the origin retry (bounded) on
							// another source; waiting subscribers stall rather than error
							// meanwhile.
							request.reject(err);
							self.state = TrackRunState::Done;
							return Poll::Ready(());
						}
					}
				}
				TrackRunState::Serve(serve_loop) => {
					let teardown = ready!(serve_loop.poll(&self.serve, waiter));
					let TrackRunState::Serve(mut serve_loop) = std::mem::replace(&mut self.state, TrackRunState::Done)
					else {
						unreachable!()
					};

					match teardown {
						ServeEnd::Idle => match serve_loop.serving.abort_unused(Error::Cancel) {
							Ok(()) => {
								tracing::debug!(broadcast = %self.serve.subscriber.log_path(&self.serve.path), track = %self.serve.name, "track released (idle)");
							}
							Err(used) => {
								serve_loop.serving = used;
								self.state = TrackRunState::Serve(serve_loop);
								continue;
							}
						},
						ServeEnd::Finished => {
							let _ = serve_loop.serving.finish();
						}
						ServeEnd::GiveBack(err) => {
							let _ = serve_loop.serving.abort(err);
						}
					}

					if let Sub::Active(active) = &mut serve_loop.sub {
						self.serve.subscriber.remove_subscribe(active.id);
						let _ = active.stream.writer.finish();
					}

					return Poll::Ready(());
				}
				TrackRunState::Done => return Poll::Ready(()),
			}
		}
	}
}

/// Opens a TRACK stream, reads the single TRACK_INFO, and maps it to the
/// model's [`track::Info`]. Lite05+ only. Bails if the session dies meanwhile.
struct TrackInfoFetch<S: crate::transport::poll::Session> {
	session: S,
	// A dedicated close-watch handle for the read.
	closed: S,
	state: TrackInfoState<S>,
}

enum TrackInfoState<S: crate::transport::poll::Session> {
	Open,
	Send { stream: Stream<S, Version> },
	Read { stream: Stream<S, Version> },
}

impl<S: crate::transport::poll::Session> TrackInfoFetch<S> {
	fn new(serve: &TrackServe<S>) -> Self {
		let session = serve.subscriber.session.clone();
		Self {
			closed: session.clone(),
			session,
			state: TrackInfoState::Open,
		}
	}

	fn poll_fetch(&mut self, serve: &TrackServe<S>, waiter: &kio::Waiter) -> Poll<Result<track::Info, Error>> {
		let mut cx = std::task::Context::from_waker(waiter.waker());
		loop {
			match &mut self.state {
				TrackInfoState::Open => {
					serve.subscriber.check_going_away()?;
					let mut stream = ready!(Stream::poll_open(&mut self.session, serve.subscriber.version, &mut cx))?;
					stream.writer.buffer(&lite::ControlType::Track)?;
					stream.writer.buffer(&lite::Track {
						broadcast: serve.path.as_path(),
						track: serve.name.as_str().into(),
					})?;
					self.state = TrackInfoState::Send { stream };
				}
				TrackInfoState::Send { stream } => {
					ready!(stream.writer.poll_flush(&mut cx))?;
					let TrackInfoState::Send { stream } = std::mem::replace(&mut self.state, TrackInfoState::Open)
					else {
						unreachable!()
					};
					self.state = TrackInfoState::Read { stream };
				}
				TrackInfoState::Read { stream } => {
					if self.closed.poll_closed(&mut cx).is_ready() {
						return Poll::Ready(Err(Error::Dropped));
					}
					let info = ready!(stream.reader.poll_decode::<lite::TrackInfo>(&mut cx))?;
					// The publisher FINs after TRACK_INFO; FIN our side too and let the
					// stream drop.
					let _ = stream.writer.finish();

					// Publisher Max Age rides on the wire, so the local retention
					// window matches what the upstream advertises (relays re-serve with
					// the same bound). `broadcast` is left at its default here;
					// `track::Request::accept` stamps the track's real broadcast.
					let model = track::Info::default()
						.with_timescale(info.timescale)
						.with_max_age(info.max_age)
						.with_priority(info.priority);
					return Poll::Ready(Ok(model));
				}
			}
		}
	}
}

/// The serve loop proper: owns this session's copy of the track (spliced into
/// the origin's logical track), driving the single upstream subscription
/// (opened lazily on the first downstream subscriber, canceled when the last
/// one leaves) concurrently with any number of one-shot fetches.
struct ServeLoop<S: crate::transport::poll::Session> {
	/// This session's copy, accepted with the resolved info. The origin splices
	/// it into the logical track; demand from the logical subscribers arrives
	/// through the producer's aggregate, sliced to this segment's bounds
	/// (including the resume floor after a source change).
	serving: track::Producer,
	/// Serve on-demand fetches of uncached groups from this session.
	dynamic: track::Dynamic,
	sub: Sub<S>,
	fetches: kio::Tasks<FetchServeRun<S>>,
	// A dedicated close-watch handle for the session-died arm.
	closed: S,
	// SUBSCRIBE_UPDATE only exists on Lite03+, so older peers can't carry a
	// preference change to an established subscription.
	supports_update: bool,
	supports_fetch: bool,
	timescale: Option<Timescale>,
	mode: ServeMode<S>,
}

// A state machine's enum is its storage: one transient instance per stream, so the
// big variant is the working state, not padding held in bulk.
#[allow(clippy::large_enum_variant)]
enum ServeMode<S: crate::transport::poll::Session> {
	/// Selecting the next event.
	Select,
	/// Driving an upstream SUBSCRIBE open. The demand arms wait meanwhile,
	/// exactly like the old inline await.
	Establish(Establish<S>),
}

impl<S: crate::transport::poll::Session> ServeLoop<S> {
	fn new(serve: &TrackServe<S>, request: track::Request, info: track::Info, timescale: Option<Timescale>) -> Self {
		// Register the fetch handler before accepting: `accept` releases the
		// request's own fetch gate, and a cache-miss fetch queued while TRACK_INFO
		// was in flight would be drained as NotFound in the gap.
		let dynamic = request.dynamic();
		let serving = request.accept(info);
		Self {
			serving,
			dynamic,
			sub: Sub::None,
			fetches: kio::Tasks::new(),
			closed: serve.subscriber.session.clone(),
			supports_update: !matches!(serve.subscriber.version, Version::Lite01 | Version::Lite02),
			supports_fetch: serve.subscriber.version.has_track_stream(),
			timescale,
			mode: ServeMode::Select,
		}
	}

	fn poll(&mut self, serve: &TrackServe<S>, waiter: &kio::Waiter) -> Poll<ServeEnd> {
		loop {
			match &mut self.mode {
				ServeMode::Establish(est) => {
					let res = ready!(est.poll(waiter));
					let id = est.id;
					self.mode = ServeMode::Select;
					match res {
						Ok(active) => self.sub = Sub::Active(active),
						Err(err) => {
							// Opening the upstream failed (usually the session dying): hand
							// the track back for another route to resume.
							serve.subscriber.remove_subscribe(id);
							return Poll::Ready(ServeEnd::GiveBack(err));
						}
					}
				}
				ServeMode::Select => {
					let mut cx = std::task::Context::from_waker(waiter.waker());

					// Deliver any buffered SUBSCRIBE_UPDATE before selecting, so the
					// demand that produced it is on the wire.
					if let Sub::Active(active) = &mut self.sub {
						match active.stream.writer.poll_flush(&mut cx) {
							Poll::Ready(Ok(())) => {}
							Poll::Ready(Err(err)) => {
								// The stream is broken; drop it (the writer resets) and
								// hand the track back.
								serve.subscriber.remove_subscribe(active.id);
								self.sub = Sub::None;
								return Poll::Ready(ServeEnd::GiveBack(err));
							}
							Poll::Pending => return Poll::Pending,
						}
					}

					// Biased: demand first, then completions, then closures.

					// (1) Track demand: a fetch, a subscription change, or the origin
					// handing the track to another route.

					// A fetch is cheap and one-shot, so serve it ahead of subscription churn.
					match self.dynamic.poll_requested_group(waiter) {
						Poll::Ready(Ok(req)) => {
							if self.supports_fetch {
								self.fetches
									.push(FetchServeRun::new(serve.clone(), req, self.timescale));
							} else {
								req.reject(Error::Version);
							}
							continue;
						}
						// Our own producer is alive (we hold it); treat as terminal anyway.
						Poll::Ready(Err(_)) => return Poll::Ready(ServeEnd::GiveBack(Error::Dropped)),
						Poll::Pending => {}
					}
					match self.serving.poll_subscription_changed(waiter) {
						Poll::Ready(Ok(pref)) => {
							match serve.begin_subscription(
								&mut self.serving,
								&mut self.sub,
								pref,
								self.supports_update,
								self.timescale,
							) {
								Ok(Begin::Establish(est)) => self.mode = ServeMode::Establish(est),
								Ok(Begin::None) => {}
								// Updating the upstream failed: hand the track back for
								// another route to resume.
								Err(err) => return Poll::Ready(ServeEnd::GiveBack(err)),
							}
							continue;
						}
						Poll::Ready(Err(_)) => return Poll::Ready(ServeEnd::GiveBack(Error::Dropped)),
						Poll::Pending => {}
					}

					// (2) In-flight fetches; completions just retire.
					let _ = self.fetches.poll(waiter);

					// (3) Nobody reads this copy anymore: the origin dropped its source
					// copy when demand ended, so drop it instead of holding the track
					// state (and its TRACK_INFO) for a reader that may never return.
					// In-flight fetches keep it alive: work already accepted still
					// gets finished.
					if self.fetches.is_empty() && self.serving.poll_unused(waiter).is_ready() {
						return Poll::Ready(ServeEnd::Idle);
					}

					// (4) The upstream subscribe stream closed, or carried a START/END/DROP.
					// Partial message bytes persist in the reader's buffer across turns.
					if let Sub::Active(active) = &mut self.sub
						&& let Poll::Ready(res) = active
							.stream
							.reader
							.poll_decode_maybe::<lite::SubscribeResponse>(&mut cx)
					{
						match res {
							Ok(Some(msg)) => {
								match &msg {
									// SUBSCRIBE_END declares the track's exclusive final
									// sequence, which may arrive while trailing groups are
									// still in flight. Record it on this segment's producer so
									// consumers learn the boundary early; the later stream FIN
									// then finds the track already finished.
									lite::SubscribeResponse::End(end) => {
										// finish_at rejects a boundary at or below the live
										// edge, which is what a peer sending an inclusive bound
										// looks like once the final group has already arrived.
										// Don't abort: the stream FIN still finishes the track,
										// so this only costs the early boundary. Warn anyway,
										// since it's our only signal that a peer disagrees
										// about the encoding.
										if let Err(err) = self.serving.finish_at(end.group) {
											tracing::warn!(track = %serve.name, group = end.group, %err, "invalid subscribe end");
										}
									}
									// SUBSCRIBE_START names the first group this feed serves:
									// the publisher skipped everything below it (e.g. it could
									// not serve the requested frame). Record it as a drop
									// signal, so a spliced reader waiting on a skipped group
									// fails over instead of stalling on a live route.
									lite::SubscribeResponse::Start(start) => {
										// A START describes the demand the SUBSCRIBE carried.
										// It applies only while the current start still matches
										// that demand (updates get no fresh START, so an update
										// that moved the start makes it stale, and one that
										// moved back restores it); elsewhere the
										// request-tracked floor stands rather than a guess.
										if active.start == active.requested {
											let _ = self.serving.start_at(start.group);
										}
									}
									// OK/DROP just resolve the range (the producer already
									// orders groups).
									_ => tracing::debug!(track = %serve.name, ?msg, "subscribe response"),
								}
								continue;
							}
							Ok(None) => {
								tracing::info!(broadcast = %serve.subscriber.log_path(&serve.path), track = %serve.name, "subscribe complete");
								// Upstream FIN'd the subscription: the publisher only FINs
								// once the track's final sequence is known and delivered, so
								// the logical track is over for good (bounded downstream
								// demand alone never FINs; the publisher parks, since a cap
								// can be raised).
								return Poll::Ready(ServeEnd::Finished);
							}
							Err(err) => {
								tracing::warn!(broadcast = %serve.subscriber.log_path(&serve.path), track = %serve.name, %err, "subscribe error");
								return Poll::Ready(ServeEnd::GiveBack(err));
							}
						}
					}

					// (5) The session died: hand the track back for another route.
					if self.closed.poll_closed(&mut cx).is_ready() {
						return Poll::Ready(ServeEnd::GiveBack(Error::Dropped));
					}

					return Poll::Pending;
				}
			}
		}
	}
}

/// Serves one downstream fetch end-to-end on its own bidi stream: send FETCH,
/// then fill the group from the bare FRAME messages that follow. The timescale
/// comes from this track's TRACK_INFO (already known), and the group sequence
/// is implicit from the request.
struct FetchServeRun<S: crate::transport::poll::Session> {
	serve: TrackServe<S>,
	session: S,
	timescale: Option<Timescale>,
	group: u64,
	state: FetchRunState<S>,
}

enum FetchRunState<S: crate::transport::poll::Session> {
	Open {
		request: Option<group::Request>,
	},
	Send {
		request: Option<group::Request>,
		stream: Stream<S, Version>,
		frame_start: u64,
	},
	Ingest {
		stream: Stream<S, Version>,
		producer: group::Producer,
		ingest: FrameIngest,
	},
	Done,
}

impl<S: crate::transport::poll::Session> FetchServeRun<S> {
	fn new(serve: TrackServe<S>, request: group::Request, timescale: Option<Timescale>) -> Self {
		let session = serve.subscriber.session.clone();
		let group = request.sequence();
		Self {
			serve,
			session,
			timescale,
			group,
			state: FetchRunState::Open { request: Some(request) },
		}
	}
}

impl<S: crate::transport::poll::Session> kio::Task for FetchServeRun<S> {
	fn poll(&mut self, waiter: &kio::Waiter) -> Poll<()> {
		let mut cx = std::task::Context::from_waker(waiter.waker());
		loop {
			match &mut self.state {
				FetchRunState::Open { request } => {
					tracing::info!(broadcast = %self.serve.subscriber.log_path(&self.serve.path), track = %self.serve.name, group = self.group, "fetch started");

					// A peer that sent GOAWAY told us to stop opening streams on this session.
					if self.serve.subscriber.going_away.is_set() {
						request.take().expect("request pending").reject(Error::GoingAway);
						self.state = FetchRunState::Done;
						return Poll::Ready(());
					}

					let mut stream = match ready!(Stream::poll_open(
						&mut self.session,
						self.serve.subscriber.version,
						&mut cx
					)) {
						Ok(stream) => stream,
						Err(err) => {
							tracing::warn!(track = %self.serve.name, %err, "fetch stream open failed");
							request.take().expect("request pending").reject(err);
							self.state = FetchRunState::Done;
							return Poll::Ready(());
						}
					};

					let request = request.take().expect("request pending");

					// A peer that predates lite-06 addresses whole groups only, so ask for
					// the whole group and number the response from 0. The wider group still
					// covers what the caller asked for (`fetch_group` positions their own
					// consumer), and is more reusable in the cache than the tail would have
					// been. Asking for the offset anyway would fail to encode and reject a
					// fetch we can serve.
					let frame_start = match self.serve.subscriber.version.has_frame_bounds() {
						true => request.frame_start(),
						false => 0,
					};

					let msg = lite::Fetch {
						broadcast: self.serve.path.as_path(),
						track: self.serve.name.as_str().into(),
						priority: request.priority(),
						group: self.group,
						start_frame: frame_start,
						// Always through the end of the group: a fetch that stopped short
						// would cache a group indistinguishable from a complete one. A
						// downstream cap is applied when serving, not when fetching.
						end_frame: None,
					};
					let buffered = stream
						.writer
						.buffer(&lite::ControlType::Fetch)
						.and_then(|()| stream.writer.buffer(&msg));
					if let Err(err) = buffered {
						stream.writer.abort(&err);
						request.reject(err);
						self.state = FetchRunState::Done;
						return Poll::Ready(());
					}
					self.state = FetchRunState::Send {
						request: Some(request),
						stream,
						frame_start,
					};
				}
				FetchRunState::Send { stream, .. } => {
					if let Err(err) = ready!(stream.writer.poll_flush(&mut cx)) {
						let FetchRunState::Send { request, stream, .. } =
							std::mem::replace(&mut self.state, FetchRunState::Done)
						else {
							unreachable!()
						};
						stream.writer.abort(&err);
						request.expect("request pending").reject(err);
						return Poll::Ready(());
					}
					let FetchRunState::Send {
						request,
						stream,
						frame_start,
					} = std::mem::replace(&mut self.state, FetchRunState::Done)
					else {
						unreachable!()
					};
					let request = request.expect("request pending");

					// Make the group available (resolving the downstream fetch) and fill
					// it. The track::Info only takes effect if the track isn't accepted yet
					// (a fetch with no live subscription); otherwise the group inherits the
					// accepted timescale. Relay-served FETCH is lite-05+, so `timescale` is
					// `Some`; fall back to the default scale defensively rather than
					// panicking.
					let group_info = track::Info::default()
						.with_timescale(self.timescale.unwrap_or_default())
						.with_max_age(self.serve.subscriber.origin.default_max_age());
					let mut producer = match request.accept(group_info) {
						Ok(producer) => producer,
						Err(err) => {
							// Already served (a concurrent fetch) or the track closed.
							tracing::debug!(track = %self.serve.name, group = self.group, %err, "fetch not served");
							stream.writer.abort(&err);
							return Poll::Ready(());
						}
					};

					// The response starts at the frame we asked for, so number it from
					// there rather than restarting the group at 0.
					if let Err(err) = producer.start_at(frame_start) {
						stream.writer.abort(&err);
						let _ = producer.abort(err);
						return Poll::Ready(());
					}

					self.state = FetchRunState::Ingest {
						stream,
						producer,
						ingest: FrameIngest::new(self.serve.subscriber.runtime.clone(), self.timescale),
					};
				}
				FetchRunState::Ingest {
					stream,
					producer,
					ingest,
				} => {
					let res = ready!(ingest.poll(&mut stream.reader, producer, waiter));
					let FetchRunState::Ingest { producer, .. } =
						std::mem::replace(&mut self.state, FetchRunState::Done)
					else {
						unreachable!()
					};
					match res {
						Ok(()) => {
							let producer = producer;
							let _ = producer.finish();
						}
						Err(err) => {
							let _ = producer.abort(err);
						}
					}
					return Poll::Ready(());
				}
				FetchRunState::Done => return Poll::Ready(()),
			}
		}
	}
}