rtc_interceptor/

noop.rs

//! Where the inbound RTCP path ends.

use crate::Interceptor;
use crate::{Attribute, Packet, StreamInfo, TaggedPacket};
use sansio::Protocol;
use shared::error::Error;
use std::collections::VecDeque;
use std::time::Instant;

/// Decides what becomes of inbound RTCP once the interceptors have had it, and passes everything
/// else through.
///
/// # Why this exists
///
/// Inbound RTCP is **for the interceptors**: a receiver report feeds the sender-side statistics, a
/// NACK is answered by the responder, transport-wide feedback drives the bandwidth estimate.
/// Handing it onward by default would give an application a stream of control traffic it did not
/// ask for and cannot act on, mixed in with its media — so by default it stops here.
///
/// An application that *does* read RTCP puts an interceptor of its own in the chain and has it
/// attach [`Attribute::DeliverToApplication`] to the packets it wants. That makes delivery a
/// per-packet judgement, made by the one component that knows which packets matter: an SFU
/// forwarding PLIs wants those and not the receiver reports its own chain is already acting on. A
/// chain-wide switch could only offer "all of it or none of it", and every application that wanted
/// one kind of packet had to take the lot and filter afterwards.
///
/// # Where this belongs in the chain
///
/// **Last**, so every interceptor that reads RTCP has already seen it by the time it is dropped.
/// Anywhere else it would starve the ones beyond it — a NACK responder placed after this would
/// never see a NACK. [`Registry::build`](crate::Registry::build) appends it for that reason,
/// rather than leaving the position to each caller.
///
/// # Why marking is the mechanism, and not re-emitting a copy
///
/// Under the nested chain an application could add an interceptor that kept a copy of each RTCP
/// packet and returned it from its own `poll_read` ahead of delegating inward. That worked because
/// a local `poll_read` queue was *terminal*: the copy bypassed everything below and escaped this
/// one.
///
/// On the belt it does not. What an interceptor emits from `poll_read` rejoins the belt **behind
/// itself**, so it arrives here like any other packet and is dropped like any other packet — the
/// original *and* the copy, twice over. Since this is always the last interceptor, no position
/// exists from which to forward past it. Marking the packet works precisely because it does not try
/// to get around this one: the packet travels the whole chain as normal, and this reads the mark
/// when it arrives.
///
/// # Naming
///
/// It was a no-op in the nested chain, where its job was to terminate the recursion and hand
/// packets back. The belt needs no terminator, so all that is left is the one decision it always
/// quietly made.
#[derive(Default)]
pub struct NoopInterceptor {
    read_queue: VecDeque<TaggedPacket>,
    write_queue: VecDeque<TaggedPacket>,
}

impl NoopInterceptor {
    /// A terminus
    pub fn new() -> Self {
        Self::default()
    }
}

impl Protocol<TaggedPacket, TaggedPacket, ()> for NoopInterceptor {
    type Rout = TaggedPacket;
    type Wout = TaggedPacket;
    type Eout = ();
    type Error = Error;
    type Time = Instant;

    fn handle_read(&mut self, mut msg: TaggedPacket) -> Result<(), Self::Error> {
        // RTP is media the application asked for; it always passes.
        if matches!(msg.message.packet, Packet::Rtp(_)) {
            self.read_queue.push_back(msg);
            return Ok(());
        }

        // Inbound RTCP is for the interceptors. Two ways past this point:
        //
        // 1. an interceptor judged this particular packet worth forwarding, by attaching
        //    `Attribute::DeliverToApplication` — the per-packet judgement, made by whichever
        //    interceptor is qualified to make it;
        // 2. it carries attributes an interceptor attached for something beyond the chain. The
        //    *payload* still stops here — nothing asked for this packet, and handing it over
        //    because an interceptor annotated it would be a surprise — but the packet carries on
        //    as an empty-RTCP carrier so the attributes reach the crate boundary.
        //
        // Anything else ends here.
        if msg.message.has(&Attribute::DeliverToApplication) {
            self.read_queue.push_back(msg);
        } else if !msg.message.attributes.is_empty() {
            msg.message.packet = Packet::Rtcp(Vec::new());
            self.read_queue.push_back(msg);
        }
        Ok(())
    }

    fn poll_read(&mut self) -> Option<Self::Rout> {
        self.read_queue.pop_front()
    }

    fn handle_write(&mut self, msg: TaggedPacket) -> Result<(), Self::Error> {
        self.write_queue.push_back(msg);
        Ok(())
    }

    fn poll_write(&mut self) -> Option<Self::Wout> {
        self.write_queue.pop_front()
    }
}

impl Interceptor for NoopInterceptor {
    fn bind_local_stream(&mut self, _info: &StreamInfo) {}
    fn unbind_local_stream(&mut self, _info: &StreamInfo) {}
    fn bind_remote_stream(&mut self, _info: &StreamInfo) {}
    fn unbind_remote_stream(&mut self, _info: &StreamInfo) {}
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::{AttributedPacket, Registry, Slot, StreamInfo};
    use sansio::Protocol;
    use shared::TransportContext;
    use shared::error::Error;
    use std::collections::VecDeque;
    use std::time::Instant;

    fn packet(message: Packet) -> TaggedPacket {
        TaggedPacket {
            now: Instant::now(),
            transport: TransportContext::default(),
            message: AttributedPacket::new(message),
        }
    }

    #[test]
    fn inbound_rtcp_does_not_reach_the_application() {
        let mut chain = Registry::new().build();

        chain.handle_read(packet(Packet::Rtcp(vec![]))).unwrap();
        assert!(chain.poll_read().is_none());
    }

    #[test]
    fn inbound_rtp_passes_through() {
        let mut chain = Registry::new().build();

        chain
            .handle_read(packet(Packet::Rtp(rtp::Packet::default())))
            .unwrap();
        assert!(chain.poll_read().is_some());
    }

    /// Outbound RTCP is untouched: this ends the *inbound* path only.
    #[test]
    fn outbound_rtcp_is_not_affected() {
        let mut chain = Registry::new().build();

        chain.handle_write(packet(Packet::Rtcp(vec![]))).unwrap();
        assert!(chain.poll_write().is_some());
    }

    /// An interceptor wire-ward of the terminus still sees inbound RTCP — that is the whole point of it
    /// being application-most.
    #[test]
    fn stages_before_it_still_see_inbound_rtcp() {
        #[derive(Default)]
        struct Counter {
            seen: std::sync::Arc<std::sync::atomic::AtomicUsize>,
            read_queue: VecDeque<TaggedPacket>,
            write_queue: VecDeque<TaggedPacket>,
        }
        impl Protocol<TaggedPacket, TaggedPacket, ()> for Counter {
            type Rout = TaggedPacket;
            type Wout = TaggedPacket;
            type Eout = ();
            type Error = Error;
            type Time = Instant;

            fn handle_read(&mut self, msg: TaggedPacket) -> Result<(), Self::Error> {
                if matches!(msg.message.packet, Packet::Rtcp(_)) {
                    self.seen.fetch_add(1, std::sync::atomic::Ordering::Relaxed);
                }
                self.read_queue.push_back(msg);
                Ok(())
            }

            fn poll_read(&mut self) -> Option<Self::Rout> {
                self.read_queue.pop_front()
            }

            fn handle_write(&mut self, msg: TaggedPacket) -> Result<(), Self::Error> {
                self.write_queue.push_back(msg);
                Ok(())
            }

            fn poll_write(&mut self) -> Option<Self::Wout> {
                self.write_queue.pop_front()
            }

            fn handle_timeout(&mut self, _now: Instant) -> Result<(), Self::Error> {
                Ok(())
            }

            fn poll_timeout(&mut self) -> Option<Self::Time> {
                None
            }
        }
        impl Interceptor for Counter {
            fn bind_local_stream(&mut self, _info: &StreamInfo) {}
            fn unbind_local_stream(&mut self, _info: &StreamInfo) {}
            fn bind_remote_stream(&mut self, _info: &StreamInfo) {}
            fn unbind_remote_stream(&mut self, _info: &StreamInfo) {}
        }
        let counter = Counter::default();
        let seen = counter.seen.clone();
        let mut chain = Registry::new().with(Slot::NackGenerator, counter).build();

        chain.handle_read(packet(Packet::Rtcp(vec![]))).unwrap();

        assert_eq!(1, seen.load(std::sync::atomic::Ordering::Relaxed));
        assert!(chain.poll_read().is_none(), "but it stops at the terminus");
    }
}

#[cfg(test)]
mod carrier_tests {
    use super::*;
    use crate::{Attribute, AttributedPacket, Registry};
    use sansio::Protocol;
    use shared::TransportContext;
    use std::time::Instant;

    fn annotated(attribute: Option<Attribute>) -> TaggedPacket {
        let mut message = AttributedPacket::new(Packet::Rtcp(vec![Box::new(
            rtcp::receiver_report::ReceiverReport::default(),
        )]));
        if let Some(attribute) = attribute {
            message.add(attribute);
        }
        TaggedPacket {
            now: Instant::now(),
            transport: TransportContext::default(),
            message,
        }
    }

    /// An annotated report is stripped, not dropped: the payload stops here — the application did
    /// not ask for RTCP — but the attributes carry on to the crate boundary, which is the only way
    /// a bandwidth estimate reaches the application.
    #[test]
    fn an_annotated_report_passes_on_as_an_empty_carrier() {
        let mut chain = Registry::new().build();
        chain
            .handle_read(annotated(Some(Attribute::TargetBitrateChanged {
                bits_per_second: 750_000.0,
            })))
            .unwrap();

        let carrier = chain.poll_read().expect("the attributes must get through");
        assert!(
            matches!(&carrier.message.packet, Packet::Rtcp(packets) if packets.is_empty()),
            "the payload must be stripped: the application did not ask for RTCP"
        );
        assert!(
            carrier.message.has(&Attribute::TargetBitrateChanged {
                bits_per_second: 0.0
            }),
            "but the attribute must survive"
        );
    }

    /// A report nobody annotated is still dropped, exactly as before.
    #[test]
    fn an_unannotated_report_still_stops_here() {
        let mut chain = Registry::new().build();
        chain.handle_read(annotated(None)).unwrap();
        assert!(chain.poll_read().is_none());
    }

    /// `DeliverToApplication` is the per-packet judgement its documentation always described:
    /// this one packet goes on **with its payload**, without turning RTCP on chain-wide.
    #[test]
    fn deliver_to_application_keeps_the_payload() {
        let mut chain = Registry::new().build();
        chain
            .handle_read(annotated(Some(Attribute::DeliverToApplication)))
            .unwrap();

        let delivered = chain.poll_read().expect("forwarded");
        assert!(
            matches!(&delivered.message.packet, Packet::Rtcp(packets) if !packets.is_empty()),
            "this packet was judged worth delivering, payload and all"
        );
    }
}