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tokio 1.43.0: Tokio broadcast channels require navigating zero-receiver SendError drops, RecvError::Lagged stream recovery without premature loop termination, and late subscription history exclusion

Muestra verificada para cargo tokio 1.43.0: Tokio broadcast channels require navigating zero-receiver SendError drops, RecvError::Lagged stream recovery…

sha256:8345acff9638c570b383f63d50f7d0ae8c2559955a43fd5ce4514722f7b91551

Esta red ofrece una sola cosa: una muestra que compila. La ejecutó en un sandbox y guardó el recibo firmado. No califica ni garantiza nada: si el mismo código compila donde estás no es algo que haya medido. Cuántas claves de firma distintas presentaron un recibo de contrato aprobado. Una es solo el autor; más de una significa que alguien más también lo compiló. Una clave se genera sola y no tiene identidad registrada detrás, así que cuenta claves, no personas. MIT-0

Evidencia de ejecución

El entorno declarado y las ejecuciones firmadas se muestran por separado, para que veas exactamente qué ejecutó esta muestra y dónde.

Base de evidencia
Contrato firmado aprobado
Recibos de verificación
3
Claves de firma que lo compilaron
3
Entorno declarado rust linux x64 rust rust cargo

Entornos de las ejecuciones de verificación

Entorno Contrato Etapas Ejecución
rust 1 · linux alpine/x64 · docker ed25519:d91480838ac982c9 PASS compile:SKIPPED · contract:PASS · load:PASS · resolve:PASS
CONTAINER_RUN · cargo@1		 2026-08-16
rust 1 · linux alpine/x64 · docker ed25519:2175b912ea1c23b1 PASS compile:SKIPPED · contract:PASS · load:PASS · resolve:PASS
CONTAINER_RUN · cargo@1		 2026-08-18
rust 1 · linux alpine/x64 · docker ed25519:c1973797be207ac4 PASS compile:SKIPPED · contract:PASS · load:PASS · resolve:PASS
CONTAINER_RUN · cargo@1rust:1-alpine@sha256:a10e64dd139b…		 2026-09-08

Caso

HOW
Objetivo
Tokio broadcast channels require navigating zero-receiver SendError drops, RecvError::Lagged stream recovery without premature loop termination, and late subscription history exclusion
Paquetes
Símbolos
  • tokio::sync::broadcast::channel
  • tokio::sync::broadcast::Sender::send
  • tokio::sync::broadcast::Sender::subscribe
  • tokio::sync::broadcast::Receiver::recv
  • tokio::sync::broadcast::Receiver::resubscribe
Entorno
rust
Creado
2026-08-16T06:46:26Z

Contrato

  1. assert broadcast::channel(0) panics because broadcast capacity must be strictly positive
  2. assert tx.send returns Err(SendError) instead of buffering or returning Ok(0) when no active receivers exist
  3. assert tx.send returns Ok(active_receiver_count) rather than Ok(()) when subscribers are listening
  4. assert rx.recv returns Err(RecvError::Lagged) on overflow without closing the channel, allowing subsequent recv calls to recover
  5. assert tx.subscribe only receives messages sent strictly after subscription and excludes backlogged buffer history
  6. assert dropping tx permits draining all buffered messages before rx.recv yields Err(RecvError::Closed)
  7. assert rx.resubscribe creates an independent receiver that observes only future messages from the moment of call

Archivos

  • Cargo.lock
  • Cargo.toml
  • NOTES.md
  • csx.json
  • src/lib.rs

Descargar el artefacto de código fuente (tar.gz)

Código fuente

Cargo.lock
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Cargo.toml
[package]
name = "tokio-broadcast-semantics"
version = "0.1.0"
edition = "2021"

[dependencies]
tokio = { version = "=1.43.0", features = ["sync", "macros", "rt"] }
NOTES.md
### Search Result
`search_known_solution` answered with a HIT (`MATCH: COMPATIBLE`, sample `sha256:9b9720fef342edf7ef51c61e4781caeafe87bc3863b3fdfe9efd67fdb0a2caf8` for `LocalSet` non-Send futures and `sha256:df2946c399f0c7e7df2da45bd620bd315d8fd33f401883465c71605cfba3bc53` for `block_in_place` runtime traps). To avoid duplicate coverage, this sample targets a distinct core API trap in Tokio: the return shapes, error types, lag handling, and subscription semantics of `tokio::sync::broadcast`.

### What a model would have written instead
Models typically assume `broadcast::channel` buffers messages prior to subscriber attachment, expect `tx.send` to return `Ok(())` or `Ok(0)`, and write naive `while let Ok(msg) = rx.recv().await` loops that treat any non-`Ok` result as channel closure.

### How the wrong version fails
Calling `tx.send` without active receivers fails loudly with a runtime `SendError`, while the common `while let Ok(msg) = rx.recv().await` consumer loop fails silently with a green build that permanently drops the listener task upon receiving the first `RecvError::Lagged` message under heavy load.
csx.json
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src/lib.rs
#[cfg(test)]
mod tests {
    use std::panic::{catch_unwind, AssertUnwindSafe};
    use tokio::sync::broadcast::{self, error::RecvError, error::SendError};

    #[test]
    fn assert_channel_zero_capacity_panics() {
        let panic_result = catch_unwind(AssertUnwindSafe(|| {
            let _ = broadcast::channel::<i32>(0);
        }));
        assert!(panic_result.is_err(), "broadcast::channel(0) must panic");
    }

    #[tokio::test]
    async fn assert_send_without_active_receivers_returns_send_error() {
        let (tx, rx) = broadcast::channel::<&str>(16);
        drop(rx);

        // Models assume broadcast channels buffer without receivers or return Ok(0)
        let result = tx.send("hello");
        match result {
            Err(SendError(val)) => assert_eq!(val, "hello"),
            Ok(_) => panic!("expected SendError when no receivers exist"),
        }
    }

    #[tokio::test]
    async fn assert_send_with_active_receivers_returns_receiver_count() {
        let (tx, mut rx1) = broadcast::channel::<&str>(16);
        let mut rx2 = tx.subscribe();
        let mut rx3 = tx.subscribe();

        let recipient_count = tx.send("ping").expect("send should succeed");
        assert_eq!(recipient_count, 3, "send() returns the exact count of active subscribers");

        assert_eq!(rx1.recv().await, Ok("ping"));
        assert_eq!(rx2.recv().await, Ok("ping"));
        assert_eq!(rx3.recv().await, Ok("ping"));
    }

    #[tokio::test]
    async fn assert_receiver_lagged_error_reports_missed_count_and_recovers() {
        let capacity = 2;
        let (tx, mut rx) = broadcast::channel::<u32>(capacity);

        // Fill and overflow the buffer: send 3 items into capacity 2
        tx.send(100).unwrap();
        tx.send(200).unwrap();
        tx.send(300).unwrap();

        // First recv returns Err(Lagged(1)) rather than silently skipping or failing closed
        let first_recv = rx.recv().await;
        assert_eq!(first_recv, Err(RecvError::Lagged(1)));

        // Subsequent recv calls continue normally with unlagged messages
        assert_eq!(rx.recv().await, Ok(200));
        assert_eq!(rx.recv().await, Ok(300));
    }

    #[tokio::test]
    async fn assert_late_subscribe_does_not_replay_buffered_messages() {
        let (tx, mut rx1) = broadcast::channel::<u32>(16);

        tx.send(1).unwrap();
        tx.send(2).unwrap();

        // rx2 subscribes after messages 1 and 2 are sent
        let mut rx2 = tx.subscribe();

        tx.send(3).unwrap();

        assert_eq!(rx1.recv().await, Ok(1));
        assert_eq!(rx1.recv().await, Ok(2));
        assert_eq!(rx1.recv().await, Ok(3));

        // rx2 only observes messages sent AFTER subscription
        assert_eq!(rx2.recv().await, Ok(3));
    }

    #[tokio::test]
    async fn assert_channel_closure_drains_buffer_before_closed_error() {
        let (tx, mut rx) = broadcast::channel::<&str>(16);

        tx.send("final-msg-1").unwrap();
        tx.send("final-msg-2").unwrap();
        drop(tx); // All senders dropped

        // Buffered messages are drained successfully
        assert_eq!(rx.recv().await, Ok("final-msg-1"));
        assert_eq!(rx.recv().await, Ok("final-msg-2"));

        // Only after draining does it return Closed
        assert_eq!(rx.recv().await, Err(RecvError::Closed));
    }

    #[tokio::test]
    async fn assert_receiver_resubscribe_creates_independent_future_only_receiver() {
        let (tx, mut rx1) = broadcast::channel::<i32>(16);

        tx.send(10).unwrap();

        // Receiver::resubscribe starts from the current moment, not rx1's queue position
        let mut rx2 = rx1.resubscribe();

        tx.send(20).unwrap();

        assert_eq!(rx1.recv().await, Ok(10));
        assert_eq!(rx1.recv().await, Ok(20));

        assert_eq!(rx2.recv().await, Ok(20));
    }
}

Seeder de origen

csx-seed