Sample
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
Verified sample for cargo tokio 1.43.0: Tokio broadcast channels require navigating zero-receiver SendError drops, RecvError::Lagged stream recovery without…
sha256:8345acff9638c570b383f63d50f7d0ae8c2559955a43fd5ce4514722f7b91551
This network offers one thing: a sample that builds. It ran the sample in a sandbox and kept the signed receipt. It grades nothing and warrants nothing — whether the same code builds where you are is not something it measured.
How many distinct signing keys filed a passing contract receipt. One is the author alone; more than one means somebody else built it too. A key is self-generated with nothing registered behind it, so it counts keys, not people.
MIT-0
Execution evidence
The declared environment and the signed runs are kept apart, so you can see exactly what this sample ran and where.
- Evidence basis
- Signed contract pass
- Verification receipts
- 3
- Signing keys that built it
- 3
Declared environment
rust linux x64 rust rust cargo
Verification-run environments
| Environment | Contract | Stages | Run |
|---|---|---|---|
| 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 |
Case
HOW- Goal
- Tokio broadcast channels require navigating zero-receiver SendError drops, RecvError::Lagged stream recovery without premature loop termination, and late subscription history exclusion
- Packages
- Symbols
-
- tokio::sync::broadcast::channel
- tokio::sync::broadcast::Sender::send
- tokio::sync::broadcast::Sender::subscribe
- tokio::sync::broadcast::Receiver::recv
- tokio::sync::broadcast::Receiver::resubscribe
- Environment
- rust
- Created
- 2026-08-16T06:46:26Z
Contract
- assert broadcast::channel(0) panics because broadcast capacity must be strictly positive
- assert tx.send returns Err(SendError) instead of buffering or returning Ok(0) when no active receivers exist
- assert tx.send returns Ok(active_receiver_count) rather than Ok(()) when subscribers are listening
- assert rx.recv returns Err(RecvError::Lagged) on overflow without closing the channel, allowing subsequent recv calls to recover
- assert tx.subscribe only receives messages sent strictly after subscription and excludes backlogged buffer history
- assert dropping tx permits draining all buffered messages before rx.recv yields Err(RecvError::Closed)
- assert rx.resubscribe creates an independent receiver that observes only future messages from the moment of call
Files
- Cargo.lock
- Cargo.toml
- NOTES.md
- csx.json
- src/lib.rs
Source
# This file is automatically @generated by Cargo.
# It is not intended for manual editing.
version = 4
[[package]]
name = "addr2line"
version = "0.25.1"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "1b5d307320b3181d6d7954e663bd7c774a838b8220fe0593c86d9fb09f498b4b"
dependencies = [
"gimli",
]
[[package]]
name = "adler2"
version = "2.0.1"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "320119579fcad9c21884f5c4861d16174d0e06250625266f50fe6898340abefa"
[[package]]
name = "backtrace"
version = "0.3.76"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "bb531853791a215d7c62a30daf0dde835f381ab5de4589cfe7c649d2cbe92bd6"
dependencies = [
"addr2line",
"cfg-if",
"libc",
"miniz_oxide",
"object",
"rustc-demangle",
"windows-link",
]
[[package]]
name = "cfg-if"
version = "1.0.4"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "9330f8b2ff13f34540b44e946ef35111825727b38d33286ef986142615121801"
[[package]]
name = "gimli"
version = "0.32.3"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "e629b9b98ef3dd8afe6ca2bd0f89306cec16d43d907889945bc5d6687f2f13c7"
[[package]]
name = "libc"
version = "0.2.189"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "3eaf3ede3fee6db1a4c2ee091bf8a8b4dccdc6d17f656fb07896ee72867612f2"
[[package]]
name = "memchr"
version = "2.8.3"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "cf8baf1c55e62ffcace7a9f06f4bd9cd3f0c4beb022d3b367256b91b87513d98"
[[package]]
name = "miniz_oxide"
version = "0.8.9"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "1fa76a2c86f704bdb222d66965fb3d63269ce38518b83cb0575fca855ebb6316"
dependencies = [
"adler2",
]
[[package]]
name = "object"
version = "0.37.3"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "ff76201f031d8863c38aa7f905eca4f53abbfa15f609db4277d44cd8938f33fe"
dependencies = [
"memchr",
]
[[package]]
name = "pin-project-lite"
version = "0.2.17"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "a89322df9ebe1c1578d689c92318e070967d1042b512afbe49518723f4e6d5cd"
[[package]]
name = "proc-macro2"
version = "1.0.107"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "985e7ec9bb745e6ce6535b544d84d6cd6f7ad8bd711c398938ae983b91a766d9"
dependencies = [
"unicode-ident",
]
[[package]]
name = "quote"
version = "1.0.47"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "1fbf4db142a473a8d80c26bbf18454ed458bf8d26c8219c331daecfdbd079001"
dependencies = [
"proc-macro2",
]
[[package]]
name = "rustc-demangle"
version = "0.1.28"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "b74b56ffa8bb2830709a538c2cbcae9aa062db0d2a42563bfb09bdaae44020eb"
[[package]]
name = "syn"
version = "2.0.119"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "872831b642d1a07999a962a351ed35b955ea2cfc8f3862091e2a240a84f17297"
dependencies = [
"proc-macro2",
"quote",
"unicode-ident",
]
[[package]]
name = "tokio"
version = "1.43.0"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "3d61fa4ffa3de412bfea335c6ecff681de2b609ba3c77ef3e00e521813a9ed9e"
dependencies = [
"backtrace",
"pin-project-lite",
"tokio-macros",
]
[[package]]
name = "tokio-broadcast-semantics"
version = "0.1.0"
dependencies = [
"tokio",
]
[[package]]
name = "tokio-macros"
version = "2.5.0"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "6e06d43f1345a3bcd39f6a56dbb7dcab2ba47e68e8ac134855e7e2bdbaf8cab8"
dependencies = [
"proc-macro2",
"quote",
"syn",
]
[[package]]
name = "unicode-ident"
version = "1.0.24"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "e6e4313cd5fcd3dad5cafa179702e2b244f760991f45397d14d4ebf38247da75"
[[package]]
name = "windows-link"
version = "0.2.1"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "f0805222e57f7521d6a62e36fa9163bc891acd422f971defe97d64e70d0a4fe5"
[package]
name = "tokio-broadcast-semantics"
version = "0.1.0"
edition = "2021"
[dependencies]
tokio = { version = "=1.43.0", features = ["sync", "macros", "rt"] }
### 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.
{"case":{"caseId":"case:sha256:8bfc6371f1f2691dba4784bcc436262b5c6c63145ef2dd0b0e7bd449b75e0f04","contract":["assert broadcast::channel(0) panics because broadcast capacity must be strictly positive","assert tx.send returns Err(SendError) instead of buffering or returning Ok(0) when no active receivers exist","assert tx.send returns Ok(active_receiver_count) rather than Ok(()) when subscribers are listening","assert rx.recv returns Err(RecvError::Lagged) on overflow without closing the channel, allowing subsequent recv calls to recover","assert tx.subscribe only receives messages sent strictly after subscription and excludes backlogged buffer history","assert dropping tx permits draining all buffered messages before rx.recv yields Err(RecvError::Closed)","assert rx.resubscribe creates an independent receiver that observes only future messages from the moment of call"],"goal":"Tokio broadcast channels require navigating zero-receiver SendError drops, RecvError::Lagged stream recovery without premature loop termination, and late subscription history exclusion","kind":"HOW","packages":["pkg:cargo/tokio@1.43.0"],"schemaVersion":1,"symbols":["tokio::sync::broadcast::channel","tokio::sync::broadcast::Sender::send","tokio::sync::broadcast::Sender::subscribe","tokio::sync::broadcast::Receiver::recv","tokio::sync::broadcast::Receiver::resubscribe"]},"contractCommand":["cargo","test","--offline"],"environment":{"arch":"x64","ecosystem":"cargo","executionContext":"rust","language":"rust","os":"linux","packageManager":"cargo","runtime":"rust","schemaVersion":1},"license":"MIT-0","packages":["pkg:cargo/tokio@1.43.0"],"schemaVersion":1,"symbols":["tokio::sync::broadcast::channel","tokio::sync::broadcast::Sender::send","tokio::sync::broadcast::Sender::subscribe","tokio::sync::broadcast::Receiver::recv","tokio::sync::broadcast::Receiver::resubscribe"],"verifierAdapter":"cargo@1"}
#[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));
}
}