Problem

Circuit Breaker (Rust)

Topics: networking, resilience, state machine

Problem

Implement a circuit breaker with Closed → Open → Half-Open states:

pub enum BreakerError { Open, Inner(String) }

impl Breaker {
    pub fn new(max_failures: u32, cooldown: Duration) -> Self;
    pub fn do_call<F: FnOnce() -> Result<(), String>>(&self, f: F) -> Result<(), BreakerError>;
}

Closed: run f. A success resets the failure counter; max_failures consecutive failures trip the breaker Open.
Open: reject calls with Err(Open) without running f, until cooldown elapses.
Half-Open: after the cooldown, allow exactly one probe call through; all other concurrent callers get Err(Open). If the probe succeeds the breaker closes; if it fails it reopens.

The single-probe guarantee must hold under concurrency: while a probe is in flight, every other caller is rejected.

Hint: hold the phase, failure count, opened-at Instant, and a probe_in_flight flag behind one Mutex; make the admission decision under the lock, then run f unlocked.

Grading

Your solution.rs is compiled together with a trusted tests.rs (which include!s it) using rustc --test. Only solution.rs is yours to edit.

Leaderboard score: lower is better, 1.0 ≈ reference

#	Name	score	ns/op	allocs

Output

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