The Half-Open Thundering Stampede

"""
Circuit Breaker State Machine
States: CLOSED (normal), OPEN (tripped), HALF_OPEN (canary testing recovery).
"""

class CircuitBreaker:
    def __init__(self, failure_threshold=3, recovery_timeout=5.0):
        self.failure_threshold = failure_threshold
        self.recovery_timeout = recovery_timeout
        self.state = "CLOSED"
        self.failure_count = 0
        self.last_state_change = 0.0
        self.trial_in_flight = False

    def allow_request(self, now):
        if self.state == "CLOSED":
            return True

        if self.state == "OPEN":
            if now - self.last_state_change >= self.recovery_timeout:
                self.state = "HALF_OPEN"
                self.last_state_change = now
                # AI Attempt: Transition to HALF_OPEN and allow request
                # BUG: Does NOT limit concurrency! All concurrent requests enter HALF_OPEN!
                return True
            return False

        if self.state == "HALF_OPEN":
            # BUG: Unconditionally returns True, stampeding the recovering upstream
            return True

        return False

    def record_success(self):
        self.state = "CLOSED"
        self.failure_count = 0
        self.trial_in_flight = False

    def record_failure(self, now):
        self.failure_count += 1
        self.trial_in_flight = False
        if self.state == "HALF_OPEN" or self.failure_count >= self.failure_threshold:
            self.state = "OPEN"
            self.last_state_change = now