import threading
import time

class MockBridge:
    def __init__(self):
        self._lock = threading.Lock()

    def busy(self):
        return self._lock.locked()

    def send_and_read_once(self, payload: bytes):
        if not self._lock.acquire(blocking=False):
            return {"busy": True, "error": "BUSY"}
        try:
            time.sleep(0.1)
            return {"ok": True, "response": "OK"}
        finally:
            self._lock.release()