import asyncio
import sys

from asyncio.locks import Lock as _Lock

# Fixes an issue with all Python versions that leaves pending waiters
# without being awakened when the first waiter is canceled.
# Code adapted from the PR https://github.com/python/cpython/pull/1031
# Waiting once it is merged to make a proper condition to relay on
# the stdlib implementation or this one patched


class Lock(_Lock):

    if sys.version_info < (3, 7, 0):
        async def acquire(self):
            """Acquire a lock.
            This method blocks until the lock is unlocked, then sets it to
            locked and returns True.
            """
            if not self._locked and all(w.cancelled() for w in self._waiters):
                self._locked = True
                return True

            fut = self._loop.create_future()

            self._waiters.append(fut)
            try:
                await fut
                self._locked = True
                return True
            except asyncio.CancelledError:
                if not self._locked:  # pragma: no cover
                    self._wake_up_first()
                raise
            finally:
                self._waiters.remove(fut)

        def _wake_up_first(self):
            """Wake up the first waiter who isn't cancelled."""
            for fut in self._waiters:
                if not fut.done():
                    fut.set_result(True)
                    break