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from __future__ import annotations
import asyncio
import contextlib
import time
from concurrent.futures import Future as ConcurrentFuture
from dataclasses import dataclass
from threading import Lock
from typing import TYPE_CHECKING, Final, Generic, Literal, NoReturn, TypeVar, cast
from weakref import WeakKeyDictionary
from ._api import _append_exception_context, _fork_transition, _raise_chained_errors, _resolve_read_write_timeout
from ._error import Timeout
if TYPE_CHECKING:
from collections.abc import AsyncGenerator, Awaitable, Callable
from concurrent.futures import Executor
from ._read_write import ReadWriteLock
from ._soft_rw import SoftReadWriteLock
_T = TypeVar("_T")
class _AsyncTransitionUnavailableError(Exception):
pass
@dataclass(frozen=True)
class _BackendOutcome(Generic[_T]):
value: _T | None = None
error: BaseException | None = None
class _AsyncTransitionGate:
def __init__(self) -> None:
self._tail_lock: Final[Lock] = Lock()
self._tail: ConcurrentFuture[None] | None = None
@contextlib.asynccontextmanager
async def hold(self) -> AsyncGenerator[None]:
ticket: ConcurrentFuture[None] = ConcurrentFuture()
with self._tail_lock:
predecessor = self._tail
self._tail = ticket
if predecessor is not None:
try:
await _wait_until_done(asyncio.wrap_future(predecessor))
except BaseException:
predecessor.add_done_callback(lambda _predecessor: self._leave(ticket))
raise
try:
yield
finally:
self._leave(ticket)
@contextlib.asynccontextmanager
async def hold_for_acquire(
self,
*,
blocking: bool,
cancel_check: Callable[[], bool] | None,
deadline: float | None,
poll_interval: float,
) -> AsyncGenerator[None]:
ticket: ConcurrentFuture[None] = ConcurrentFuture()
with self._tail_lock:
predecessor = self._tail
self._tail = ticket
if predecessor is not None and not predecessor.done():
try:
await self._wait_for_predecessor(
predecessor,
blocking=blocking,
cancel_check=cancel_check,
deadline=deadline,
poll_interval=poll_interval,
)
except BaseException:
predecessor.add_done_callback(lambda _predecessor: self._leave(ticket))
raise
try:
yield
finally:
self._leave(ticket)
@staticmethod
async def _wait_for_predecessor(
predecessor: ConcurrentFuture[None],
*,
blocking: bool,
cancel_check: Callable[[], bool] | None,
deadline: float | None,
poll_interval: float,
) -> None:
if not blocking:
raise _AsyncTransitionUnavailableError
waiter = asyncio.wrap_future(predecessor)
while not predecessor.done():
if cancel_check is not None and cancel_check():
raise _AsyncTransitionUnavailableError
if deadline is not None:
if (remaining := deadline - time.perf_counter()) <= 0:
raise _AsyncTransitionUnavailableError
wait_interval = min(poll_interval, remaining) if cancel_check is not None else remaining
else:
wait_interval = poll_interval if cancel_check is not None else None
await asyncio.wait((waiter,), timeout=wait_interval)
def _leave(self, ticket: ConcurrentFuture[None]) -> None:
with self._tail_lock:
if self._tail is ticket:
self._tail = None
ticket.set_result(None)
class _TaskOwners:
"""
Per-task ownership in front of a sync reader/writer lock that tracks threads.
An async wrapper runs each call on an executor thread, and that thread does not identify the calling task. Waiting
writers block new readers so a stream of readers cannot starve them. A thread lock guards the table because tasks on
different event loops share it.
"""
def __init__(self) -> None:
self._lock: Final[Lock] = Lock()
self._mode: Literal["read", "write"] | None = None
self._depths: dict[asyncio.Task[object], int] = {}
self._transitioning = False
self._waiting_writers = 0
self._changed: ConcurrentFuture[None] = ConcurrentFuture()
#: The executor thread that took the sync lock's level for every task, which may leave on another worker.
self.backend_thread: int | None = None
async def acquire(
self,
mode: Literal["read", "write"],
*,
timeout: float,
blocking: bool,
lock_file: str,
enter: Callable[[float], Awaitable[None]],
) -> None:
task = _current_task()
timeout = _resolve_read_write_timeout(timeout, blocking=blocking)
deadline = None if not blocking or timeout < 0 else time.perf_counter() + timeout
waiting_writer = False
try:
while isinstance(
admission := self._admit(task, mode, lock_file, waiting_writer=waiting_writer), ConcurrentFuture
):
if not blocking or (deadline is not None and time.perf_counter() >= deadline):
raise Timeout(lock_file)
if mode == "write" and not waiting_writer:
waiting_writer = True
with self._lock:
self._waiting_writers += 1
wait = None if deadline is None else deadline - time.perf_counter()
await asyncio.wait((asyncio.wrap_future(admission),), timeout=wait)
finally:
if waiting_writer:
with self._lock:
self._waiting_writers -= 1
self._notify()
if admission == "held":
return
try:
await enter(-1 if deadline is None else max(0.0, deadline - time.perf_counter()))
except BaseException:
self._finish_transition(mode=None)
raise
self._finish_transition(mode=mode, holder=task)
def _admit(
self,
task: asyncio.Task[object],
mode: Literal["read", "write"],
lock_file: str,
*,
waiting_writer: bool,
) -> Literal["held", "enter"] | ConcurrentFuture[None]:
with self._lock:
if (depth := self._depths.get(task)) is not None:
if self._mode != mode:
msg = (
f"Cannot acquire {mode} lock on {lock_file}: already holding a "
f"{'write' if mode == 'read' else 'read'} lock "
f"({'downgrade' if mode == 'read' else 'upgrade'} not allowed)"
)
raise RuntimeError(msg)
self._depths[task] = depth + 1
return "held"
writers_ahead = self._waiting_writers - waiting_writer
if not self._transitioning:
if mode == "read" and self._mode == "read" and not writers_ahead:
self._depths[task] = 1
return "held"
if not self._depths and (mode == "write" or not writers_ahead):
self._transitioning, self._mode = True, mode
return "enter"
return self._changed
async def release(
self, *, force: bool, lock_file: str, leave: Callable[[], asyncio.Future[_BackendOutcome[None]]]
) -> None:
task = _current_task()
with self._lock:
if (depth := self._depths.get(task)) is None:
if force:
return
msg = f"Cannot release a lock on {lock_file} that is not held by this task"
raise RuntimeError(msg)
if not force and depth > 1:
self._depths[task] = depth - 1
return
del self._depths[task]
if self._depths:
return
self._transitioning = True
mode = self._mode
try:
cancellation = await _drain_cancellation(leave())
except BaseException:
# The sync lock may still hold the transaction, so keep the hold for a retried release to find.
self._finish_transition(mode=mode, holder=task)
raise
self._finish_transition(mode=None)
if cancellation is not None:
# The release succeeded, so a restored hold would be a phantom no backend call can clear.
raise cancellation
def reset(self) -> None:
with self._lock:
self._depths.clear()
self._mode, self._transitioning = None, False
self._notify()
def _finish_transition(
self, *, mode: Literal["read", "write"] | None, holder: asyncio.Task[object] | None = None
) -> None:
with self._lock:
self._transitioning, self._mode = False, mode
if holder is not None:
self._depths[holder] = 1
self._notify()
def _notify(self) -> None:
changed, self._changed = self._changed, ConcurrentFuture()
changed.set_result(None)
_TASK_OWNERS: Final[WeakKeyDictionary[ReadWriteLock | SoftReadWriteLock, _TaskOwners]] = WeakKeyDictionary()
_TASK_OWNERS_LOCK: Final[Lock] = Lock()
def _task_owners_for(sync_lock: ReadWriteLock | SoftReadWriteLock) -> _TaskOwners:
"""Singleton wrappers share one sync lock, so they must share its task holds too."""
with _fork_transition(), _TASK_OWNERS_LOCK:
if (owners := _TASK_OWNERS.get(sync_lock)) is None:
owners = _TASK_OWNERS[sync_lock] = _TaskOwners()
return owners
def _current_task() -> asyncio.Task[object]:
if (task := asyncio.current_task()) is None:
msg = "an async reader/writer lock must be used from inside an asyncio task"
raise RuntimeError(msg)
return task
async def _drain_cancellation(future: asyncio.Future[_BackendOutcome[None]]) -> asyncio.CancelledError | None:
# Returned, not raised, so the caller can tell a call that completed under cancellation from one that failed.
try:
await _wait_until_done(future)
except asyncio.CancelledError as cancellation:
try:
await _drain_future(future)
except BaseException as error: # ruff:ignore[blind-except] # reported with the cancellation below
_raise_cancelled_error(cancellation, error)
return cancellation
_future_result(future)
return None
async def _drain_future(future: asyncio.Future[_BackendOutcome[_T]]) -> _T:
while not future.done():
with contextlib.suppress(asyncio.CancelledError):
await _wait_until_done(future)
return _future_result(future)
async def _wait_until_done(future: asyncio.Future[_T]) -> None:
if not future.done():
await asyncio.wait((future,))
def _future_result(future: asyncio.Future[_BackendOutcome[_T]]) -> _T:
outcome = future.result()
if (error := outcome.error) is None:
return cast("_T", outcome.value)
context = error.__context__
try:
raise error # ruff:ignore[raise-within-try] # the handler restores context changed across the async boundary
except BaseException:
error.__context__ = context
raise
def _run_in_executor(
loop: asyncio.AbstractEventLoop, executor: Executor | None, func: Callable[[], _T]
) -> asyncio.Future[_BackendOutcome[_T]]:
outcome: asyncio.Future[_BackendOutcome[_T]] = loop.create_future()
def settle(call: asyncio.Future[_BackendOutcome[_T]]) -> None:
try:
result = call.result()
except asyncio.CancelledError:
# shutdown(cancel_futures=True) dropped the call unrun: a CancelledError would cancel a task nobody canceled
# and hide that the lock state never changed.
msg = "the executor shut down before running the lock backend call"
result = _BackendOutcome(error=RuntimeError(msg))
outcome.set_result(result)
loop.run_in_executor(executor, _capture_call, func).add_done_callback(settle)
return outcome
def _capture_call(func: Callable[[], _T]) -> _BackendOutcome[_T]:
try:
return _BackendOutcome(value=func())
except BaseException as error: # ruff:ignore[blind-except] # backend control-flow exceptions are operation results
return _BackendOutcome(error=error)
def _raise_cancelled_error(cancellation: asyncio.CancelledError, error: BaseException) -> NoReturn:
# A reconciliation step failed while unwinding a cancellation, so keep both exception chains. Splice the error's
# existing context onto the cancellation, then make the cancellation the error's context, so both the failure and
# the cancellation that triggered it survive. Shared by the async wrappers so cancellations report the same way.
if (context := error.__context__) is not None and context is not cancellation:
if (cancellation_context := cancellation.__context__) is not None:
_append_exception_context(context, cancellation_context)
cancellation.__context__ = context
error.__context__ = cancellation
_raise_chained_errors(error)
async def _capture_awaitable(awaitable: Awaitable[_T]) -> _BackendOutcome[_T]:
try:
return _BackendOutcome(value=await awaitable)
except BaseException as error: # ruff:ignore[blind-except] # backend cancellation must remain distinct from caller cancellation
return _BackendOutcome(error=error)
__all__ = [
"_AsyncTransitionGate",
"_AsyncTransitionUnavailableError",
"_BackendOutcome",
"_capture_awaitable",
"_capture_call",
"_drain_future",
"_future_result",
"_raise_cancelled_error",
"_run_in_executor",
"_task_owners_for",
"_wait_until_done",
]
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