Use ThreadSelectorEventLoop on Windows with ProactorEventLoop

docs/versionhistory.rst

@@ -7,6 +7,8 @@ This library adheres to `Semantic Versioning 2.0 <http://semver.org/>`_.
 
 - Fixed a misleading ``ValueError`` in the context of DNS failures
   (`#815 <https://github.com/agronholm/anyio/issues/815>`_; PR by @graingert)
+- Ported ``ThreadSelectorEventLoop`` from Tornado to allow
+  ``anyio.wait_socket_readable(sock)`` to work on Windows with a ``ProactorEventLoop``.
 
 **4.6.2**
 

src/anyio/_backends/_asyncio.py

@@ -98,6 +98,7 @@
 from ..abc._eventloop import StrOrBytesPath
 from ..lowlevel import RunVar
 from ..streams.memory import MemoryObjectReceiveStream, MemoryObjectSendStream
+from ._selector_thread import _get_selector_windows
 
 if sys.version_info >= (3, 10):
     from typing import ParamSpec
@@ -2683,13 +2684,24 @@ async def wait_socket_readable(cls, sock: socket.socket) -> None:
             raise BusyResourceError("reading from") from None
 
         loop = get_running_loop()
+        if (
+            sys.platform == "win32"
+            and asyncio.get_event_loop_policy().__class__.__name__
+            == "WindowsProactorEventLoopPolicy"
+        ):
+            selector = _get_selector_windows(loop)
+            add_reader = selector.add_reader
+            remove_reader = selector.remove_reader
+        else:
+            add_reader = loop.add_reader
+            remove_reader = loop.remove_reader
         event = read_events[sock] = asyncio.Event()
-        loop.add_reader(sock, event.set)
+        add_reader(sock, event.set)
         try:
             await event.wait()
         finally:
             if read_events.pop(sock, None) is not None:
-                loop.remove_reader(sock)
+                remove_reader(sock)
                 readable = True
             else:
                 readable = False

src/anyio/_backends/_selector_thread.py

@@ -0,0 +1,315 @@
+"""Ensure asyncio selector methods (add_reader, etc.) are available.
+Running select in a thread and defining these methods on the running event loop.
+Originally in tornado.platform.asyncio.
+Redistributed under license Apache-2.0
+"""
+
+from __future__ import annotations
+
+import asyncio
+import atexit
+import errno
+import functools
+import select
+import socket
+import threading
+import typing
+from typing import (
+    Any,
+    Callable,
+    Union,
+)
+from weakref import WeakKeyDictionary
+
+if typing.TYPE_CHECKING:
+    from typing_extensions import Protocol
+
+    class _HasFileno(Protocol):
+        def fileno(self) -> int:
+            pass
+
+    _FileDescriptorLike = Union[int, _HasFileno]
+
+
+# registry of asyncio loop : selector thread
+_selectors: WeakKeyDictionary = WeakKeyDictionary()
+
+# Collection of selector thread event loops to shut down on exit.
+_selector_loops: set[SelectorThread] = set()
+
+
+def _atexit_callback() -> None:
+    for loop in _selector_loops:
+        with loop._select_cond:
+            loop._closing_selector = True
+            loop._select_cond.notify()
+        try:
+            loop._waker_w.send(b"a")
+        except BlockingIOError:
+            pass
+        # If we don't join our (daemon) thread here, we may get a deadlock
+        # during interpreter shutdown. I don't really understand why. This
+        # deadlock happens every time in CI (both travis and appveyor) but
+        # I've never been able to reproduce locally.
+        assert loop._thread is not None
+        loop._thread.join()
+    _selector_loops.clear()
+
+
+atexit.register(_atexit_callback)
+
+
+# SelectorThread from tornado 6.4.0
+
+
+class SelectorThread:
+    """Define ``add_reader`` methods to be called in a background select thread.
+
+    Instances of this class start a second thread to run a selector.
+    This thread is completely hidden from the user;
+    all callbacks are run on the wrapped event loop's thread.
+
+    Typically used via ``AddThreadSelectorEventLoop``,
+    but can be attached to a running asyncio loop.
+    """
+
+    _closed = False
+
+    def __init__(self, real_loop: asyncio.AbstractEventLoop) -> None:
+        self._real_loop = real_loop
+
+        self._select_cond = threading.Condition()
+        self._select_args: (
+            tuple[list[_FileDescriptorLike], list[_FileDescriptorLike]] | None
+        ) = None
+        self._closing_selector = False
+        self._thread: threading.Thread | None = None
+        self._thread_manager_handle = self._thread_manager()
+
+        async def thread_manager_anext() -> None:
+            # the anext builtin wasn't added until 3.10. We just need to iterate
+            # this generator one step.
+            await self._thread_manager_handle.__anext__()
+
+        # When the loop starts, start the thread. Not too soon because we can't
+        # clean up if we get to this point but the event loop is closed without
+        # starting.
+        self._real_loop.call_soon(
+            lambda: self._real_loop.create_task(thread_manager_anext())
+        )
+
+        self._readers: dict[_FileDescriptorLike, Callable] = {}
+        self._writers: dict[_FileDescriptorLike, Callable] = {}
+
+        # Writing to _waker_w will wake up the selector thread, which
+        # watches for _waker_r to be readable.
+        self._waker_r, self._waker_w = socket.socketpair()
+        self._waker_r.setblocking(False)
+        self._waker_w.setblocking(False)
+        _selector_loops.add(self)
+        self.add_reader(self._waker_r, self._consume_waker)
+
+    def close(self) -> None:
+        if self._closed:
+            return
+        with self._select_cond:
+            self._closing_selector = True
+            self._select_cond.notify()
+        self._wake_selector()
+        if self._thread is not None:
+            self._thread.join()
+        _selector_loops.discard(self)
+        self.remove_reader(self._waker_r)
+        self._waker_r.close()
+        self._waker_w.close()
+        self._closed = True
+
+    async def _thread_manager(self) -> typing.AsyncGenerator[None, None]:
+        # Create a thread to run the select system call. We manage this thread
+        # manually so we can trigger a clean shutdown from an atexit hook. Note
+        # that due to the order of operations at shutdown, only daemon threads
+        # can be shut down in this way (non-daemon threads would require the
+        # introduction of a new hook: https://bugs.python.org/issue41962)
+        self._thread = threading.Thread(
+            name="Tornado selector",
+            daemon=True,
+            target=self._run_select,
+        )
+        self._thread.start()
+        self._start_select()
+        try:
+            # The presense of this yield statement means that this coroutine
+            # is actually an asynchronous generator, which has a special
+            # shutdown protocol. We wait at this yield point until the
+            # event loop's shutdown_asyncgens method is called, at which point
+            # we will get a GeneratorExit exception and can shut down the
+            # selector thread.
+            yield
+        except GeneratorExit:
+            self.close()
+            raise
+
+    def _wake_selector(self) -> None:
+        if self._closed:
+            return
+        try:
+            self._waker_w.send(b"a")
+        except BlockingIOError:
+            pass
+
+    def _consume_waker(self) -> None:
+        try:
+            self._waker_r.recv(1024)
+        except BlockingIOError:
+            pass
+
+    def _start_select(self) -> None:
+        # Capture reader and writer sets here in the event loop
+        # thread to avoid any problems with concurrent
+        # modification while the select loop uses them.
+        with self._select_cond:
+            assert self._select_args is None
+            self._select_args = (list(self._readers.keys()), list(self._writers.keys()))
+            self._select_cond.notify()
+
+    def _run_select(self) -> None:
+        while True:
+            with self._select_cond:
+                while self._select_args is None and not self._closing_selector:
+                    self._select_cond.wait()
+                if self._closing_selector:
+                    return
+                assert self._select_args is not None
+                to_read, to_write = self._select_args
+                self._select_args = None
+
+            # We use the simpler interface of the select module instead of
+            # the more stateful interface in the selectors module because
+            # this class is only intended for use on windows, where
+            # select.select is the only option. The selector interface
+            # does not have well-documented thread-safety semantics that
+            # we can rely on so ensuring proper synchronization would be
+            # tricky.
+            try:
+                # On windows, selecting on a socket for write will not
+                # return the socket when there is an error (but selecting
+                # for reads works). Also select for errors when selecting
+                # for writes, and merge the results.
+                #
+                # This pattern is also used in
+                # https://github.com/python/cpython/blob/v3.8.0/Lib/selectors.py#L312-L317
+                rs, ws, xs = select.select(to_read, to_write, to_write)
+                ws = ws + xs
+            except OSError as e:
+                # After remove_reader or remove_writer is called, the file
+                # descriptor may subsequently be closed on the event loop
+                # thread. It's possible that this select thread hasn't
+                # gotten into the select system call by the time that
+                # happens in which case (at least on macOS), select may
+                # raise a "bad file descriptor" error. If we get that
+                # error, check and see if we're also being woken up by
+                # polling the waker alone. If we are, just return to the
+                # event loop and we'll get the updated set of file
+                # descriptors on the next iteration. Otherwise, raise the
+                # original error.
+                if e.errno == getattr(errno, "WSAENOTSOCK", errno.EBADF):
+                    rs, _, _ = select.select([self._waker_r.fileno()], [], [], 0)
+                    if rs:
+                        ws = []
+                    else:
+                        raise
+                else:
+                    raise
+
+            try:
+                self._real_loop.call_soon_threadsafe(self._handle_select, rs, ws)
+            except RuntimeError:
+                # "Event loop is closed". Swallow the exception for
+                # consistency with PollIOLoop (and logical consistency
+                # with the fact that we can't guarantee that an
+                # add_callback that completes without error will
+                # eventually execute).
+                pass
+            except AttributeError:
+                # ProactorEventLoop may raise this instead of RuntimeError
+                # if call_soon_threadsafe races with a call to close().
+                # Swallow it too for consistency.
+                pass
+
+    def _handle_select(
+        self, rs: list[_FileDescriptorLike], ws: list[_FileDescriptorLike]
+    ) -> None:
+        for r in rs:
+            self._handle_event(r, self._readers)
+        for w in ws:
+            self._handle_event(w, self._writers)
+        self._start_select()
+
+    def _handle_event(
+        self,
+        fd: _FileDescriptorLike,
+        cb_map: dict[_FileDescriptorLike, Callable],
+    ) -> None:
+        try:
+            callback = cb_map[fd]
+        except KeyError:
+            return
+        callback()
+
+    def add_reader(
+        self, fd: _FileDescriptorLike, callback: Callable[..., None], *args: Any
+    ) -> None:
+        self._readers[fd] = functools.partial(callback, *args)
+        self._wake_selector()
+
+    def add_writer(
+        self, fd: _FileDescriptorLike, callback: Callable[..., None], *args: Any
+    ) -> None:
+        self._writers[fd] = functools.partial(callback, *args)
+        self._wake_selector()
+
+    def remove_reader(self, fd: _FileDescriptorLike) -> bool:
+        try:
+            del self._readers[fd]
+        except KeyError:
+            return False
+        self._wake_selector()
+        return True
+
+    def remove_writer(self, fd: _FileDescriptorLike) -> bool:
+        try:
+            del self._writers[fd]
+        except KeyError:
+            return False
+        self._wake_selector()
+        return True
+
+
+def _get_selector_windows(
+    asyncio_loop: asyncio.AbstractEventLoop,
+) -> SelectorThread:
+    """Get selector-compatible loop.
+
+    Sets ``add_reader`` family of methods on the asyncio loop.
+
+    Workaround Windows proactor removal of *reader methods.
+    """
+
+    if asyncio_loop in _selectors:
+        return _selectors[asyncio_loop]
+
+    selector_thread = _selectors[asyncio_loop] = SelectorThread(asyncio_loop)
+
+    # patch loop.close to also close the selector thread
+    loop_close = asyncio_loop.close
+
+    def _close_selector_and_loop() -> None:
+        # restore original before calling selector.close,
+        # which in turn calls eventloop.close!
+        asyncio_loop.close = loop_close  # type: ignore[method-assign]
+        _selectors.pop(asyncio_loop, None)
+        selector_thread.close()
+
+    asyncio_loop.close = _close_selector_and_loop  # type: ignore[method-assign]
+
+    return selector_thread

src/anyio/_core/_sockets.py

@@ -595,9 +595,6 @@ def wait_socket_readable(sock: socket.socket) -> Awaitable[None]:
     """
     Wait until the given socket has data to be read.
 
-    This does **NOT** work on Windows when using the asyncio backend with a proactor
-    event loop (default on py3.8+).
-
     .. warning:: Only use this on raw sockets that have not been wrapped by any higher
         level constructs like socket streams!