import tornado selector thread

v6.5.1
License: Apache-2.0
Copyright (c) 2025 The Tornado Authors

unmodified from original, so we can track changes

Author: minrk

Lib/asyncio/_selector_thread.py

@@ -0,0 +1,302 @@
+"""
+Compatibility for [add|remove]_[reader|writer] where unavailable (Proactor).
+
+Runs select in a background thread.
+
+Adapted from Tornado 6.5.1
+
+:copyright: 2025, The Tornado Authors
+:license: Apache-2.0
+"""
+
+import asyncio
+import atexit
+import contextvars
+import errno
+import functools
+import select
+import socket
+import threading
+import typing
+
+from typing import (
+    Any,
+    Callable,
+    Dict,
+    List,
+    Optional,
+    Protocol,
+    Set,
+    Tuple,
+    TypeVar,
+    Union,
+)
+
+if typing.TYPE_CHECKING:
+    from typing_extensions import TypeVarTuple, Unpack
+
+
+class _HasFileno(Protocol):
+    def fileno(self) -> int:
+        pass
+
+
+_FileDescriptorLike = Union[int, _HasFileno]
+
+_T = TypeVar("_T")
+
+if typing.TYPE_CHECKING:
+    _Ts = TypeVarTuple("_Ts")
+
+# 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 loop._thread is not None:
+            # 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.
+            loop._thread.join()
+    _selector_loops.clear()
+
+
+atexit.register(_atexit_callback)
+
+
+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._main_thread_ctx = contextvars.copy_context()
+
+        self._real_loop = real_loop
+
+        self._select_cond = threading.Condition()
+        self._select_args: Optional[
+            Tuple[List[_FileDescriptorLike], List[_FileDescriptorLike]]
+        ] = None
+        self._closing_selector = False
+        self._thread: Optional[threading.Thread] = 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()),
+            context=self._main_thread_ctx,
+        )
+
+        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, context=self._main_thread_ctx
+                )
+            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
+