bpo-32309: Implement asyncio.ThreadPool

Doc/library/asyncio-pools.rst

@@ -0,0 +1,95 @@
+.. currentmodule:: asyncio
+
+.. versionadded:: 3.9
+
+.. _asyncio-pools:
+
+=====
+Pools
+=====
+
+**Source code:** :source:`Lib/asyncio/pools.py`
+
+-----------------------------------------------
+
+.. note::
+    This section of the documentation and all of its members have been
+    added *provisionally* to asyncio's API. For more details, see
+    :term:`provisional api`.
+
+Asyncio pools are high-level, asynchronous context managers that can be used
+to concurrently run blocking functions and methods.
+
+There are many potential use cases, but a particularly useful one is for
+combining libraries without asyncio support with existing asyncio programs.
+Normally, calling a non-async function within the event loop would will result
+in blocking the event loop until the function returns. However, by using a
+pool to run the function, it can be executed in a separate worker (such as a
+thread or process) without blocking the event loop.
+
+.. class:: ThreadPool(concurrency=None)
+
+   An asynchronous thread pool that provides methods to concurrently
+   run IO-bound functions, without blocking the event loop.
+
+   *concurrency* is an optional argument that limits the number of
+   threads to utilize in the thread pool. With the default value of
+   ``None``, the amount of threads used will scale based on the
+   number of processors.
+
+   .. coroutinemethod:: run(func, /, *args, **kwargs)
+
+      Asynchronously run *func* with its arguments and keyword-arguments
+      within the thread pool, and return a :class:`asyncio.Future` object
+      that represents the eventual result of its execution. ::
+
+          async with asyncio.ThreadPool() as pool:
+              await pool.run(time.sleep, 1)
+
+      Raises a :exc:`RuntimeError` if the thread pool is *not* running.
+
+   .. coroutinemethod:: astart()
+
+      Start the thread pool and spawn its threads. Note that
+      this function is called automatically when using ``asyncio.ThreadPool``
+      as an asynchronous context manager, and does not need to be called
+      directly.
+
+      Raises a :exc:`RuntimeError` if the thread pool is already running or
+      if it's been closed.
+
+   .. coroutinemethod:: aclose()
+
+      Close the thread pool. Note that this function is
+      called automatically when using ``asyncio.ThreadPool`` as an
+      asynchronous context manager, and does not need to be called directly.
+
+      Raises a :exc:`RuntimeError` if the thread pool has already been closed.
+
+Examples
+========
+
+Here's an example of concurrently running two IO-bound functions using
+:class:`asyncio.ThreadPool`::
+
+    import asyncio
+
+    def blocking_io():
+        print("start blocking_io")
+        with open('/dev/urandom', 'rb') as f:
+            f.read(100_000)
+        print("blocking_io complete")
+
+    def other_blocking_io():
+        print("start other_blocking_io")
+        with open('/dev/zero', 'rb') as f:
+            f.read(10)
+        print("other_blocking_io complete")
+
+    async def main():
+        async with asyncio.ThreadPool() as pool:
+            await asyncio.gather(
+                pool.run(blocking_io),
+                pool.run(other_blocking_io))
+
+    asyncio.run(main())

Doc/library/asyncio.rst

@@ -43,6 +43,8 @@ asyncio provides a set of **high-level** APIs to:
 
 * :ref:`synchronize <asyncio-sync>` concurrent code;
 
+* concurrently run blocking functions in a :ref:`pool <asyncio-pools>`;
+
 Additionally, there are **low-level** APIs for
 *library and framework developers* to:
 
@@ -73,6 +75,7 @@ Additionally, there are **low-level** APIs for
    asyncio-subprocess.rst
    asyncio-queue.rst
    asyncio-exceptions.rst
+   asyncio-pools.rst
 
 .. toctree::
    :caption: Low-level APIs

Doc/whatsnew/3.9.rst

@@ -137,6 +137,13 @@ details, see the documentation for ``loop.create_datagram_endpoint()``.
 (Contributed by Kyle Stanley, Antoine Pitrou, and Yury Selivanov in
 :issue:`37228`.)
 
+Added :class:`asyncio.ThreadPool`, an asynchronous context manager for
+concurrently running IO-bound functions without blocking the event loop.
+It essentially works as a higher-level version of
+:meth:`asyncio.loop.run_in_executor` that can take keyword arguments and
+be used as a context manager using ``async with``.
+(Contributed by Kyle Stanley in :issue:`32309`.)
+
 Added a new :term:`coroutine` :meth:`~asyncio.loop.shutdown_default_executor`
 that schedules a shutdown for the default executor that waits on the
 :class:`~concurrent.futures.ThreadPoolExecutor` to finish closing. Also,

Lib/asyncio/__init__.py

@@ -11,6 +11,7 @@
 from .exceptions import *
 from .futures import *
 from .locks import *
+from .pools import *
 from .protocols import *
 from .runners import *
 from .queues import *
@@ -29,6 +30,7 @@
            exceptions.__all__ +
            futures.__all__ +
            locks.__all__ +
+           pools.__all__ +
            protocols.__all__ +
            runners.__all__ +
            queues.__all__ +

Lib/asyncio/pools.py

@@ -0,0 +1,173 @@
+"""Support for high-level asynchronous pools in asyncio."""
+
+__all__ = 'ThreadPool',
+
+
+import concurrent.futures
+import functools
+import threading
+import os
+
+from abc import ABC, abstractmethod
+
+from . import events
+from . import exceptions
+from . import futures
+
+
+class AbstractPool(ABC):
+    """Abstract base class for asynchronous pools."""
+
+    @abstractmethod
+    async def astart(self):
+        raise NotImplementedError
+
+    @abstractmethod
+    async def __aenter__(self):
+        await self.astart()
+        return self
+
+    @abstractmethod
+    async def aclose(self):
+        raise NotImplementedError
+
+    @abstractmethod
+    async def __aexit__(self, exc_type, exc_value, exc_traceback):
+        await self.aclose()
+
+    @abstractmethod
+    async def run(self, func, /, *args, **kwargs):
+        """Asynchronously run function *func* using the pool.
+
+        Return a future, representing the eventual result of *func*.
+        """
+        raise NotImplementedError
+
+
+class ThreadPool(AbstractPool):
+    """Asynchronous threadpool for running IO-bound functions.
+
+    Directly calling an IO-bound function within the main thread will block
+    other operations from occurring until it is completed. By using a
+    threadpool, several IO-bound functions can be ran concurrently within
+    their own threads, without blocking other operations.
+
+    The optional argument *concurrency* sets the number of threads within the
+    threadpool. If *concurrency* is `None`, the maximum number of threads will
+    be used; based on the number of CPU cores.
+
+    This threadpool is intended to be used as an asynchronous context manager,
+    using the `async with` syntax, which provides automatic initialization and
+    finalization of resources. For example:
+
+    import asyncio
+
+    def blocking_io():
+        print("start blocking_io")
+        with open('/dev/urandom', 'rb') as f:
+            f.read(100_000)
+        print("blocking_io complete")
+
+    def other_blocking_io():
+        print("start other_blocking_io")
+        with open('/dev/zero', 'rb') as f:
+            f.read(10)
+        print("other_blocking_io complete")
+
+    async def main():
+        async with asyncio.ThreadPool() as pool:
+            await asyncio.gather(
+                pool.run(blocking_io),
+                pool.run(other_blocking_io))
+
+    asyncio.run(main())
+    """
+
+    def __init__(self, concurrency=None):
+        if concurrency is None:
+            concurrency = min(32, (os.cpu_count() or 1) + 4)
+
+        self._concurrency = concurrency
+        self._running = False
+        self._closed = False
+        self._loop = None
+        self._pool = None
+
+    async def astart(self):
+        self._loop = events.get_running_loop()
+        await self._spawn_threadpool()
+
+    async def __aenter__(self):
+        await self.astart()
+        return self
+
+    async def aclose(self):
+        await self._shutdown_threadpool()
+
+    async def __aexit__(self, exc_type, exc_value, exc_traceback):
+        await self.aclose()
+
+    async def run(self, /, func, *args, **kwargs):
+        if not self._running:
+            raise RuntimeError(f"unable to run {func!r}, "
+                               "threadpool is not running")
+
+        func_call = functools.partial(func, *args, **kwargs)
+        executor = self._pool
+        return await futures.wrap_future(
+            executor.submit(func_call), loop=self._loop)
+
+    async def _spawn_threadpool(self):
+        """Schedule the spawning of the threadpool.
+
+        Asynchronously spawns a threadpool with *concurrency* threads.
+        """
+        if self._running:
+            raise RuntimeError("threadpool is already running")
+
+        if self._closed:
+            raise RuntimeError("threadpool is closed")
+
+        future = self._loop.create_future()
+        thread = threading.Thread(target=self._do_spawn, args=(future,))
+        thread.start()
+        try:
+            await future
+        finally:
+            thread.join()
+
+    def _do_spawn(self, future):
+        try:
+            self._pool = concurrent.futures.ThreadPoolExecutor(
+                                                max_workers=self._concurrency)
+            self._running = True
+            self._loop.call_soon_threadsafe(future.set_result, None)
+        except Exception as ex:
+            self._loop.call_soon_threadsafe(future.set_exception, ex)
+
+    async def _shutdown_threadpool(self):
+        """Schedule the shutdown of the threadpool.
+
+        Asynchronously joins all of the threads in the threadpool.
+        """
+        if self._closed:
+            raise RuntimeError("threadpool is already closed")
+
+        # Set _running to False as early as possible
+        self._running = False
+
+        future = self._loop.create_future()
+        thread = threading.Thread(target=self._do_shutdown, args=(future,))
+        thread.start()
+        try:
+            await future
+        finally:
+            thread.join()
+
+    def _do_shutdown(self, future):
+        try:
+            self._pool.shutdown()
+            self._closed = True
+            self._loop.call_soon_threadsafe(future.set_result, None)
+        except Exception as ex:
+            self._loop.call_soon_threadsafe(future.set_exception, ex)