Add run_sync and ensure_async functions (jupyter#315)

(cherry picked from commit 51b8a12)

jupyter_core/tests/test_async.py

@@ -0,0 +1,34 @@
+"""Tests for async helper functions"""
+
+# Copyright (c) Jupyter Development Team.
+# Distributed under the terms of the Modified BSD License.
+
+import asyncio
+
+from jupyter_core.utils import ensure_async, run_sync
+
+
+async def afunc():
+    return "afunc"
+
+
+def func():
+    return "func"
+
+
+sync_afunc = run_sync(afunc)
+
+
+def test_ensure_async():
+    async def main():
+        assert await ensure_async(afunc()) == "afunc"
+        assert await ensure_async(func()) == "func"
+
+    asyncio.run(main())
+
+
+def test_run_sync():
+    async def main():
+        assert sync_afunc() == "afunc"
+
+    asyncio.run(main())

jupyter_core/utils/__init__.py

@@ -1,5 +1,13 @@
+# Copyright (c) Jupyter Development Team.
+# Distributed under the terms of the Modified BSD License.
+
+import asyncio
+import atexit
 import errno
+import inspect
 import os
+import threading
+from typing import Any, Awaitable, Callable, Optional, TypeVar, Union
 
 
 def ensure_dir_exists(path, mode=0o777):
@@ -15,3 +23,96 @@ def ensure_dir_exists(path, mode=0o777):
             raise
     if not os.path.isdir(path):
         raise OSError("%r exists but is not a directory" % path)
+
+
+T = TypeVar("T")
+
+
+class _TaskRunner:
+    """A task runner that runs an asyncio event loop on a background thread."""
+
+    def __init__(self):
+        self.__io_loop: Optional[asyncio.AbstractEventLoop] = None
+        self.__runner_thread: Optional[threading.Thread] = None
+        self.__lock = threading.Lock()
+        atexit.register(self._close)
+
+    def _close(self):
+        if self.__io_loop:
+            self.__io_loop.stop()
+
+    def _runner(self):
+        loop = self.__io_loop
+        assert loop is not None
+        try:
+            loop.run_forever()
+        finally:
+            loop.close()
+
+    def run(self, coro):
+        """Synchronously run a coroutine on a background thread."""
+        with self.__lock:
+            name = f"{threading.current_thread().name} - runner"
+            if self.__io_loop is None:
+                self.__io_loop = asyncio.new_event_loop()
+                self.__runner_thread = threading.Thread(target=self._runner, daemon=True, name=name)
+                self.__runner_thread.start()
+        fut = asyncio.run_coroutine_threadsafe(coro, self.__io_loop)
+        return fut.result(None)
+
+
+_runner_map = {}
+_loop_map = {}
+
+
+def run_sync(coro: Callable[..., Awaitable[T]]) -> Callable[..., T]:
+    """Runs a coroutine and blocks until it has executed.
+
+    Parameters
+    ----------
+    coro : coroutine
+        The coroutine to be executed.
+    Returns
+    -------
+    result :
+        Whatever the coroutine returns.
+    """
+
+    def wrapped(*args, **kwargs):
+        name = threading.current_thread().name
+        inner = coro(*args, **kwargs)
+        try:
+            # If a loop is currently running in this thread,
+            # use a task runner.
+            asyncio.get_running_loop()
+            if name not in _runner_map:
+                _runner_map[name] = _TaskRunner()
+            return _runner_map[name].run(inner)
+        except RuntimeError:
+            pass
+
+        # Run the loop for this thread.
+        if name not in _loop_map:
+            _loop_map[name] = asyncio.new_event_loop()
+        loop = _loop_map[name]
+        return loop.run_until_complete(inner)
+
+    wrapped.__doc__ = coro.__doc__
+    return wrapped
+
+
+async def ensure_async(obj: Union[Awaitable[Any], Any]) -> Any:
+    """Convert a non-awaitable object to a coroutine if needed,
+    and await it if it was not already awaited.
+    """
+    if inspect.isawaitable(obj):
+        try:
+            result = await obj
+        except RuntimeError as e:
+            if str(e) == "cannot reuse already awaited coroutine":
+                # obj is already the coroutine's result
+                return obj
+            raise
+        return result
+    # obj doesn't need to be awaited
+    return obj