Add simple web loop (pyodide#1158)

jmsmdy · Feb 5, 2021 · a4fad9b · a4fad9b
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diff --git a/docs/project/changelog.md b/docs/project/changelog.md
@@ -58,6 +58,14 @@
   raise a `KeyboardInterrupt` by writing to the interrupt buffer.
   [#1148](https://github.com/iodide-project/pyodide/pull/1148) and
   [#1173](https://github.com/iodide-project/pyodide/pull/1173)
+- A `JsProxy` of a Javascript `Promise` or other awaitable object is now a 
+  Python awaitable.
+  [#880](https://github.com/iodide-project/pyodide/pull/880)
+- Added a Python event loop to support asyncio by scheduling coroutines to run 
+  as jobs on the browser event loop. This event loop is available by default and 
+  automatically enabled by any relevant asyncio API, so for instance 
+  `asyncio.ensure_future` works without any configuration.
+  [#1158](https://github.com/iodide-project/pyodide/pull/1158)
 - Made PyProxy of an iterable Python object an iterable Js object: defined the
   `[Symbol.iterator]` method, can be used like `for(let x of proxy)`.
   Made a PyProxy of a Python iterator an iterator: `proxy.next()` is

diff --git a/docs/usage/api-reference.md b/docs/usage/api-reference.md
@@ -21,6 +21,7 @@ Backward compatibility of the API is not guaranteed at this point.
    pyodide.console.InteractiveConsole
    pyodide.console.repr_shorten
    pyodide.console.displayhook
+   pyodide.webloop.WebLoop
 ```
 
 

diff --git a/src/pyodide-py/pyodide/__init__.py b/src/pyodide-py/pyodide/__init__.py
@@ -1,13 +1,19 @@
 from ._base import open_url, eval_code, find_imports, as_nested_list
 from ._core import JsException  # type: ignore
 from ._importhooks import JsFinder
+from .webloop import WebLoopPolicy
+import asyncio
 import sys
+import platform
 
 jsfinder = JsFinder()
 register_js_module = jsfinder.register_js_module
 unregister_js_module = jsfinder.unregister_js_module
 sys.meta_path.append(jsfinder)  # type: ignore
 
+if platform.system() == "Emscripten":
+    asyncio.set_event_loop_policy(WebLoopPolicy())
+
 
 __version__ = "0.16.1"
 

diff --git a/src/pyodide-py/pyodide/webloop.py b/src/pyodide-py/pyodide/webloop.py
@@ -0,0 +1,247 @@
+import asyncio
+from asyncio import tasks, futures
+import time
+import contextvars
+
+
+from typing import Awaitable, Callable
+
+
+class WebLoop(asyncio.AbstractEventLoop):
+    """A custom event loop for use in Pyodide.
+
+    Schedules tasks on the browser event loop. Does no lifecycle management and runs
+    forever.
+
+    ``run_forever`` and ``run_until_complete`` cannot block like a normal event loop would
+    because we only have one thread so blocking would stall the browser event loop
+    and prevent anything from ever happening.
+
+    We defer all work to the browser event loop using the setTimeout function.
+    To ensure that this event loop doesn't stall out UI and other browser handling,
+    we want to make sure that each task is scheduled on the browser event loop as a
+    task not as a microtask. ``setTimeout(callback, 0)`` enqueues the callback as a
+    task so it works well for our purposes.
+    """
+
+    def __init__(self):
+        self._task_factory = None
+        asyncio._set_running_loop(self)
+
+    def get_debug(self):
+        return False
+
+    #
+    # Lifecycle methods: We ignore all lifecycle management
+    #
+
+    def is_running(self) -> bool:
+        """Returns ``True`` if the event loop is running.
+
+        Always returns ``True`` because WebLoop has no lifecycle management.
+        """
+        return True
+
+    def is_closed(self) -> bool:
+        """Returns ``True`` if the event loop was closed.
+
+        Always returns ``False`` because WebLoop has no lifecycle management.
+        """
+        return False
+
+    def _check_closed(self):
+        """Used in create_task.
+
+        Would raise an error if ``self.is_closed()``, but we are skipping all lifecycle stuff.
+        """
+        pass
+
+    def run_forever(self):
+        """Run the event loop forever. Does nothing in this implementation.
+
+        We cannot block like a normal event loop would
+        because we only have one thread so blocking would stall the browser event loop
+        and prevent anything from ever happening.
+        """
+        pass
+
+    def run_until_complete(self, future: Awaitable):
+        """Run until future is done.
+
+        If the argument is a coroutine, it is wrapped in a Task.
+
+        The native event loop `run_until_complete` blocks until evaluation of the
+        future is complete and then returns the result of the future.
+        Since we cannot block, we just ensure that the future is scheduled and
+        return the future. This makes this method a bit useless. Instead, use
+        `future.add_done_callback(do_something_with_result)` or:
+        ```python
+        async def wrapper():
+            result = await future
+            do_something_with_result(result)
+        ```
+        """
+        return asyncio.ensure_future(future)
+
+    #
+    # Scheduling methods: use browser.setTimeout to schedule tasks on the browser event loop.
+    #
+
+    def call_soon(self, callback: Callable, *args, context: contextvars.Context = None):
+        """Arrange for a callback to be called as soon as possible.
+
+        Any positional arguments after the callback will be passed to
+        the callback when it is called.
+
+        This schedules the callback on the browser event loop using ``setTimeout(callback, 0)``.
+        """
+        delay = 0
+        return self.call_later(delay, callback, *args, context=context)
+
+    def call_soon_threadsafe(
+        callback: Callable, *args, context: contextvars.Context = None
+    ):
+        """Like ``call_soon()``, but thread-safe.
+
+        We have no threads so everything is "thread safe", and we just use ``call_soon``.
+        """
+        return self.call_soon(callback, *args, context=context)
+
+    def call_later(
+        self,
+        delay: float,
+        callback: Callable,
+        *args,
+        context: contextvars.Context = None
+    ):
+        """Arrange for a callback to be called at a given time.
+
+        Return a Handle: an opaque object with a cancel() method that
+        can be used to cancel the call.
+
+        The delay can be an int or float, expressed in seconds.  It is
+        always relative to the current time.
+
+        Each callback will be called exactly once.  If two callbacks
+        are scheduled for exactly the same time, it undefined which
+        will be called first.
+
+        Any positional arguments after the callback will be passed to
+        the callback when it is called.
+
+        This uses `setTimeout(callback, delay)`
+        """
+        from js import setTimeout
+
+        if delay < 0:
+            raise ValueError("Can't schedule in the past")
+        h = asyncio.Handle(callback, args, self, context=context)
+        setTimeout(h._run, delay * 1000)
+        return h
+
+    def call_at(
+        self,
+        when: float,
+        callback: Callable,
+        *args,
+        context: contextvars.Context = None
+    ):
+        """Like ``call_later()``, but uses an absolute time.
+
+        Absolute time corresponds to the event loop's ``time()`` method.
+
+        This uses ``setTimeout(callback, when - cur_time)``
+        """
+        cur_time = self.time()
+        delay = when - cur_time
+        return self.call_later(delay, callback, *args, context=context)
+
+    #
+    # The remaining methods are copied directly from BaseEventLoop
+    #
+
+    def time(self):
+        """Return the time according to the event loop's clock.
+
+        This is a float expressed in seconds since an epoch, but the
+        epoch, precision, accuracy and drift are unspecified and may
+        differ per event loop.
+
+        Copied from ``BaseEventLoop.time``
+        """
+        return time.monotonic()
+
+    def create_future(self):
+        """Create a Future object attached to the loop.
+
+        Copied from ``BaseEventLoop.create_future``
+        """
+        return futures.Future(loop=self)
+
+    def create_task(self, coro, *, name=None):
+        """Schedule a coroutine object.
+
+        Return a task object.
+
+        Copied from ``BaseEventLoop.create_task``
+        """
+        self._check_closed()
+        if self._task_factory is None:
+            task = tasks.Task(coro, loop=self, name=name)
+            if task._source_traceback:
+                # Added comment:
+                # this only happens if get_debug() returns True.
+                # In that case, remove create_task from _source_traceback.
+                del task._source_traceback[-1]
+        else:
+            task = self._task_factory(self, coro)
+            tasks._set_task_name(task, name)
+
+        return task
+
+    def set_task_factory(self, factory):
+        """Set a task factory that will be used by loop.create_task().
+
+        If factory is None the default task factory will be set.
+
+        If factory is a callable, it should have a signature matching
+        '(loop, coro)', where 'loop' will be a reference to the active
+        event loop, 'coro' will be a coroutine object.  The callable
+        must return a Future.
+
+        Copied from ``BaseEventLoop.set_task_factory``
+        """
+        if factory is not None and not callable(factory):
+            raise TypeError("task factory must be a callable or None")
+        self._task_factory = factory
+
+    def get_task_factory(self):
+        """Return a task factory, or None if the default one is in use.
+
+        Copied from ``BaseEventLoop.get_task_factory``
+        """
+        return self._task_factory
+
+
+class WebLoopPolicy(asyncio.DefaultEventLoopPolicy):
+    """
+    A simple event loop policy for managing WebLoop based event loops.
+    """
+
+    def __init__(self):
+        self._default_loop = None
+
+    def get_event_loop(self):
+        """Get the current event loop"""
+        if self._default_loop:
+            return self._default_loop
+        return self.new_event_loop()
+
+    def new_event_loop(self):
+        """Create a new event loop"""
+        self._default_loop = WebLoop()
+        return self._default_loop
+
+    def set_event_loop(self, loop: asyncio.AbstractEventLoop):
+        """Set the current event loop"""
+        self._default_loop = loop
diff --git a/src/pyodide.js b/src/pyodide.js
@@ -491,8 +491,9 @@ globalThis.languagePluginLoader = new Promise((resolve, reject) => {
     let response = await fetch(`${baseURL}packages.json`);
     let json = await response.json();
     fixRecursionLimit(self.pyodide);
-    self.pyodide.registerJsModule("js", globalThis);
     self.pyodide = makePublicAPI(self.pyodide, PUBLIC_API);
+    self.pyodide.registerJsModule("js", globalThis);
+    self.pyodide.registerJsModule("pyodide_js", self.pyodide);
     self.pyodide._module.packages = json;
     resolve();
   };