Docs and one small improvement for issue #25304, by Vincent Michel.

Doc/library/asyncio-dev.rst

@@ -96,10 +96,9 @@ the same thread. But when the task uses ``yield from``, the task is suspended
 and the event loop executes the next task.
 
 To schedule a callback from a different thread, the
-:meth:`BaseEventLoop.call_soon_threadsafe` method should be used. Example to
-schedule a coroutine from a different thread::
+:meth:`BaseEventLoop.call_soon_threadsafe` method should be used. Example::
 
-    loop.call_soon_threadsafe(asyncio.async, coro_func())
+    loop.call_soon_threadsafe(callback, *args)
 
 Most asyncio objects are not thread safe. You should only worry if you access
 objects outside the event loop. For example, to cancel a future, don't call
@@ -107,6 +106,13 @@ directly its :meth:`Future.cancel` method, but::
 
     loop.call_soon_threadsafe(fut.cancel)
 
+To schedule a coroutine object from a different thread, the
+:func:`run_coroutine_threadsafe` function should be used. It returns a
+:class:`concurrent.futures.Future` to access the result::
+
+     future = asyncio.run_coroutine_threadsafe(coro_func(), loop)
+     result = future.result(timeout)  # Wait for the result with a timeout
+
 To handle signals and to execute subprocesses, the event loop must be run in
 the main thread.
 

Doc/library/asyncio-task.rst

@@ -661,3 +661,42 @@ Task functions
    .. versionchanged:: 3.4.3
       If the wait is cancelled, the future *fut* is now also cancelled.
 
+
+.. function:: run_coroutine_threadsafe(coro, loop)
+
+   Submit a :ref:`coroutine object <coroutine>` to a given event loop.
+
+   Return a :class:`concurrent.futures.Future` to access the result.
+
+   This function is meant to be called from a different thread than the one
+   where the event loop is running. Usage::
+
+     # Create a coroutine
+     coro = asyncio.sleep(1, result=3)
+     # Submit the coroutine to a given loop
+     future = asyncio.run_coroutine_threadsafe(coro, loop)
+     # Wait for the result with an optional timeout argument
+     assert future.result(timeout) == 3
+
+   If an exception is raised in the coroutine, the returned future will be
+   notified. It can also be used to cancel the task in the event loop::
+
+     try:
+         result = future.result(timeout)
+     except asyncio.TimeoutError:
+         print('The coroutine took too long, cancelling the task...')
+         future.cancel()
+     except Exception as exc:
+         print('The coroutine raised an exception: {!r}'.format(exc))
+     else:
+         print('The coroutine returned: {!r}'.format(result))
+
+   See the :ref:`concurrency and multithreading <asyncio-multithreading>`
+   section of the documentation.
+
+   .. note::
+
+      Unlike the functions above, :func:`run_coroutine_threadsafe` requires the
+      *loop* argument to be passed explicitely.
+
+   .. versionadded:: 3.4.4

Lib/asyncio/tasks.py

@@ -704,7 +704,12 @@ def run_coroutine_threadsafe(coro, loop):
     future = concurrent.futures.Future()
 
     def callback():
-        futures._chain_future(ensure_future(coro, loop=loop), future)
+        try:
+            futures._chain_future(ensure_future(coro, loop=loop), future)
+        except Exception as exc:
+            if future.set_running_or_notify_cancel():
+                future.set_exception(exc)
+            raise
 
     loop.call_soon_threadsafe(callback)
     return future

Lib/test/test_asyncio/test_tasks.py

@@ -2166,6 +2166,27 @@ class RunCoroutineThreadsafeTests(test_utils.TestCase):
         with self.assertRaises(asyncio.CancelledError):
             self.loop.run_until_complete(future)
 
+    def test_run_coroutine_threadsafe_task_factory_exception(self):
+        """Test coroutine submission from a tread to an event loop
+        when the task factory raise an exception."""
+        # Clear the time generator
+        asyncio.ensure_future(self.add(1, 2), loop=self.loop)
+        # Schedule the target
+        future = self.loop.run_in_executor(None, self.target)
+        # Set corrupted task factory
+        self.loop.set_task_factory(lambda loop, coro: wrong_name)
+        # Set exception handler
+        callback = test_utils.MockCallback()
+        self.loop.set_exception_handler(callback)
+        # Run event loop
+        with self.assertRaises(NameError) as exc_context:
+            self.loop.run_until_complete(future)
+        # Check exceptions
+        self.assertIn('wrong_name', exc_context.exception.args[0])
+        self.assertEqual(len(callback.call_args_list), 1)
+        (loop, context), kwargs = callback.call_args
+        self.assertEqual(context['exception'], exc_context.exception)
+
 
 if __name__ == '__main__':
     unittest.main()