Use greenlets rather than threads for sync loop

docs-requirements.txt

@@ -5,3 +5,4 @@ towncrier
 trio >= 0.15.0
 outcome
 attrs
+greenlet

docs/source/principles.rst

@@ -168,10 +168,11 @@ this gap by providing two event loop implementations.
   installed a custom event loop policy, calling :func:`asyncio.new_event_loop`
   (including the implicit call made by the first :func:`asyncio.get_event_loop`
   in the main thread) will give you an event loop that transparently runs
-  in a separate thread in order to support multiple
+  in a separate greenlet in order to support multiple
   calls to :meth:`~asyncio.loop.run_until_complete`,
   :meth:`~asyncio.loop.run_forever`, and :meth:`~asyncio.loop.stop`.
   Sync loops are intended to allow trio-asyncio to run the existing
   test suites of large asyncio libraries, which often call
   :meth:`~asyncio.loop.run_until_complete` on the same loop multiple times.
-  Using them for other purposes is deprecated.
+  Using them for other purposes is not recommended (it is better to refactor
+  so you can use an async loop) but will probably work.

docs/source/usage.rst

@@ -104,7 +104,7 @@ Asyncio main loop
 +++++++++++++++++
 
 Sometimes you instead start with asyncio code which you wish to extend
-with some Trio portions. By far the best-supported approach here is to
+with some Trio portions. The best-supported approach here is to
 wrap your entire asyncio program in a Trio event loop. In other words,
 you should transform this code::
 
@@ -123,27 +123,18 @@ to this::
         trio_asyncio.run(trio_asyncio.aio_as_trio(async_main))
 
 If your program makes multiple calls to ``run_until_complete()`` and/or
-``run_forever()``, this may be a somewhat challenging transformation.
-In theory, you can instead keep the old approach (``get_event_loop()`` +
+``run_forever()``, or if the call to :func:`asyncio.run` is hidden inside
+a library you're using, then this may be a somewhat challenging transformation.
+In such cases, you can instead keep the old approach (``get_event_loop()`` +
 ``run_until_complete()``) unchanged, and if you've imported ``trio_asyncio``
 (and not changed the asyncio event loop policy) you'll still be able to use
 :func:`~trio_asyncio.trio_as_aio` to run Trio code from within your
-asyncio-flavored functions. In practice, this is not recommended, because:
-
-* It's implemented by running the contents of the loop in an
-  additional thread, so anything that expects to run on the main
-  thread (such as a signal handler) won't be happy.
-
-* The implementation is kind of a terrible hack.
-
-For these reasons, obtaining a new Trio-enabled asyncio event loop
-using the standard asyncio functions (:func:`asyncio.get_event_loop`,
-etc), rather than :func:`trio_asyncio.open_loop`, will raise a
-deprecation warning. (Except when running under pytest, because
-support for ``run_until_complete()`` is often needed to test asyncio
-libraries' test suites against trio-asyncio.) asyncio is transitioning
-towards the model of using a single top-level :func:`asyncio.run` call
-anyway, so the effort you spend on conversion won't be wasted.
+asyncio-flavored functions. This is referred to internally as a "sync loop"
+(``SyncTripEventLoop``), as contrasted with the "async loop" that you use
+when you start from an existing Trio run. The sync loop is implemented using
+the ``greenlet`` library to switch out of a Trio run that has not yet completed,
+so it is less well-supported than the approach where you start in Trio.
+But as of trio-asyncio 0.14.0, we do think it should generally work.
 
 Compatibility issues
 ++++++++++++++++++++
@@ -166,7 +157,7 @@ Interrupting the asyncio loop
 
 A trio-asyncio event loop created with :func:`open_loop` does not support
 ``run_until_complete`` or ``run_forever``. If you need these features,
-you might be able to get away with using a (deprecated) "sync loop" as
+you might be able to get away with using a "sync loop" as
 explained :ref:`above <asyncio-loop>`, but it's better to refactor
 your program so all of its async code runs within a single event loop
 invocation. For example, you might replace::
@@ -180,7 +171,7 @@ invocation. For example, you might replace::
     loop = asyncio.get_event_loop()
     loop.run_until_complete(setup)
     loop.run_forever()
-        
+
 with::
 
     stopped_event = trio.Event()
@@ -202,9 +193,7 @@ Detecting the current function's flavor
 
 :func:`sniffio.current_async_library` correctly reports "asyncio" or
 "trio" when called from a trio-asyncio program, based on the flavor of
-function that's calling it. (Some corner cases
-might not work on Pythons below 3.7 where asyncio doesn't support
-context variables.)
+function that's calling it.
 
 However, this feature should generally not be necessary, because you
 should know whether each function in your program is asyncio-flavored

newsfragments/137.feature.rst

@@ -0,0 +1,10 @@
+trio-asyncio now implements its :ref:`synchronous event loop <asyncio-loop>`
+(which is used when the top-level of your program is an asyncio call such as
+:func:`asyncio.run`, rather than a Trio call such as :func:`trio.run`)
+using the ``greenlet`` library rather than a separate thread. This provides
+some better theoretical grounding and fixes various edge cases around signal
+handling and other integrations; in particular, recent versions of IPython
+will no longer crash when importing trio-asyncio. Synchronous event loops have
+been un-deprecated with this change, though we still recommend using an
+async loop (``async with trio_asyncio.open_loop():`` from inside a Trio run)
+where possible.

setup.py

@@ -67,6 +67,7 @@
         "outcome",
         "sniffio >= 1.3.0",
         "exceptiongroup >= 1.0.0; python_version < '3.11'",
+        "greenlet",
     ],
     # This means, just install *everything* you see under trio/, even if it
     # doesn't look like a source file, so long as it appears in MANIFEST.in:

tests/conftest.py

@@ -123,10 +123,6 @@ def xfail(rel_id):
         def skip(rel_id):
             mark(pytest.mark.skip, rel_id)
 
-        # This hangs, probably due to the thread shenanigans (it works
-        # fine with a greenlet-based sync loop)
-        skip("test_base_events.py::RunningLoopTests::test_running_loop_within_a_loop")
-
         # Remainder of these have unclear issues
         if sys.version_info < (3, 8):
             xfail(
@@ -201,23 +197,7 @@ def skip(rel_id):
                 may_be_absent=True,
             )
 
-        if sys.version_info >= (3, 9):
-            # This tries to create a new loop from within an existing one,
-            # which we don't support.
-            xfail("test_locks.py::ConditionTests::test_ambiguous_loops")
-
         if sys.version_info >= (3, 12):
-            # This test sets signal handlers from within a coroutine,
-            # which doesn't work for us because SyncTrioEventLoop runs on
-            # a non-main thread.
-            xfail("test_unix_events.py::TestFork::test_fork_signal_handling")
-
-            # This test explicitly uses asyncio.tasks._c_current_task,
-            # bypassing our monkeypatch.
-            xfail(
-                "test_tasks.py::CCurrentLoopTests::test_current_task_with_implicit_loop"
-            )
-
             # These tests assume asyncio.sleep(0) is sufficient to run all pending tasks
             xfail(
                 "test_futures2.py::PyFutureTests::test_task_exc_handler_correct_context"

trio_asyncio/_base.py

@@ -492,41 +492,57 @@ def add_reader(self, fd, callback, *args):
         self._ensure_fd_no_transport(fd)
         return self._add_reader(fd, callback, *args)
 
-    def _add_reader(self, fd, callback, *args):
+    # Local helper to factor out common logic between _add_reader/_add_writer
+    def _add_io_handler(self, set_handle, wait_ready, fd, callback, args):
         self._check_closed()
         handle = ScopedHandle(callback, args, self)
-        reader = self._set_read_handle(fd, handle)
-        if reader is not None:
-            reader.cancel()
-        if self._token is None:
-            return
-        self._nursery.start_soon(self._reader_loop, fd, handle)
+        old_handle = set_handle(fd, handle)
 
-    def _set_read_handle(self, fd, handle):
-        try:
-            key = self._selector.get_key(fd)
-        except KeyError:
-            self._selector.register(fd, EVENT_READ, (handle, None))
+        if old_handle is not None:
+            old_handle.cancel()
+        if self._token is None:
             return None
-        else:
-            mask, (reader, writer) = key.events, key.data
-            self._selector.modify(fd, mask | EVENT_READ, (handle, writer))
-            return reader
+        self._nursery.start_soon(self._io_task, fd, handle, wait_ready)
+        return handle
 
-    async def _reader_loop(self, fd, handle):
+    async def _io_task(self, fd, handle, wait_ready):
         with handle._scope:
             try:
                 while True:
                     if handle._cancelled:
                         break
-                    await _wait_readable(fd)
+                    try:
+                        await wait_ready(fd)
+                    except OSError:
+                        # maybe someone did
+                        #   h = add_reader(sock); h.cancel(); sock.close()
+                        # without yielding to the event loop
+                        if handle._cancelled:
+                            break
+                        raise
                     if handle._cancelled:
                         break
                     handle._run()
                     await self.synchronize()
             except Exception as exc:
                 handle._raise(exc)
 
+    def _add_reader(self, fd, callback, *args):
+        return self._add_io_handler(
+            self._set_read_handle, _wait_readable, fd, callback, args
+        )
+
+    def _set_read_handle(self, fd, handle):
+        try:
+            key = self._selector.get_key(fd)
+        except KeyError:
+            self._selector.register(fd, EVENT_READ, (handle, None))
+            return None
+        else:
+            mask, (reader, writer) = key.events, key.data
+            self._selector.modify(fd, mask | EVENT_READ, (handle, writer))
+            return reader
+
     # writing to a file descriptor
 
     def add_writer(self, fd, callback, *args):
@@ -546,15 +562,10 @@ def add_writer(self, fd, callback, *args):
 
     # remove_writer: unchanged from asyncio
 
-    def _add_writer(self, fd, callback, *args):
-        self._check_closed()
-        handle = ScopedHandle(callback, args, self)
-        writer = self._set_write_handle(fd, handle)
-        if writer is not None:
-            writer.cancel()
-        if self._token is None:
-            return
-        self._nursery.start_soon(self._writer_loop, fd, handle)
+    def _add_writer(self, fd, callback, *args, _defer_start=False):
+        return self._add_io_handler(
+            self._set_write_handle, _wait_writable, fd, callback, args
+        )
 
     def _set_write_handle(self, fd, handle):
         try:
@@ -566,20 +577,6 @@ def _set_write_handle(self, fd, handle):
             self._selector.modify(fd, mask | EVENT_WRITE, (reader, handle))
             return writer
 
-    async def _writer_loop(self, fd, handle):
-        with handle._scope:
-            try:
-                while True:
-                    if handle._cancelled:
-                        break
-                    await _wait_writable(fd)
-                    if handle._cancelled:
-                        break
-                    handle._run()
-                    await self.synchronize()
-            except Exception as exc:
-                handle._raise(exc)
-
     def autoclose(self, fd):
         """
         Mark a file descriptor so that it's auto-closed along with this loop.
@@ -752,6 +749,7 @@ async def _main_loop_exit(self):
         # clean core fields
         self._nursery = None
         self._task = None
+        self._token = None
 
     def is_running(self):
         if self._stopped is None:
@@ -778,6 +776,11 @@ async def wait_stopped(self):
         """
         await self._stopped.wait()
 
+    def _trio_io_cancel(self, cancel_scope):
+        """Called when a ScopedHandle representing an I/O reader or writer
+        has its cancel() method called."""
+        cancel_scope.cancel()
+
     def stop(self):
         """Halt the main loop.
 

trio_asyncio/_handles.py

@@ -31,7 +31,7 @@ def __init__(self, *args, **kw):
 
     def cancel(self):
         super().cancel()
-        self._scope.cancel()
+        self._loop._trio_io_cancel(self._scope)
 
     def _repr_info(self):
         return super()._repr_info() + ["scope={!r}".format(self._scope)]

trio_asyncio/_loop.py

@@ -11,7 +11,6 @@
 
 from ._async import TrioEventLoop
 from ._util import run_aio_future
-from ._deprecate import warn_deprecated
 
 try:
     from trio.lowlevel import wait_for_child
@@ -106,26 +105,29 @@ def _in_trio_context():
         return True
 
 
+_sync_loop_task_name = "trio_asyncio sync loop task"
+
+
+def _in_trio_context_other_than_sync_loop():
+    try:
+        return trio.lowlevel.current_task().name != _sync_loop_task_name
+    except RuntimeError:
+        return False
+
+
 class _TrioPolicy(asyncio.events.BaseDefaultEventLoopPolicy):
     @staticmethod
     def _loop_factory():
         raise RuntimeError("Event loop creations shouldn't get here")
 
     def new_event_loop(self):
-        if _in_trio_context():
+        if _in_trio_context_other_than_sync_loop():
             raise RuntimeError(
                 "You're within a Trio environment.\n"
                 "Use 'async with open_loop()' instead."
             )
         if _faked_policy.policy is not None:
             return _faked_policy.policy.new_event_loop()
-        if "pytest" not in sys.modules:
-            warn_deprecated(
-                "Using trio-asyncio outside of a Trio event loop",
-                "0.10.0",
-                issue=None,
-                instead=None,
-            )
 
         from ._sync import SyncTrioEventLoop
 
@@ -220,7 +222,7 @@ def _new_policy_get():
 def _new_policy_set(new_policy):
     if isinstance(new_policy, TrioPolicy):
         raise RuntimeError("You can't set the Trio loop policy manually")
-    if _in_trio_context():
+    if _in_trio_context_other_than_sync_loop():
         raise RuntimeError("You can't change the event loop policy in Trio context")
     if new_policy is not None and not isinstance(
         new_policy, asyncio.AbstractEventLoopPolicy