Merge pull request #95 from goodboy/try_trip

Support trio-run-in-process as process spawning backend

.travis.yml

@@ -4,7 +4,7 @@ sudo: required
 
 matrix:
     include:
-        - name: "Windows, Python Latest"
+        - name: "Windows, Python Latest: multiprocessing"
           os: windows
           language: sh
           python: 3.x  # only works on linux
@@ -13,7 +13,7 @@ matrix:
               - export PATH="/c/Python:/c/Python/Scripts:$PATH"
               - python -m pip install --upgrade pip wheel
 
-        - name: "Windows, Python 3.7"
+        - name: "Windows, Python 3.7: multiprocessing"
           os: windows
           python: 3.7  # only works on linux
           language: sh
@@ -22,8 +22,19 @@ matrix:
               - export PATH="/c/Python:/c/Python/Scripts:$PATH"
               - python -m pip install --upgrade pip wheel
 
-        - python: 3.7  # this works for Linux but is ignored on macOS or Windows
+        - name: "Python 3.7: multiprocessing"
-        - python: 3.8
+          python: 3.7  # this works for Linux but is ignored on macOS or Windows
+          env: SPAWN_BACKEND="mp"
+        - name: "Python 3.7: trio-run-in-process"
+          python: 3.7  # this works for Linux but is ignored on macOS or Windows
+          env: SPAWN_BACKEND="trio_run_in_process"
+
+        - name: "Python 3.8: multiprocessing"
+          python: 3.8  # this works for Linux but is ignored on macOS or Windows
+          env: SPAWN_BACKEND="mp"
+        - name: "Python 3.8: trio-run-in-process"
+          python: 3.8  # this works for Linux but is ignored on macOS or Windows
+          env: SPAWN_BACKEND="trio_run_in_process"
 
 install:
     - cd $TRAVIS_BUILD_DIR
@@ -32,4 +43,4 @@ install:
 
 script:
     - mypy tractor/ --ignore-missing-imports
-    - pytest tests/ --no-print-logs
+    - pytest tests/ --no-print-logs --spawn-backend=${SPAWN_BACKEND}

README.rst

@@ -83,8 +83,6 @@ Its tenets non-comprehensively include:
     are greatly appreciated!
 
 .. _concept-in-progress: https://trio.discourse.group/t/structured-concurrency-kickoff/55
-.. _pulsar: http://quantmind.github.io/pulsar/design.html
-.. _execnet: https://codespeak.net/execnet/
 
 
 Install
@@ -357,14 +355,15 @@ Depending on the function type ``Portal.run()`` tries to
 correctly interface exactly like a local version of the remote
 built-in Python *function type*. Currently async functions, generators,
 and regular functions are supported. Inspiration for this API comes
-from the way execnet_ does `remote function execution`_ but without
+`remote function execution`_ but without the client code being
-the client code (necessarily) having to worry about the underlying
+concerned about the underlying channels_ system or shipping code
-channels_ system or shipping code over the network.
+over the network.
 
 This *portal* approach turns out to be paricularly exciting with the
 introduction of `asynchronous generators`_ in Python 3.6! It means that
 actors can compose nicely in a data streaming pipeline.
 
+.. _exactly like trio: https://trio.readthedocs.io/en/latest/reference-core.html#cancellation-semantics
 
 Streaming
 *********
@@ -703,20 +702,54 @@ need to hop into a debugger. You just need to pass the existing
     tractor.run(main, arbiter_addr=('192.168.0.10', 1616))
 
 
-Choosing a ``multiprocessing`` *start method*
+Choosing a process spawning backend
-*********************************************
+***********************************
-``tractor`` supports selection of the `multiprocessing start method`_ via
+``tractor`` is architected to support multiple actor (sub-process)
-a ``start_method`` kwarg to ``tractor.run()``. Note that on Windows
+spawning backends. Specific defaults are chosen based on your system
-*spawn* it the only supported method and on nix systems *forkserver* is
+but you can also explicitly select a backend of choice at startup
-selected by default for speed.
+via a ``start_method`` kwarg to ``tractor.run()``.
 
-.. _multiprocessing start method: https://docs.python.org/3/library/multiprocessing.html#contexts-and-start-methods
+Currently the options available are:
 
+- ``trio_run_in_process``: a ``trio``-native spawner from the `Ethereum community`_
+- ``spawn``: one of the stdlib's ``multiprocessing`` `start methods`_
+- ``forkserver``: a faster ``multiprocessing`` variant that is Unix only
+
+.. _start methods: https://docs.python.org/3.8/library/multiprocessing.html#contexts-and-start-methods
+.. _Ethereum community : https://github.com/ethereum/trio-run-in-process
+
+
+``trio-run-in-process``
++++++++++++++++++++++++
+`trio-run-in-process`_ is a young "pure ``trio``" process spawner
+which utilizes the native `trio subprocess APIs`_. It has shown great
+reliability under testing for predictable teardown when launching
+recursive pools of actors (multiple nurseries deep) and as such has been
+chosen as the default backend on \*nix systems.
+
+.. _trio-run-in-process: https://github.com/ethereum/trio-run-in-process
+.. _trio subprocess APIs : https://trio.readthedocs.io/en/stable/reference-io.html#spawning-subprocesses
+
+
+``multiprocessing``
++++++++++++++++++++
+There is support for the stdlib's ``multiprocessing`` `start methods`_.
+Note that on Windows *spawn* it the only supported method and on \*nix
+systems *forkserver* is the best method for speed but has the caveat
+that it will break easily (hangs due to broken pipes) if spawning actors
+using nested nurseries.
+
+In general, the ``multiprocessing`` backend **has not proven reliable**
+for handling errors from actors more then 2 nurseries *deep* (see `#89`_).
+If you for some reason need this consider sticking with alternative
+backends.
+
+.. _#89: https://github.com/goodboy/tractor/issues/89
 
 Windows "gotchas"
-*****************
+^^^^^^^^^^^^^^^^^
-`tractor` internally uses the stdlib's `multiprocessing` package which
+On Windows (which requires the use of the stdlib's `multiprocessing`
-*can* have some gotchas on Windows. Namely, the need for calling
+package) there are some gotchas. Namely, the need for calling
 `freeze_support()`_ inside the ``__main__`` context.  Additionally you
 may need place you `tractor` program entry point in a seperate
 `__main__.py` module in your package in order to avoid an error like the

mypy.ini

@@ -0,0 +1,2 @@
+[mypy]
+plugins = trio_typing.plugin

setup.py

@@ -39,6 +39,7 @@ setup(
     ],
     install_requires=[
         'msgpack', 'trio>0.8', 'async_generator', 'colorlog', 'wrapt',
+        'trio_typing', 'trio-run-in-process',
     ],
     tests_require=['pytest'],
     python_requires=">=3.7",

tests/conftest.py

@@ -2,6 +2,7 @@
 ``tractor`` testing!!
 """
 import random
+import platform
 
 import pytest
 import tractor
@@ -13,8 +14,28 @@ _arb_addr = '127.0.0.1', random.randint(1000, 9999)
 
 
 def pytest_addoption(parser):
-    parser.addoption("--ll", action="store", dest='loglevel',
+    parser.addoption(
-                     default=None, help="logging level to set when testing")
+        "--ll", action="store", dest='loglevel',
+        default=None, help="logging level to set when testing"
+    )
+
+    parser.addoption(
+        "--spawn-backend", action="store", dest='spawn_backend',
+        default='trio_run_in_process',
+        help="Processing spawning backend to use for test run",
+    )
+
+
+def pytest_configure(config):
+    backend = config.option.spawn_backend
+
+    if platform.system() == "Windows":
+        backend = 'mp'
+
+    if backend == 'mp':
+        tractor._spawn.try_set_start_method('spawn')
+    elif backend == 'trio_run_in_process':
+        tractor._spawn.try_set_start_method(backend)
 
 
 @pytest.fixture(scope='session', autouse=True)
@@ -31,11 +52,26 @@ def arb_addr():
 
 
 def pytest_generate_tests(metafunc):
+    spawn_backend = metafunc.config.option.spawn_backend
+    if not spawn_backend:
+        # XXX some weird windows bug with `pytest`?
+        spawn_backend = 'mp'
+    assert spawn_backend in ('mp', 'trio_run_in_process')
+
     if 'start_method' in metafunc.fixturenames:
+        if spawn_backend == 'mp':
             from multiprocessing import get_all_start_methods
             methods = get_all_start_methods()
-        if 'fork' in methods:  # fork not available on windows, so check before removing
+            if 'fork' in methods:
-            # XXX: the fork method is in general incompatible with
+                # fork not available on windows, so check before
-            # trio's global scheduler state
+                # removing XXX: the fork method is in general
+                # incompatible with trio's global scheduler state
                 methods.remove('fork')
+        elif spawn_backend == 'trio_run_in_process':
+            if platform.system() == "Windows":
+                pytest.fail(
+                    "Only `--spawn-backend=mp` is supported on Windows")
+
+            methods = ['trio_run_in_process']
+
         metafunc.parametrize("start_method", methods, scope='module')

tests/test_cancellation.py

@@ -289,11 +289,18 @@ async def test_nested_multierrors(loglevel, start_method):
     This test goes only 2 nurseries deep but we should eventually have tests
     for arbitrary n-depth actor trees.
     """
-    # XXX: forkserver can't seem to handle any more then 2 depth
+    if start_method == 'trio_run_in_process':
-    # process trees for whatever reason.
-    # Any more process levels then this and we start getting pretty slow anyway
         depth = 3
         subactor_breadth = 2
+    else:
+        # XXX: multiprocessing can't seem to handle any more then 2 depth
+        # process trees for whatever reason.
+        # Any more process levels then this and we see bugs that cause
+        # hangs and broken pipes all over the place...
+        if start_method == 'forkserver':
+            pytest.skip("Forksever sux hard at nested spawning...")
+        depth = 2
+        subactor_breadth = 2
 
     with trio.fail_after(120):
         try:

tests/test_rpc.py

@@ -60,7 +60,7 @@ def test_rpc_errors(arb_addr, to_call, testdir):
     if exposed_mods == ['tmp_mod']:
         # create an importable module with a bad import
         testdir.syspathinsert()
-        # module should cause raise a ModuleNotFoundError at import
+        # module should raise a ModuleNotFoundError at import
         testdir.makefile('.py', tmp_mod=funcname)
 
         # no need to exposed module to the subactor
@@ -69,7 +69,9 @@ def test_rpc_errors(arb_addr, to_call, testdir):
         func_defined = False
         # subactor should not try to invoke anything
         subactor_requests_to = None
-        remote_err = trio.MultiError
+        # the module will be attempted to be imported locally but will
+        # fail in the initial local instance of the actor
+        remote_err = inside_err
 
     async def main():
         actor = tractor.current_actor()
@@ -100,7 +102,7 @@ def test_rpc_errors(arb_addr, to_call, testdir):
     if exposed_mods and func_defined:
         run()
     else:
-        # underlying errors are propagated upwards (yet)
+        # underlying errors aren't propagated upwards (yet)
         with pytest.raises(remote_err) as err:
             run()
 
@@ -114,4 +116,5 @@ def test_rpc_errors(arb_addr, to_call, testdir):
                 value.exceptions
             ))
 
+        if getattr(value, 'type', None):
             assert value.type is inside_err

tractor/__init__.py

@@ -99,15 +99,17 @@ def run(
     name: Optional[str] = None,
     arbiter_addr: Tuple[str, int] = (
         _default_arbiter_host, _default_arbiter_port),
-    # the `multiprocessing` start method:
+    # either the `multiprocessing` start method:
     # https://docs.python.org/3/library/multiprocessing.html#contexts-and-start-methods
-    start_method: str = 'forkserver',
+    # OR `trio_run_in_process` (the new default).
+    start_method: Optional[str] = None,
     **kwargs,
 ) -> Any:
     """Run a trio-actor async function in process.
 
     This is tractor's main entry and the start point for any async actor.
     """
+    if start_method is not None:
         _spawn.try_set_start_method(start_method)
     return trio.run(_main, async_fn, args, kwargs, arbiter_addr, name)
 

tractor/_actor.py

@@ -5,10 +5,14 @@ from collections import defaultdict
 from functools import partial
 from itertools import chain
 import importlib
+import importlib.util
 import inspect
 import uuid
 import typing
 from typing import Dict, List, Tuple, Any, Optional
+from types import ModuleType
+import sys
+import os
 
 import trio  # type: ignore
 from trio_typing import TaskStatus
@@ -25,6 +29,7 @@ from ._exceptions import (
 from ._discovery import get_arbiter
 from ._portal import Portal
 from . import _state
+from . import _mp_fixup_main
 
 
 log = get_logger('tractor')
@@ -148,6 +153,10 @@ async def _invoke(
             actor._ongoing_rpc_tasks.set()
 
 
+def _get_mod_abspath(module):
+    return os.path.abspath(module.__file__)
+
+
 class Actor:
     """The fundamental concurrency primitive.
 
@@ -162,6 +171,14 @@ class Actor:
     _root_nursery: trio.Nursery
     _server_nursery: trio.Nursery
 
+    # marked by the process spawning backend at startup
+    # will be None for the parent most process started manually
+    # by the user (currently called the "arbiter")
+    _spawn_method: Optional[str] = None
+
+    # Information about `__main__` from parent
+    _parent_main_data: Dict[str, str]
+
     def __init__(
         self,
         name: str,
@@ -171,10 +188,23 @@ class Actor:
         loglevel: str = None,
         arbiter_addr: Optional[Tuple[str, int]] = None,
     ) -> None:
+        """This constructor is called in the parent actor **before** the spawning
+        phase (aka before a new process is executed).
+        """
         self.name = name
         self.uid = (name, uid or str(uuid.uuid4()))
-        self.rpc_module_paths = rpc_module_paths
+
-        self._mods: dict = {}
+        # retreive and store parent `__main__` data which
+        # will be passed to children
+        self._parent_main_data = _mp_fixup_main._mp_figure_out_main()
+
+        mods = {}
+        for name in rpc_module_paths or ():
+            mod = importlib.import_module(name)
+            mods[name] = _get_mod_abspath(mod)
+
+        self.rpc_module_paths = mods
+        self._mods: Dict[str, ModuleType] = {}
 
         # TODO: consider making this a dynamically defined
         # @dataclass once we get py3.7
@@ -225,11 +255,34 @@ class Actor:
         the original nursery we need to try and load the local module
         code (if it exists).
         """
-        for path in self.rpc_module_paths:
+        try:
-            log.debug(f"Attempting to import {path}")
+            if self._spawn_method == 'trio_run_in_process':
-            self._mods[path] = importlib.import_module(path)
+                parent_data = self._parent_main_data
+                if 'init_main_from_name' in parent_data:
+                    _mp_fixup_main._fixup_main_from_name(
+                        parent_data['init_main_from_name'])
+                elif 'init_main_from_path' in parent_data:
+                    _mp_fixup_main._fixup_main_from_path(
+                        parent_data['init_main_from_path'])
+
+            for modpath, filepath in self.rpc_module_paths.items():
+                # XXX append the allowed module to the python path which
+                # should allow for relative (at least downward) imports.
+                sys.path.append(os.path.dirname(filepath))
+                log.debug(f"Attempting to import {modpath}@{filepath}")
+                self._mods[modpath] = importlib.import_module(modpath)
+        except ModuleNotFoundError:
+            # it is expected the corresponding `ModuleNotExposed` error
+            # will be raised later
+            log.error(f"Failed to import {modpath} in {self.name}")
+            raise
 
     def _get_rpc_func(self, ns, funcname):
+        if ns == "__mp_main__":
+            # In subprocesses, `__main__` will actually map to
+            # `__mp_main__` which should be the same entry-point-module
+            # as the parent.
+            ns = "__main__"
         try:
             return getattr(self._mods[ns], funcname)
         except KeyError as err:
@@ -488,7 +541,7 @@ class Actor:
                 f"Exiting msg loop for {chan} from {chan.uid} "
                 f"with last msg:\n{msg}")
 
-    def _fork_main(
+    def _mp_main(
         self,
         accept_addr: Tuple[str, int],
         forkserver_info: Tuple[Any, Any, Any, Any, Any],
@@ -500,13 +553,18 @@ class Actor:
         self._forkserver_info = forkserver_info
         from ._spawn import try_set_start_method
         spawn_ctx = try_set_start_method(start_method)
+
         if self.loglevel is not None:
             log.info(
                 f"Setting loglevel for {self.uid} to {self.loglevel}")
             get_console_log(self.loglevel)
+
+        assert spawn_ctx
         log.info(
             f"Started new {spawn_ctx.current_process()} for {self.uid}")
+
         _state._current_actor = self
+
         log.debug(f"parent_addr is {parent_addr}")
         try:
             trio.run(partial(
@@ -515,6 +573,26 @@ class Actor:
             pass  # handle it the same way trio does?
         log.info(f"Actor {self.uid} terminated")
 
+    async def _trip_main(
+        self,
+        accept_addr: Tuple[str, int],
+        parent_addr: Tuple[str, int] = None
+    ) -> None:
+        """Entry point for a `trio_run_in_process` subactor.
+
+        Here we don't need to call `trio.run()` since trip does that as
+        part of its subprocess startup sequence.
+        """
+        if self.loglevel is not None:
+            log.info(
+                f"Setting loglevel for {self.uid} to {self.loglevel}")
+            get_console_log(self.loglevel)
+
+        log.info(f"Started new TRIP process for {self.uid}")
+        _state._current_actor = self
+        await self._async_main(accept_addr, parent_addr=parent_addr)
+        log.info(f"Actor {self.uid} terminated")
+
     async def _async_main(
         self,
         accept_addr: Tuple[str, int],
@@ -598,9 +676,13 @@ class Actor:
                         f"Failed to ship error to parent "
                         f"{self._parent_chan.uid}, channel was closed")
                     log.exception("Actor errored:")
+
+            if isinstance(err, ModuleNotFoundError):
+                raise
             else:
                 # XXX wait, why?
                 # causes a hang if I always raise..
+                # A parent process does something weird here?
                 raise
 
         finally:
@@ -649,8 +731,8 @@ class Actor:
                     port=accept_port, host=accept_host,
                 )
             )
-            log.debug(
+            log.debug(f"Started tcp server(s) on"
-                    f"Started tcp server(s) on {[l.socket for l in listeners]}")  # type: ignore
+                      " {[l.socket for l in listeners]}")  # type: ignore
             self._listeners.extend(listeners)
             task_status.started()
 

tractor/_mp_fixup_main.py

@@ -0,0 +1,100 @@
+"""
+Helpers pulled mostly verbatim from ``multiprocessing.spawn``
+to aid with "fixing up" the ``__main__`` module in subprocesses.
+
+These helpers are needed for any spawing backend that doesn't already handle this.
+For example when using ``trio_run_in_process`` it is needed but obviously not when
+we're already using ``multiprocessing``.
+"""
+import os
+import sys
+import platform
+import types
+import runpy
+from typing import Dict
+
+
+ORIGINAL_DIR = os.path.abspath(os.getcwd())
+
+
+def _mp_figure_out_main() -> Dict[str, str]:
+    """Taken from ``multiprocessing.spawn.get_preparation_data()``.
+
+    Retrieve parent actor `__main__` module data.
+    """
+    d = {}
+    # Figure out whether to initialise main in the subprocess as a module
+    # or through direct execution (or to leave it alone entirely)
+    main_module = sys.modules['__main__']
+    main_mod_name = getattr(main_module.__spec__, "name", None)
+    if main_mod_name is not None:
+        d['init_main_from_name'] = main_mod_name
+    # elif sys.platform != 'win32' or (not WINEXE and not WINSERVICE):
+    elif platform.system() != 'Windows':
+        main_path = getattr(main_module, '__file__', None)
+        if main_path is not None:
+            if (
+                not os.path.isabs(main_path) and (
+                    ORIGINAL_DIR is not None)
+            ):
+                # process.ORIGINAL_DIR is not None):
+                # main_path = os.path.join(process.ORIGINAL_DIR, main_path)
+                main_path = os.path.join(ORIGINAL_DIR, main_path)
+            d['init_main_from_path'] = os.path.normpath(main_path)
+
+    return d
+
+
+# Multiprocessing module helpers to fix up the main module in
+# spawned subprocesses
+def _fixup_main_from_name(mod_name: str) -> None:
+    # __main__.py files for packages, directories, zip archives, etc, run
+    # their "main only" code unconditionally, so we don't even try to
+    # populate anything in __main__, nor do we make any changes to
+    # __main__ attributes
+    current_main = sys.modules['__main__']
+    if mod_name == "__main__" or mod_name.endswith(".__main__"):
+        return
+
+    # If this process was forked, __main__ may already be populated
+    if getattr(current_main.__spec__, "name", None) == mod_name:
+        return
+
+    # Otherwise, __main__ may contain some non-main code where we need to
+    # support unpickling it properly. We rerun it as __mp_main__ and make
+    # the normal __main__ an alias to that
+    # old_main_modules.append(current_main)
+    main_module = types.ModuleType("__mp_main__")
+    main_content = runpy.run_module(mod_name,
+                                    run_name="__mp_main__",
+                                    alter_sys=True)
+    main_module.__dict__.update(main_content)
+    sys.modules['__main__'] = sys.modules['__mp_main__'] = main_module
+
+
+def _fixup_main_from_path(main_path: str) -> None:
+    # If this process was forked, __main__ may already be populated
+    current_main = sys.modules['__main__']
+
+    # Unfortunately, the main ipython launch script historically had no
+    # "if __name__ == '__main__'" guard, so we work around that
+    # by treating it like a __main__.py file
+    # See https://github.com/ipython/ipython/issues/4698
+    main_name = os.path.splitext(os.path.basename(main_path))[0]
+    if main_name == 'ipython':
+        return
+
+    # Otherwise, if __file__ already has the setting we expect,
+    # there's nothing more to do
+    if getattr(current_main, '__file__', None) == main_path:
+        return
+
+    # If the parent process has sent a path through rather than a module
+    # name we assume it is an executable script that may contain
+    # non-main code that needs to be executed
+    # old_main_modules.append(current_main)
+    main_module = types.ModuleType("__mp_main__")
+    main_content = runpy.run_path(main_path,
+                                  run_name="__mp_main__")
+    main_module.__dict__.update(main_content)
+    sys.modules['__main__'] = sys.modules['__mp_main__'] = main_module

tractor/_spawn.py

@@ -1,9 +1,14 @@
 """
-Process spawning.
+Machinery for actor process spawning using multiple backends.
-
-Mostly just wrapping around ``multiprocessing``.
 """
+import inspect
 import multiprocessing as mp
+import platform
+from typing import Any, Dict, Optional
+
+import trio
+from trio_typing import TaskStatus
+from async_generator import aclosing
 
 try:
     from multiprocessing import semaphore_tracker  # type: ignore
@@ -18,34 +23,65 @@ from typing import Tuple
 
 from . import _forkserver_override
 from ._state import current_actor
-from ._actor import Actor
+from .log import get_logger
+from ._portal import Portal
+from ._actor import Actor, ActorFailure
 
 
-_ctx: mp.context.BaseContext = mp.get_context("spawn")  # type: ignore
+log = get_logger('tractor')
+
+# use trip as our default on *nix systems for now
+if platform.system() != 'Windows':
+    _spawn_method: str = "trio_run_in_process"
+else:
+    _spawn_method = "spawn"
+
+_ctx: Optional[mp.context.BaseContext] = None
 
 
-def try_set_start_method(name: str) -> mp.context.BaseContext:
+if platform.system() == 'Windows':
-    """Attempt to set the start method for ``multiprocess.Process`` spawning.
+    async def proc_waiter(proc: mp.Process) -> None:
+        await trio.hazmat.WaitForSingleObject(proc.sentinel)
+else:
+    import trio_run_in_process
 
-    If the desired method is not supported the sub-interpreter (aka "spawn"
+    async def proc_waiter(proc: mp.Process) -> None:
-    method) is used.
+        await trio.hazmat.wait_readable(proc.sentinel)
+
+
+def try_set_start_method(name: str) -> Optional[mp.context.BaseContext]:
+    """Attempt to set the start method for process starting, aka the "actor
+    spawning backend".
+
+    If the desired method is not supported this function will error. On Windows
+    the only supported option is the ``multiprocessing`` "spawn" method. The default
+    on *nix systems is ``trio_run_in_process``.
     """
     global _ctx
+    global _spawn_method
 
-    allowed = mp.get_all_start_methods()
+    methods = mp.get_all_start_methods()
+    if 'fork' in methods:
+        # forking is incompatible with ``trio``s global task tree
+        methods.remove('fork')
 
-    if name not in allowed:
+    # no Windows support for trip yet
-        name = 'spawn'
+    if platform.system() != 'Windows':
-    elif name == 'fork':
+        methods += ['trio_run_in_process']
+
+    if name not in methods:
         raise ValueError(
-            "`fork` is unsupported due to incompatibility with `trio`"
+            f"Spawn method `{name}` is invalid please choose one of {methods}"
         )
     elif name == 'forkserver':
         _forkserver_override.override_stdlib()
-
-    assert name in allowed
-
         _ctx = mp.get_context(name)
+    elif name == 'trio_run_in_process':
+        _ctx = None
+    else:
+        _ctx = mp.get_context(name)
+
+    _spawn_method = name
     return _ctx
 
 
@@ -55,16 +91,116 @@ def is_main_process() -> bool:
     return mp.current_process().name == 'MainProcess'
 
 
-def new_proc(
+async def exhaust_portal(
-    name: str,
+    portal: Portal,
+    actor: Actor
+) -> Any:
+    """Pull final result from portal (assuming it has one).
+
+    If the main task is an async generator do our best to consume
+    what's left of it.
+    """
+    try:
+        log.debug(f"Waiting on final result from {actor.uid}")
+        final = res = await portal.result()
+        # if it's an async-gen then alert that we're cancelling it
+        if inspect.isasyncgen(res):
+            final = []
+            log.warning(
+                f"Blindly consuming asyncgen for {actor.uid}")
+            with trio.fail_after(1):
+                async with aclosing(res) as agen:
+                    async for item in agen:
+                        log.debug(f"Consuming item {item}")
+                        final.append(item)
+    except (Exception, trio.MultiError) as err:
+        # we reraise in the parent task via a ``trio.MultiError``
+        return err
+    else:
+        return final
+
+
+async def cancel_on_completion(
+    portal: Portal,
     actor: Actor,
+    errors: Dict[Tuple[str, str], Exception],
+    task_status: TaskStatus[trio.CancelScope] = trio.TASK_STATUS_IGNORED,
+) -> None:
+    """Cancel actor gracefully once it's "main" portal's
+    result arrives.
+
+    Should only be called for actors spawned with `run_in_actor()`.
+    """
+    with trio.CancelScope() as cs:
+        task_status.started(cs)
+        # if this call errors we store the exception for later
+        # in ``errors`` which will be reraised inside
+        # a MultiError and we still send out a cancel request
+        result = await exhaust_portal(portal, actor)
+        if isinstance(result, Exception):
+            errors[actor.uid] = result
+            log.warning(
+                f"Cancelling {portal.channel.uid} after error {result}"
+            )
+        else:
+            log.info(
+                f"Cancelling {portal.channel.uid} gracefully "
+                "after result {result}")
+
+        # cancel the process now that we have a final result
+        await portal.cancel_actor()
+
+
+async def new_proc(
+    name: str,
+    actor_nursery: 'ActorNursery',  # type: ignore
+    subactor: Actor,
+    errors: Dict[Tuple[str, str], Exception],
     # passed through to actor main
     bind_addr: Tuple[str, int],
     parent_addr: Tuple[str, int],
-) -> mp.Process:
+    use_trio_run_in_process: bool = False,
+    task_status: TaskStatus[Portal] = trio.TASK_STATUS_IGNORED
+) -> None:
     """Create a new ``multiprocessing.Process`` using the
     spawn method as configured using ``try_set_start_method()``.
     """
+    cancel_scope = None
+
+    # mark the new actor with the global spawn method
+    subactor._spawn_method = _spawn_method
+
+    async with trio.open_nursery() as nursery:
+        if use_trio_run_in_process or _spawn_method == 'trio_run_in_process':
+            # trio_run_in_process
+            async with trio_run_in_process.open_in_process(
+                subactor._trip_main,
+                bind_addr,
+                parent_addr,
+            ) as proc:
+                log.info(f"Started {proc}")
+
+                # wait for actor to spawn and connect back to us
+                # channel should have handshake completed by the
+                # local actor by the time we get a ref to it
+                event, chan = await actor_nursery._actor.wait_for_peer(
+                    subactor.uid)
+                portal = Portal(chan)
+                actor_nursery._children[subactor.uid] = (
+                    subactor, proc, portal)
+                task_status.started(portal)
+
+                # wait for ActorNursery.wait() to be called
+                await actor_nursery._join_procs.wait()
+
+                if portal in actor_nursery._cancel_after_result_on_exit:
+                    cancel_scope = await nursery.start(
+                        cancel_on_completion, portal, subactor, errors)
+
+                # TRIP blocks here until process is complete
+        else:
+            # `multiprocessing`
+            assert _ctx
             start_method = _ctx.get_start_method()
             if start_method == 'forkserver':
                 # XXX do our hackery on the stdlib to avoid multiple
@@ -72,15 +208,17 @@ def new_proc(
                 fs = forkserver._forkserver
                 curr_actor = current_actor()
                 if is_main_process() and not curr_actor._forkserver_info:
-            # if we're the "main" process start the forkserver only once
+                    # if we're the "main" process start the forkserver
-            # and pass its ipc info to downstream children
+                    # only once and pass its ipc info to downstream
+                    # children
                     # forkserver.set_forkserver_preload(rpc_module_paths)
                     forkserver.ensure_running()
                     fs_info = (
                         fs._forkserver_address,
                         fs._forkserver_alive_fd,
                         getattr(fs, '_forkserver_pid', None),
-                getattr(resource_tracker._resource_tracker, '_pid', None),
+                        getattr(
+                            resource_tracker._resource_tracker, '_pid', None),
                         resource_tracker._resource_tracker._fd,
                     )
                 else:
@@ -95,8 +233,8 @@ def new_proc(
             else:
                 fs_info = (None, None, None, None, None)
 
-    return _ctx.Process(  # type: ignore
+            proc = _ctx.Process(  # type: ignore
-        target=actor._fork_main,
+                target=subactor._mp_main,
                 args=(
                     bind_addr,
                     fs_info,
@@ -106,3 +244,55 @@ def new_proc(
                 # daemon=True,
                 name=name,
             )
+            # `multiprocessing` only (since no async interface):
+            # register the process before start in case we get a cancel
+            # request before the actor has fully spawned - then we can wait
+            # for it to fully come up before sending a cancel request
+            actor_nursery._children[subactor.uid] = (subactor, proc, None)
+
+            proc.start()
+            if not proc.is_alive():
+                raise ActorFailure("Couldn't start sub-actor?")
+
+            log.info(f"Started {proc}")
+
+            # wait for actor to spawn and connect back to us
+            # channel should have handshake completed by the
+            # local actor by the time we get a ref to it
+            event, chan = await actor_nursery._actor.wait_for_peer(
+                subactor.uid)
+            portal = Portal(chan)
+            actor_nursery._children[subactor.uid] = (subactor, proc, portal)
+
+            # unblock parent task
+            task_status.started(portal)
+
+            # wait for ``ActorNursery`` block to signal that
+            # subprocesses can be waited upon.
+            # This is required to ensure synchronization
+            # with user code that may want to manually await results
+            # from nursery spawned sub-actors. We don't want the
+            # containing nurseries here to collect results or error
+            # while user code is still doing it's thing. Only after the
+            # nursery block closes do we allow subactor results to be
+            # awaited and reported upwards to the supervisor.
+            await actor_nursery._join_procs.wait()
+
+            if portal in actor_nursery._cancel_after_result_on_exit:
+                cancel_scope = await nursery.start(
+                    cancel_on_completion, portal, subactor, errors)
+
+            # TODO: timeout block here?
+            if proc.is_alive():
+                await proc_waiter(proc)
+            proc.join()
+
+        log.debug(f"Joined {proc}")
+        # pop child entry to indicate we are no longer managing this subactor
+        subactor, proc, portal = actor_nursery._children.pop(subactor.uid)
+        # cancel result waiter that may have been spawned in
+        # tandem if not done already
+        if cancel_scope:
+            log.warning(
+                f"Cancelling existing result waiter task for {subactor.uid}")
+            cancel_scope.cancel()

tractor/_trionics.py

@@ -1,39 +1,37 @@
 """
 ``trio`` inspired apis and helpers
 """
-import inspect
-import platform
 import multiprocessing as mp
 from typing import Tuple, List, Dict, Optional, Any
 import typing
 
 import trio
-from async_generator import asynccontextmanager, aclosing
+from async_generator import asynccontextmanager
 
 from ._state import current_actor
 from .log import get_logger, get_loglevel
-from ._actor import Actor, ActorFailure
+from ._actor import Actor  # , ActorFailure
 from ._portal import Portal
 from . import _spawn
 
 
-if platform.system() == 'Windows':
-    async def proc_waiter(proc: mp.Process) -> None:
-        await trio.hazmat.WaitForSingleObject(proc.sentinel)
-else:
-    async def proc_waiter(proc: mp.Process) -> None:
-        await trio.hazmat.wait_readable(proc.sentinel)
-
-
 log = get_logger('tractor')
 
 
 class ActorNursery:
     """Spawn scoped subprocess actors.
     """
-    def __init__(self, actor: Actor) -> None:
+    def __init__(
+        self,
+        actor: Actor,
+        ria_nursery: trio.Nursery,
+        da_nursery: trio.Nursery,
+        errors: Dict[Tuple[str, str], Exception],
+    ) -> None:
         # self.supervisor = supervisor  # TODO
         self._actor: Actor = actor
+        self._ria_nursery = ria_nursery
+        self._da_nursery = da_nursery
         self._children: Dict[
             Tuple[str, str],
             Tuple[Actor, mp.Process, Optional[Portal]]
@@ -42,9 +40,8 @@ class ActorNursery:
         # cancelled when their "main" result arrives
         self._cancel_after_result_on_exit: set = set()
         self.cancelled: bool = False
-
+        self._join_procs = trio.Event()
-    async def __aenter__(self):
+        self.errors = errors
-        return self
 
     async def start_actor(
         self,
@@ -53,9 +50,11 @@ class ActorNursery:
         statespace: Optional[Dict[str, Any]] = None,
         rpc_module_paths: List[str] = None,
         loglevel: str = None,  # set log level per subactor
+        nursery: trio.Nursery = None,
     ) -> Portal:
         loglevel = loglevel or self._actor.loglevel or get_loglevel()
-        actor = Actor(
+
+        subactor = Actor(
             name,
             # modules allowed to invoked funcs from
             rpc_module_paths=rpc_module_paths or [],
@@ -64,37 +63,29 @@ class ActorNursery:
             arbiter_addr=current_actor()._arb_addr,
         )
         parent_addr = self._actor.accept_addr
-        proc = _spawn.new_proc(
+        assert parent_addr
+
+        # start a task to spawn a process
+        # blocks until process has been started and a portal setup
+        nursery = nursery or self._da_nursery
+
+        # XXX: the type ignore is actually due to a `mypy` bug
+        return await nursery.start(  # type: ignore
+            _spawn.new_proc,
             name,
-            actor,
+            self,
+            subactor,
+            self.errors,
             bind_addr,
             parent_addr,
         )
-        # register the process before start in case we get a cancel
-        # request before the actor has fully spawned - then we can wait
-        # for it to fully come up before sending a cancel request
-        self._children[actor.uid] = (actor, proc, None)
-
-        proc.start()
-        if not proc.is_alive():
-            raise ActorFailure("Couldn't start sub-actor?")
-
-        log.info(f"Started {proc}")
-        # wait for actor to spawn and connect back to us
-        # channel should have handshake completed by the
-        # local actor by the time we get a ref to it
-        event, chan = await self._actor.wait_for_peer(actor.uid)
-        portal = Portal(chan)
-        self._children[actor.uid] = (actor, proc, portal)
-
-        return portal
 
     async def run_in_actor(
         self,
         name: str,
         fn: typing.Callable,
         bind_addr: Tuple[str, int] = ('127.0.0.1', 0),
-        rpc_module_paths: List[str] = [],
+        rpc_module_paths: Optional[List[str]] = None,
         statespace: Dict[str, Any] = None,
         loglevel: str = None,  # set log level per subactor
         **kwargs,  # explicit args to ``fn``
@@ -109,13 +100,15 @@ class ActorNursery:
         mod_path = fn.__module__
         portal = await self.start_actor(
             name,
-            rpc_module_paths=[mod_path] + rpc_module_paths,
+            rpc_module_paths=[mod_path] + (rpc_module_paths or []),
             bind_addr=bind_addr,
             statespace=statespace,
             loglevel=loglevel,
+            # use the run_in_actor nursery
+            nursery=self._ria_nursery,
         )
         # this marks the actor to be cancelled after its portal result
-        # is retreived, see ``wait()`` below.
+        # is retreived, see logic in `open_nursery()` below.
         self._cancel_after_result_on_exit.add(portal)
         await portal._submit_for_result(
             mod_path,
@@ -124,136 +117,6 @@ class ActorNursery:
         )
         return portal
 
-    async def wait(self) -> None:
-        """Wait for all subactors to complete.
-
-        This is probably the most complicated (and confusing, sorry)
-        function that does all the clever crap to deal with cancellation,
-        error propagation, and graceful subprocess tear down.
-        """
-        async def exhaust_portal(portal, actor):
-            """Pull final result from portal (assuming it has one).
-
-            If the main task is an async generator do our best to consume
-            what's left of it.
-            """
-            try:
-                log.debug(f"Waiting on final result from {actor.uid}")
-                final = res = await portal.result()
-                # if it's an async-gen then alert that we're cancelling it
-                if inspect.isasyncgen(res):
-                    final = []
-                    log.warning(
-                        f"Blindly consuming asyncgen for {actor.uid}")
-                    with trio.fail_after(1):
-                        async with aclosing(res) as agen:
-                            async for item in agen:
-                                log.debug(f"Consuming item {item}")
-                                final.append(item)
-            except (Exception, trio.MultiError) as err:
-                # we reraise in the parent task via a ``trio.MultiError``
-                return err
-            else:
-                return final
-
-        async def cancel_on_completion(
-            portal: Portal,
-            actor: Actor,
-            task_status=trio.TASK_STATUS_IGNORED,
-        ) -> None:
-            """Cancel actor gracefully once it's "main" portal's
-            result arrives.
-
-            Should only be called for actors spawned with `run_in_actor()`.
-            """
-            with trio.CancelScope() as cs:
-                task_status.started(cs)
-                # if this call errors we store the exception for later
-                # in ``errors`` which will be reraised inside
-                # a MultiError and we still send out a cancel request
-                result = await exhaust_portal(portal, actor)
-                if isinstance(result, Exception):
-                    errors.append(result)
-                    log.warning(
-                        f"Cancelling {portal.channel.uid} after error {result}"
-                    )
-                else:
-                    log.info(f"Cancelling {portal.channel.uid} gracefully")
-
-                # cancel the process now that we have a final result
-                await portal.cancel_actor()
-
-            # XXX: lol, this will never get run without a shield above..
-            # if cs.cancelled_caught:
-            #     log.warning(
-            #         "Result waiter was cancelled, process may have died")
-
-        async def wait_for_proc(
-            proc: mp.Process,
-            actor: Actor,
-            cancel_scope: Optional[trio.CancelScope] = None,
-        ) -> None:
-            # TODO: timeout block here?
-            if proc.is_alive():
-                await proc_waiter(proc)
-
-            # please god don't hang
-            proc.join()
-            log.debug(f"Joined {proc}")
-            # indicate we are no longer managing this subactor
-            self._children.pop(actor.uid)
-
-            # proc terminated, cancel result waiter that may have
-            # been spawned in tandem if not done already
-            if cancel_scope:
-                log.warning(
-                    f"Cancelling existing result waiter task for {actor.uid}")
-                cancel_scope.cancel()
-
-        log.debug(f"Waiting on all subactors to complete")
-        # since we pop each child subactor on termination,
-        # iterate a copy
-        children = self._children.copy()
-        errors: List[Exception] = []
-        # wait on run_in_actor() tasks, unblocks when all complete
-        async with trio.open_nursery() as nursery:
-            for subactor, proc, portal in children.values():
-                cs = None
-                # portal from ``run_in_actor()``
-                if portal in self._cancel_after_result_on_exit:
-                    assert portal
-                    cs = await nursery.start(
-                        cancel_on_completion, portal, subactor)
-                    # TODO: how do we handle remote host spawned actors?
-                    nursery.start_soon(
-                        wait_for_proc, proc, subactor, cs)
-
-        if errors:
-            if not self.cancelled:
-                # bubble up error(s) here and expect to be called again
-                # once the nursery has been cancelled externally (ex.
-                # from within __aexit__() if an error is caught around
-                # ``self.wait()`` then, ``self.cancel()`` is called
-                # immediately, in the default supervisor strat, after
-                # which in turn ``self.wait()`` is called again.)
-                raise trio.MultiError(errors)
-
-        # wait on all `start_actor()` subactors to complete
-        # if errors were captured above and we have not been cancelled
-        # then these ``start_actor()`` spawned actors will block until
-        # cancelled externally
-        children = self._children.copy()
-        async with trio.open_nursery() as nursery:
-            for subactor, proc, portal in children.values():
-                # TODO: how do we handle remote host spawned actors?
-                assert portal
-                nursery.start_soon(wait_for_proc, proc, subactor, cs)
-
-        log.debug(f"All subactors for {self} have terminated")
-        if errors:
-            # always raise any error if we're also cancelled
-            raise trio.MultiError(errors)
-
     async def cancel(self, hard_kill: bool = False) -> None:
         """Cancel this nursery by instructing each subactor to cancel
         itself and wait for all subactors to terminate.
@@ -270,7 +133,7 @@ class ActorNursery:
 
         log.debug(f"Cancelling nursery")
         with trio.move_on_after(3) as cs:
-            async with trio.open_nursery() as n:
+            async with trio.open_nursery() as nursery:
                 for subactor, proc, portal in self._children.values():
                     if hard_kill:
                         do_hard_kill(proc)
@@ -297,68 +160,137 @@ class ActorNursery:
 
                         # spawn cancel tasks for each sub-actor
                         assert portal
-                        n.start_soon(portal.cancel_actor)
+                        nursery.start_soon(portal.cancel_actor)
 
         # if we cancelled the cancel (we hung cancelling remote actors)
         # then hard kill all sub-processes
         if cs.cancelled_caught:
             log.error(f"Failed to gracefully cancel {self}, hard killing!")
-            async with trio.open_nursery() as n:
+            async with trio.open_nursery():
                 for subactor, proc, portal in self._children.values():
-                    n.start_soon(do_hard_kill, proc)
+                    nursery.start_soon(do_hard_kill, proc)
 
         # mark ourselves as having (tried to have) cancelled all subactors
         self.cancelled = True
-        await self.wait()
+        self._join_procs.set()
 
-    async def __aexit__(self, etype, value, tb):
+
-        """Wait on all subactor's main routines to complete.
+@asynccontextmanager
+async def open_nursery() -> typing.AsyncGenerator[ActorNursery, None]:
+    """Create and yield a new ``ActorNursery`` to be used for spawning
+    structured concurrent subactors.
+
+    When an actor is spawned a new trio task is started which
+    invokes one of the process spawning backends to create and start
+    a new subprocess. These tasks are started by one of two nurseries
+    detailed below. The reason for spawning processes from within
+    a new task is because ``trio_run_in_process`` itself creates a new
+    internal nursery and the same task that opens a nursery **must**
+    close it. It turns out this approach is probably more correct
+    anyway since it is more clear from the following nested nurseries
+    which cancellation scopes correspond to each spawned subactor set.
     """
-        # XXX: this is effectively the (for now) lone
+    actor = current_actor()
-        # cancellation/supervisor strategy (one-cancels-all)
+    if not actor:
-        # which exactly mimicks trio's behaviour
+        raise RuntimeError("No actor instance has been defined yet?")
-        if etype is not None:
+
+    # the collection of errors retreived from spawned sub-actors
+    errors: Dict[Tuple[str, str], Exception] = {}
+
+    # This is the outermost level "deamon actor" nursery. It is awaited
+    # **after** the below inner "run in actor nursery". This allows for
+    # handling errors that are generated by the inner nursery in
+    # a supervisor strategy **before** blocking indefinitely to wait for
+    # actors spawned in "daemon mode" (aka started using
+    # ``ActorNursery.start_actor()``).
+    async with trio.open_nursery() as da_nursery:
+        try:
+            # This is the inner level "run in actor" nursery. It is
+            # awaited first since actors spawned in this way (using
+            # ``ActorNusery.run_in_actor()``) are expected to only
+            # return a single result and then complete (i.e. be canclled
+            # gracefully). Errors collected from these actors are
+            # immediately raised for handling by a supervisor strategy.
+            # As such if the strategy propagates any error(s) upwards
+            # the above "daemon actor" nursery will be notified.
+            async with trio.open_nursery() as ria_nursery:
+                anursery = ActorNursery(
+                    actor, ria_nursery, da_nursery, errors
+                )
+                try:
+                    # spawning of actors happens in the caller's scope
+                    # after we yield upwards
+                    yield anursery
+                    log.debug(
+                        f"Waiting on subactors {anursery._children}"
+                        "to complete"
+                    )
+                except (BaseException, Exception) as err:
+                    # if the caller's scope errored then we activate our
+                    # one-cancels-all supervisor strategy (don't
+                    # worry more are coming).
+                    anursery._join_procs.set()
                     try:
                         # XXX: hypothetically an error could be raised and then
                         # a cancel signal shows up slightly after in which case
                         # the `else:` block here might not complete?
                         # For now, shield both.
                         with trio.CancelScope(shield=True):
-                    if etype in (trio.Cancelled, KeyboardInterrupt):
+                            if err in (trio.Cancelled, KeyboardInterrupt):
                                 log.warning(
                                     f"Nursery for {current_actor().uid} was "
-                            f"cancelled with {etype}")
+                                    f"cancelled with {err}")
                             else:
                                 log.exception(
                                     f"Nursery for {current_actor().uid} "
-                            f"errored with {etype}, ")
+                                    f"errored with {err}, ")
-                    await self.cancel()
+
+                            # cancel all subactors
+                            await anursery.cancel()
+
                     except trio.MultiError as merr:
-                if value not in merr.exceptions:
+                        # If we receive additional errors while waiting on
-                    raise trio.MultiError(merr.exceptions + [value])
+                        # remaining subactors that were cancelled,
-                raise
+                        # aggregate those errors with the original error
+                        # that triggered this teardown.
+                        if err not in merr.exceptions:
+                            raise trio.MultiError(merr.exceptions + [err])
                     else:
-            log.debug(f"Waiting on subactors {self._children} to complete")
-            try:
-                await self.wait()
-            except (Exception, trio.MultiError) as err:
-                log.warning(f"Nursery cancelling due to {err}")
-                if self._children:
-                    with trio.CancelScope(shield=True):
-                        await self.cancel()
                         raise
 
+                # Last bit before first nursery block ends in the case
+                # where we didn't error in the caller's scope
+                log.debug(f"Waiting on all subactors to complete")
+                anursery._join_procs.set()
+
+                # ria_nursery scope end
+
+        except (Exception, trio.MultiError) as err:
+            # If actor-local error was raised while waiting on
+            # ".run_in_actor()" actors then we also want to cancel all
+            # remaining sub-actors (due to our lone strategy:
+            # one-cancels-all).
+            log.warning(f"Nursery cancelling due to {err}")
+            if anursery._children:
+                with trio.CancelScope(shield=True):
+                    await anursery.cancel()
+            raise
+        finally:
+            # No errors were raised while awaiting ".run_in_actor()"
+            # actors but those actors may have returned remote errors as
+            # results (meaning they errored remotely and have relayed
+            # those errors back to this parent actor). The errors are
+            # collected in ``errors`` so cancel all actors, summarize
+            # all errors and re-raise.
+            if errors:
+                if anursery._children:
+                    with trio.CancelScope(shield=True):
+                        await anursery.cancel()
+                if len(errors) > 1:
+                    raise trio.MultiError(tuple(errors.values()))
+                else:
+                    raise list(errors.values())[0]
+
+        # ria_nursery scope end
+
     log.debug(f"Nursery teardown complete")
-
-
-@asynccontextmanager
-async def open_nursery() -> typing.AsyncGenerator[ActorNursery, None]:
-    """Create and yield a new ``ActorNursery``.
-    """
-    actor = current_actor()
-    if not actor:
-        raise RuntimeError("No actor instance has been defined yet?")
-
-    # TODO: figure out supervisors from erlang
-    async with ActorNursery(actor) as nursery:
-        yield nursery

tractor/testing/_tractor_test.py

@@ -1,4 +1,5 @@
 import inspect
+import platform
 from functools import partial, wraps
 
 from .. import run
@@ -19,6 +20,7 @@ def tractor_test(fn):
 
         - ``arb_addr`` (a socket addr tuple where arbiter is listening)
         - ``loglevel`` (logging level passed to tractor internals)
+        - ``start_method`` (subprocess spawning backend)
 
     are defined in the `pytest` fixture space they will be automatically
     injected to tests declaring these funcargs.
@@ -28,7 +30,7 @@ def tractor_test(fn):
         *args,
         loglevel=None,
         arb_addr=None,
-        start_method='forkserver',
+        start_method=None,
         **kwargs
     ):
         # __tracebackhide__ = True
@@ -40,9 +42,15 @@ def tractor_test(fn):
             # allows test suites to define a 'loglevel' fixture
             # that activates the internal logging
             kwargs['loglevel'] = loglevel
+
+        if start_method is None:
+            if platform.system() == "Windows":
+                start_method = 'spawn'
+            else:
+                start_method = 'trio_run_in_process'
+
         if 'start_method' in inspect.signature(fn).parameters:
-            # allows test suites to define a 'loglevel' fixture
+            # set of subprocess spawning backends
-            # that activates the internal logging
             kwargs['start_method'] = start_method
         return run(
             partial(fn, *args, **kwargs),