Add "spawn" start method support

Add full support for using the "spawn" process starting method as per:
https://docs.python.org/3/library/multiprocessing.html#contexts-and-start-methods

Add a  `spawn_method` argument to `tractor.run()` for specifying the
desired method explicitly. By default use the "fastest" method available.
On *nix systems this is the original "forkserver" method.

This should be the solution to getting windows support!

Resolves #60

tractor/__init__.py

@@ -19,6 +19,7 @@ from ._trionics import open_nursery
 from ._state import current_actor
 from ._exceptions import RemoteActorError, ModuleNotExposed
 from . import msg
+from . import _spawn
 
 
 __all__ = [
@@ -92,12 +93,14 @@ def run(
     name: str = None,
     arbiter_addr: Tuple[str, int] = (
         _default_arbiter_host, _default_arbiter_port),
+    spawn_method: str = 'forkserver',
     **kwargs: typing.Dict[str, typing.Any],
 ) -> Any:
     """Run a trio-actor async function in process.
 
     This is tractor's main entry and the start point for any async actor.
     """
+    _spawn.try_set_start_method(spawn_method)
     return trio.run(_main, async_fn, args, kwargs, name, arbiter_addr)
 
 

tractor/_actor.py

@@ -391,7 +391,8 @@ class Actor:
                                 f" {chan} from {chan.uid}")
                         break
 
-                    log.trace(f"Received msg {msg} from {chan.uid}")  # type: ignore
+                    log.trace(   # type: ignore
+                        f"Received msg {msg} from {chan.uid}")
                     if msg.get('cid'):
                         # deliver response to local caller/waiter
                         await self._push_result(chan, msg)
@@ -478,18 +479,20 @@ class Actor:
         self,
         accept_addr: Tuple[str, int],
         forkserver_info: Tuple[Any, Any, Any, Any, Any],
+        start_method: str,
         parent_addr: Tuple[str, int] = None
     ) -> None:
         """The routine called *after fork* which invokes a fresh ``trio.run``
         """
         self._forkserver_info = forkserver_info
-        from ._spawn import ctx
+        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)
         log.info(
-            f"Started new {ctx.current_process()} for {self.uid}")
+            f"Started new {spawn_ctx.current_process()} for {self.uid}")
         _state._current_actor = self
         log.debug(f"parent_addr is {parent_addr}")
         try:

tractor/_spawn.py

@@ -5,15 +5,35 @@ Mostly just wrapping around ``multiprocessing``.
 """
 import multiprocessing as mp
 from multiprocessing import forkserver, semaphore_tracker  # type: ignore
-from typing import Tuple
+from typing import Tuple, Optional
 
 from . import _forkserver_hackzorz
 from ._state import current_actor
 from ._actor import Actor
 
 
-_forkserver_hackzorz.override_stdlib()
-ctx = mp.get_context("forkserver")
+_ctx: mp.context.BaseContext = mp.get_context("spawn")
+
+
+def try_set_start_method(name: str) -> mp.context.BaseContext:
+    """Attempt to set the start method for ``multiprocess.Process`` spawning.
+
+    If the desired method is not supported the sub-interpreter (aka "spawn"
+    method) is used.
+    """
+    global _ctx
+
+    allowed = mp.get_all_start_methods()
+    if name not in allowed:
+        name == 'spawn'
+
+    assert name in allowed
+
+    if name == 'forkserver':
+        _forkserver_hackzorz.override_stdlib()
+
+    _ctx = mp.get_context(name)
+    return _ctx
 
 
 def is_main_process() -> bool:
@@ -29,6 +49,13 @@ def new_proc(
     bind_addr: Tuple[str, int],
     parent_addr: Tuple[str, int],
 ) -> mp.Process:
+    """Create a new ``multiprocessing.Process`` using the
+    spawn method as configured using ``try_set_start_method()``.
+    """
+    start_method = _ctx.get_start_method()
+    if start_method == 'forkserver':
+        # XXX do our hackery on the stdlib to avoid multiple
+        # forkservers (one at each subproc layer).
         fs = forkserver._forkserver
         curr_actor = current_actor()
         if is_main_process() and not curr_actor._forkserver_info:
@@ -52,10 +79,17 @@ def new_proc(
                 semaphore_tracker._semaphore_tracker._pid,
                 semaphore_tracker._semaphore_tracker._fd,
              ) = curr_actor._forkserver_info
+    else:
+        fs_info = (None, None, None, None, None)
 
-    return ctx.Process(
+    return _ctx.Process(
         target=actor._fork_main,
-        args=(bind_addr, fs_info, parent_addr),
+        args=(
+            bind_addr,
+            fs_info,
+            start_method,
+            parent_addr
+        ),
         # daemon=True,
         name=name,
     )