Be explicit about the spawning backend default

Set `trio-run-in-process` as the default on *nix systems and
`multiprocessing`'s spawn method on Windows. Enable overriding the
default choice using `tractor._spawn.try_set_start_method()`. Allows
for easy runs of the test suite using a user chosen backend.

tractor/__init__.py

@@ -4,7 +4,6 @@ tractor: An actor model micro-framework built on
 """
 import importlib
 from functools import partial
-import platform
 from typing import Tuple, Any, Optional
 import typing
 
@@ -103,15 +102,14 @@ def run(
     # either the `multiprocessing` start method:
     # https://docs.python.org/3/library/multiprocessing.html#contexts-and-start-methods
     # OR `trio_run_in_process` (the new default).
-    start_method: str = 'trio_run_in_process',
+    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 platform.system() == 'Windows':
-        start_method = 'spawn'  # only one supported for now
+    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

@@ -537,6 +537,7 @@ class Actor:
                 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}")
 
@@ -555,6 +556,11 @@ class Actor:
         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}")

tractor/_spawn.py

@@ -6,7 +6,7 @@ Mostly just wrapping around ``multiprocessing``.
 import inspect
 import multiprocessing as mp
 import platform
-from typing import Any, List, Dict
+from typing import Any, Dict, Optional
 
 import trio
 from trio_typing import TaskStatus
@@ -32,8 +32,13 @@ from ._actor import Actor, ActorFailure
 
 log = get_logger('tractor')
 
-_ctx: mp.context.BaseContext = mp.get_context("spawn")  # type: ignore
-_spawn_method: str = "spawn"
+# use trip as our default for now
+if platform.system() != 'Windows':
+    _spawn_method: str = "trio_run_in_process"
+else:
+    _spawn_method = "spawn"
+
+_ctx: Optional[mp.context.BaseContext] = None
 
 
 if platform.system() == 'Windows':
@@ -46,7 +51,7 @@ else:
         await trio.hazmat.wait_readable(proc.sentinel)
 
 
-def try_set_start_method(name: str) -> mp.context.BaseContext:
+def try_set_start_method(name: str) -> Optional[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"
@@ -55,15 +60,16 @@ def try_set_start_method(name: str) -> mp.context.BaseContext:
     global _ctx
     global _spawn_method
 
-    allowed = mp.get_all_start_methods()
+    methods = mp.get_all_start_methods()
+    methods.remove('fork')
 
-    # no Windows support for trip yet (afaik)
+    # no Windows support for trip yet
     if platform.system() != 'Windows':
-        allowed += ['trio_run_in_process']
+        methods += ['trio_run_in_process']
 
-    if name not in allowed:
+    if name not in methods:
         raise ValueError(
-            f"Spawn method {name} is unsupported please choose one of {allowed}"
+            f"Spawn method `{name}` is invalid please choose one of {methods}"
         )
 
     elif name == 'fork':
@@ -73,6 +79,10 @@ def try_set_start_method(name: str) -> mp.context.BaseContext:
     elif name == 'forkserver':
         _forkserver_override.override_stdlib()
         _ctx = mp.get_context(name)
+    elif name == 'trio_run_in_process':
+        _ctx = None
+    else:
+        _ctx = mp.get_context(name)
 
     _spawn_method = name
     return _ctx
@@ -191,6 +201,7 @@ async def new_proc(
                 # 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