Run Windows on trio and mp backends

The new pure trio spawning backend uses `subprocess` internally which is
also supported on windows so let's run it in CI.

.travis.yml

@@ -8,6 +8,17 @@ matrix:
           os: windows
           language: sh
           python: 3.x  # only works on linux
+          env: SPAWN_BACKEND="mp"
+          before_install:
+              - choco install python3 --params "/InstallDir:C:\\Python"
+              - export PATH="/c/Python:/c/Python/Scripts:$PATH"
+              - python -m pip install --upgrade pip wheel
+
+        - name: "Windows, Python Latest: trio"
+          os: windows
+          language: sh
+          python: 3.x  # only works on linux
+          env: SPAWN_BACKEND="trio"
           before_install:
               - choco install python3 --params "/InstallDir:C:\\Python"
               - export PATH="/c/Python:/c/Python/Scripts:$PATH"
@@ -16,6 +27,17 @@ matrix:
         - name: "Windows, Python 3.7: multiprocessing"
           os: windows
           python: 3.7  # only works on linux
+          env: SPAWN_BACKEND="mp"
+          language: sh
+          before_install:
+              - choco install python3 --version 3.7.4 --params "/InstallDir:C:\\Python"
+              - export PATH="/c/Python:/c/Python/Scripts:$PATH"
+              - python -m pip install --upgrade pip wheel
+
+        - name: "Windows, Python 3.7: trio"
+          os: windows
+          python: 3.7  # only works on linux
+          env: SPAWN_BACKEND="trio"
           language: sh
           before_install:
               - choco install python3 --version 3.7.4 --params "/InstallDir:C:\\Python"

tests/conftest.py

@@ -6,13 +6,21 @@ import platform
 
 import pytest
 import tractor
-from tractor.testing import tractor_test
+
+# export for tests
+from tractor.testing import tractor_test  # noqa
 
 
 pytest_plugins = ['pytester']
 _arb_addr = '127.0.0.1', random.randint(1000, 9999)
 
 
+no_windows = pytest.mark.skipif(
+    platform.system() == "Windows",
+    reason="Test is unsupported on windows",
+)
+
+
 def pytest_addoption(parser):
     parser.addoption(
         "--ll", action="store", dest='loglevel',
@@ -29,9 +37,6 @@ def pytest_addoption(parser):
 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':
@@ -68,10 +73,6 @@ def pytest_generate_tests(metafunc):
                 # incompatible with trio's global scheduler state
                 methods.remove('fork')
         elif spawn_backend == 'trio':
-            if platform.system() == "Windows":
-                pytest.fail(
-                    "Only `--spawn-backend=mp` is supported on Windows")
-
             methods = ['trio']
 
         metafunc.parametrize("start_method", methods, scope='module')

tests/test_cancellation.py

@@ -10,7 +10,7 @@ import pytest
 import trio
 import tractor
 
-from conftest import tractor_test
+from conftest import tractor_test, no_windows
 
 
 async def assert_err(delay=0):
@@ -339,6 +339,7 @@ async def test_nested_multierrors(loglevel, start_method):
                         subexc.type is trio.Cancelled)
 
 
+@no_windows
 def test_open_in_proc_cancel_via_SIGINT(loglevel, start_method):
     """Ensure that a control-C (SIGINT) signal cancels both the parent and
     child processes in trionic fashion
@@ -356,6 +357,7 @@ def test_open_in_proc_cancel_via_SIGINT(loglevel, start_method):
         tractor.run(main)
 
 
+@no_windows
 def test_open_in_proc_cancel_via_SIGINT_other_task(
     loglevel,
     start_method

tractor/_spawn.py

@@ -55,12 +55,13 @@ else:
 
 
 def try_set_start_method(name: str) -> Optional[mp.context.BaseContext]:
-    """Attempt to set the start method for process starting, aka the "actor
+    """Attempt to set the 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``.
+    If the desired method is not supported this function will error.
+    On Windows only the ``multiprocessing`` "spawn" method is offered
+    besides the default ``trio`` which uses async wrapping around
+    ``subprocess.Popen``.
     """
     global _ctx
     global _spawn_method
@@ -70,9 +71,8 @@ def try_set_start_method(name: str) -> Optional[mp.context.BaseContext]:
         # forking is incompatible with ``trio``s global task tree
         methods.remove('fork')
 
-    # no Windows support for trip yet
-    if platform.system() != 'Windows':
-        methods += ['trio']
+    # supported on all platforms
+    methods += ['trio']
 
     if name not in methods:
         raise ValueError(