Allow choosing the spawn backend per test session

Add a `--spawn-backend` option which can be set to one of {'mp',
'trio_run_in_process'} which will either run the test suite using the
`multiprocessing` or `trio-run-in-process` backend respectively.
Currently trying to run both in the same session can result in hangs
seemingly due to a lack of cleanup of forkservers / resource trackers
from `multiprocessing` which cause broken pipe errors on occasion (no
idea on the details).

For `test_cancellation.py::test_nested_multierrors`, use less nesting
when mp is used since it breaks if we push it too hard with the
whole recursive subprocess spawning thing...

tests/conftest.py

@@ -14,8 +14,24 @@ _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 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)
@@ -32,16 +48,21 @@ def arb_addr():
 
 
 def pytest_generate_tests(metafunc):
-    if 'start_method' in metafunc.fixturenames:
+    spawn_backend = metafunc.config.getoption("spawn_backend")
+    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 platform.system() != "Windows":
-            methods = ['trio_run_in_process']
-
             if 'fork' in methods:  # fork not available on windows, so check before 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

@@ -284,21 +284,21 @@ async def spawn_and_error(breadth, depth) -> None:
 
 
 @tractor_test
-async def test_nested_multierrors(loglevel):
+async def test_nested_multierrors(loglevel, start_method):
     """Test that failed actor sets are wrapped in `trio.MultiError`s.
     This test goes only 2 nurseries deep but we should eventually have tests
     for arbitrary n-depth actor trees.
     """
-    # if start_method == 'trio_run_in_process':
+    if start_method == 'trio_run_in_process':
+        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...
         depth = 2
-    subactor_breadth = 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...
-    #     depth = 1
-    #     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 a raise of 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

tractor/testing/_tractor_test.py

@@ -19,6 +19,7 @@ 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.
@@ -41,8 +42,7 @@ def tractor_test(fn):
             # that activates the internal logging
             kwargs['loglevel'] = loglevel
         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),