Auto-enable xdist when it's supported

requirements/dev.txt

@@ -12,6 +12,8 @@ aiosignal==1.1.2
     # via -r requirements/base.txt
 alabaster==0.7.12
     # via sphinx
+apipkg==1.5
+    # via execnet
 appdirs==1.4.4
     # via
     #   black
@@ -66,6 +68,8 @@ distlib==0.3.1
     # via virtualenv
 docutils==0.16
     # via sphinx
+execnet==1.7.1
+    # via pytest-xdist
 filelock==3.0.12
     # via virtualenv
 flake8-pyi==20.10.0
@@ -137,7 +141,9 @@ pluggy==0.13.1
 pre-commit==2.9.3
     # via -r requirements/lint.txt
 py==1.10.0
-    # via pytest
+    # via
+    #   pytest
+    #   pytest-forked
 pycares==3.1.1
     # via aiodns
 pycodestyle==2.6.0
@@ -158,13 +164,19 @@ pyparsing==2.4.7
     # via packaging
 pytest-cov==2.10.1
     # via -r requirements/test.txt
+pytest-forked==1.3.0
+    # via pytest-xdist
 pytest-mock==3.5.1
     # via -r requirements/test.txt
+pytest-xdist==2.2.0 ; python_version >= "3.8" or sys_platform == "win32"
+    # via -r requirements/test.txt
 pytest==6.1.2
     # via
     #   -r requirements/test.txt
     #   pytest-cov
+    #   pytest-forked
     #   pytest-mock
+    #   pytest-xdist
 python-dateutil==2.8.1
     # via freezegun
 pytz==2020.5

requirements/test.txt

@@ -9,6 +9,7 @@ mypy-extensions==0.4.3; implementation_name=="cpython"
 pytest==6.2.1
 pytest-cov==2.11.1
 pytest-mock==3.5.1
+pytest-xdist==2.2.0; python_version>="3.8" or sys_platform == "win32"
 re-assert==1.1.0
 setuptools-git==1.2
 trustme==0.6.0; platform_machine!="i686"    # no 32-bit wheels

setup.cfg

@@ -36,6 +36,11 @@ omit = site-packages
 
 [tool:pytest]
 addopts =
+    # `pytest-xdist`:
+    #--numprocesses=auto
+    # ^ this is being added dynamically by tests._pytest_plugin instead
+    # Ref: https://github.com/pytest-dev/pytest-xdist/issues/620
+
     # show 10 slowest invocations:
     --durations=10
 
@@ -50,6 +55,9 @@ addopts =
 
     # `pytest-cov`:
     --cov=aiohttp
+
+    # pre-load an in-tree plugin
+    -p tests._pytest_plugin
 filterwarnings =
     error
     ignore:module 'ssl' has no attribute 'OP_NO_COMPRESSION'. The Python interpreter is compiled against OpenSSL < 1.0.0. Ref. https.//docs.python.org/3/library/ssl.html#ssl.OP_NO_COMPRESSION:UserWarning

tests/_pytest_plugin.py

@@ -0,0 +1,37 @@
+"""An early plugin enabling pytest-xdist in supported envs."""
+
+from contextlib import suppress
+from sys import platform
+from typing import TYPE_CHECKING, List
+
+try:
+    from asyncio import ThreadedChildWatcher
+except ImportError:
+    SUPPORTS_XDIST = platform == "win32"
+else:
+    SUPPORTS_XDIST = True
+    del ThreadedChildWatcher
+
+
+if TYPE_CHECKING:
+    from _pytest.config import Config
+    from _pytest.config.argparsing import Parser
+
+
+def pytest_load_initial_conftests(
+    early_config: "Config",
+    parser: "Parser",
+    args: List[str],
+) -> None:
+    """Auto-enable pytest-xdist when supported and not disabled."""
+    if not SUPPORTS_XDIST:
+        return
+
+    with suppress(ValueError):
+        p_pos = args.index("no:xdist") - 1
+        if p_pos >= 0:
+            # NOTE: allow disabling xdist with `-p no:xdist`
+            return
+
+    xdist_args = "-n", "auto"
+    args[:0] = xdist_args  # Prepend to allow user-supplied overrides

tests/test_loop.py

@@ -7,13 +7,14 @@
 
 from aiohttp import web
 from aiohttp.test_utils import AioHTTPTestCase, unittest_run_loop
+from tests._pytest_plugin import SUPPORTS_XDIST  # type: ignore[import]
 
 
 @pytest.mark.skipif(
     platform.system() == "Windows", reason="the test is not valid for Windows"
 )
 async def test_subprocess_co(loop: Any) -> None:
-    assert threading.current_thread() is threading.main_thread()
+    assert threading.current_thread() is threading.main_thread() or SUPPORTS_XDIST
     proc = await asyncio.create_subprocess_shell(
         "exit 0",
         stdin=asyncio.subprocess.DEVNULL,