test: more focused .pth file creation

This removes our custom balanced_xdist plugin.  We couldn't force a set of tests
onto one worked, and xdist now lets us do that with xdist_group().  The tests
run about 5% slower.

Author: nedbat

pyproject.toml

@@ -111,7 +111,7 @@ ignored-argument-names = "_|unused|.*_unused"
 ## PYTEST
 
 [tool.pytest.ini_options]
-addopts = "-q -n auto -p no:legacypath --strict-markers --no-flaky-report -rfEX --failed-first"
+addopts = "-q -n auto --dist=loadgroup -p no:legacypath --strict-markers --no-flaky-report -rfEX --failed-first"
 python_classes = "*Test"
 markers = [
     "expensive: too slow to run during \"make smoke\"",
@@ -138,15 +138,6 @@ xfail_strict = true
 # https://docs.pytest.org/en/stable/reference/reference.html#confval-verbosity_assertions
 verbosity_assertions = 5
 
-balanced_clumps = [
-    # Because of expensive session-scoped fixture:
-    "VirtualenvTest",
-    # Because of shared-file manipulations (~/tests/actual/testing):
-    "CompareTest",
-    # No idea why this one fails if run on separate workers:
-    "GetZipBytesTest",
-]
-
 ## Scriv
 
 [tool.scriv]

tests/balance_xdist_plugin.py

@@ -30,7 +30,6 @@
 # $set_env.py: PYTEST_ADDOPTS - Extra arguments to pytest.
 
 pytest_plugins = [
-    "tests.balance_xdist_plugin",
     "tests.select_plugin",
 ]
 
@@ -94,29 +93,22 @@ def force_local_pyc_files() -> None:
     sys.pycache_prefix = None
 
 
-WORKER = os.getenv("PYTEST_XDIST_WORKER", "none")
-
-def pytest_sessionstart() -> None:
-    """Run once at the start of the test session."""
-    warnings.filterwarnings("ignore", r".*no-sysmon")
-    # Only in the main process...
-    if WORKER == "none":
-        # Create a .pth file for measuring subprocess coverage.
-        pth_dir = find_writable_pth_directory()
-        assert pth_dir
-        sub_dir = pth_dir / "subcover.pth"
-        sub_dir.write_text("import coverage; coverage.process_startup()\n", encoding="utf-8")
-        # subcover.pth is deleted by pytest_sessionfinish below.
-
-
-def pytest_sessionfinish() -> None:
-    """Hook the end of a test session, to clean up."""
-    # This is called by each of the workers and by the main process.
-    if WORKER == "none":
-        for pth_dir in possible_pth_dirs():             # pragma: part covered
-            pth_file = pth_dir / "subcover.pth"
-            if pth_file.exists():
-                pth_file.unlink()
+# Give this an underscored name so pylint won't complain when we use the fixture.
+@pytest.fixture(name="_create_pth_file")
+def create_pth_file() -> Iterator[None]:
+    """Create and clean up a .pth file for tests that need it for subprocesses."""
+    for pth_dir in possible_pth_dirs():             # pragma: part covered
+        pth_file = pth_dir / "subcover.pth"
+        try:
+            pth_file.write_text("import coverage; coverage.process_startup()\n", encoding="utf-8")
+        except OSError:                             # pragma: cant happen
+            continue
+        else:
+            break
+    try:
+        yield
+    finally:
+        pth_file.unlink()
 
 
 def possible_pth_dirs() -> Iterator[Path]:
@@ -131,18 +123,3 @@ def possible_pth_dirs() -> Iterator[Path]:
     # If we're still looking, then try the Python library directory.
     # https://github.com/nedbat/coveragepy/issues/339
     yield Path(sysconfig.get_path("purelib"))       # pragma: cant happen
-
-
-def find_writable_pth_directory() -> Path | None:
-    """Find a place to write a .pth file."""
-    for pth_dir in possible_pth_dirs():             # pragma: part covered
-        try_it = pth_dir / f"touch_{WORKER}.it"
-        try:
-            try_it.write_text("foo", encoding="utf-8")
-        except OSError:                             # pragma: cant happen
-            continue
-
-        os.remove(try_it)
-        return pth_dir
-
-    return None                                     # pragma: cant happen

tests/conftest.py

@@ -47,6 +47,7 @@ def path_regex(path: str) -> str:
 GOLD_PATH_RX = path_regex("/tests/gold/testing/getty/gettysburg.txt")
 OUT_PATH_RX = path_regex("out/gettysburg.txt")
 
+@pytest.mark.xdist_group(name="compare_test")
 class CompareTest(CoverageTest):
     """Tests of goldtest.py:compare()"""
 

tests/test_goldtest.py

@@ -1306,7 +1306,8 @@ def setUp(self) -> None:
             f.close()
             """)
 
-    def test_subprocess_with_pth_files(self) -> None:
+    @pytest.mark.xdist_group(name="needs_pth")
+    def test_subprocess_with_pth_files(self, _create_pth_file: None) -> None:
         # An existing data file should not be read when a subprocess gets
         # measured automatically.  Create the data file here with bogus data in
         # it.
@@ -1330,7 +1331,8 @@ def test_subprocess_with_pth_files(self) -> None:
         data.read()
         assert line_counts(data)['sub.py'] == 3
 
-    def test_subprocess_with_pth_files_and_parallel(self) -> None:
+    @pytest.mark.xdist_group(name="needs_pth")
+    def test_subprocess_with_pth_files_and_parallel(self, _create_pth_file: None) -> None:
         # https://github.com/nedbat/coveragepy/issues/492
         self.make_file("coverage.ini", """\
             [run]
@@ -1377,7 +1379,14 @@ class ProcessStartupWithSourceTest(CoverageTest):
     @pytest.mark.parametrize("dashm", ["-m", ""])
     @pytest.mark.parametrize("package", ["pkg", ""])
     @pytest.mark.parametrize("source", ["main", "sub"])
-    def test_pth_and_source_work_together(self, dashm: str, package: str, source: str) -> None:
+    @pytest.mark.xdist_group(name="needs_pth")
+    def test_pth_and_source_work_together(
+        self,
+        dashm: str,
+        package: str,
+        source: str,
+        _create_pth_file: None,
+    ) -> None:
         """Run the test for a particular combination of factors.
 
         The arguments are all strings:

tests/test_process.py

@@ -17,6 +17,7 @@
 from tests.helpers import os_sep
 
 
+@pytest.mark.xdist_group(name="get_zip_bytes_test")
 class GetZipBytesTest(CoverageTest):
     """Tests of `get_zip_bytes`."""
 

tests/test_python.py

@@ -162,6 +162,7 @@ def coverage_command_fixture(request: pytest.FixtureRequest) -> str:
     return cast(str, request.param)
 
 
+@pytest.mark.xdist_group(name="virtualenv_test")
 class VirtualenvTest(CoverageTest):
     """Tests of virtualenv considerations."""