CI: install pytest-xdist; fix cross-drive Windows failure in new tests

pytest-xdist was never installed by tests.yml on any platform, so every
test in test_xdist.py (importorskip("xdist")) -- including the new
test_xdist_fail_under_uses_merged_coverage -- was silently skipped in
CI, on every run. Add it alongside pytest in the cross-platform
install step (test_xdist.py already self-skips on Windows via its own
pytestmark, so this activates it for Linux/macOS).

Also fixes a real Windows CI failure this surfaced in the sibling
test_fail_under_precedence_with_failing_pytest_run: GitHub Actions
Windows runners check the repo out to D:\, while tmp_path resolves
under C:\Users\...\Temp. Passing an absolute C:\ file to `-m pytest`
while cwd sat on D:\ made pytest's rootdir/common-ancestor computation
fall back to scanning a drive root, hitting the classic
"C:\Documents and Settings" permission-denied junction. Fixed by
running the inner subprocess with cwd=tmp_path and a relative filename
instead, which also lets the now-unnecessary --source argument be
dropped (default cwd-relative file matching covers it). Applied the
same pattern to test_xdist_fail_under_uses_merged_coverage for
consistency, now that it actually runs in CI.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: jaltmayerpizzorno

.github/workflows/tests.yml

@@ -47,7 +47,7 @@ jobs:
     - name: install prereqs
       run: |
         python3 -m pip install -U pip  # to avoid warnings
-        python3 -m pip install pytest
+        python3 -m pip install pytest pytest-xdist
 
     - name: install Unix dependencies
       if: matrix.os != 'windows-latest'

tests/test_coverage.py

@@ -757,12 +757,16 @@ def test_fail():
     """))
 
     # coverage is fine -- pytest's own failure exit code (1) must be preserved
-    p = subprocess.run(f"{sys.executable} -m slipcover --source {tmp_path} --fail-under 1 -m pytest {test_file}".split(), check=False)
+    p = subprocess.run(
+        [sys.executable, '-m', 'slipcover', '--fail-under', '1', '-m', 'pytest', test_file.name],
+        cwd=str(tmp_path), check=False)
     assert 1 == p.returncode
 
     # coverage is below threshold -- fail-under (2) must override pytest's
     # own exit code
-    p = subprocess.run(f"{sys.executable} -m slipcover --source {tmp_path} --fail-under 100 -m pytest {test_file}".split(), check=False)
+    p = subprocess.run(
+        [sys.executable, '-m', 'slipcover', '--fail-under', '100', '-m', 'pytest', test_file.name],
+        cwd=str(tmp_path), check=False)
     assert 2 == p.returncode
 
 

tests/test_xdist.py

@@ -248,13 +248,13 @@ def test_bar():
     # real merged coverage is 10/12 = 83.3% (the "return 1"/"return 3"
     # branches are never taken) -- comfortably above 50, so this must pass.
     # The coordinator's own local, unmerged view would incorrectly see 0%
-    # (it discovers the file via --source but never executes it itself,
+    # (it discovers the file via cwd-matching but never executes it itself,
     # since actual test execution happens only in the xdist workers), which
     # would incorrectly fail this same check.
     result = subprocess.run(
-        [sys.executable, '-m', 'slipcover', '--source', str(tmp_path),
-         '--fail-under', '50',
-         '-m', 'pytest', '-n', '2', str(test_file)],
+        [sys.executable, '-m', 'slipcover', '--fail-under', '50',
+         '-m', 'pytest', '-n', '2', test_file.name],
+        cwd=str(tmp_path),
         capture_output=True,
         text=True
     )