fix(test): make .pth files once per session

The old way, we made and deleted .pth file around each test.  This
caused problems because pth files are written to a common location
shared by all test workers, so tests would fail because pth files were
being deleted as other workers were trying to use them.

Now we make the pth file once per session.  There's still a chance that
a worker will be using a pth file just as another worker is ending and
removing it.

1 files changed, 49 insertions, 2 deletions

diff --git a/tests/conftest.py b/tests/conftest.py
index 581329bb..1bf37298 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -7,9 +7,11 @@ Pytest auto configuration.
 This module is run automatically by pytest, to define and enable fixtures.
 """
 
+import glob
 import itertools
 import os
 import sys
+import sysconfig
 import warnings
 
 import pytest
@@ -99,13 +101,14 @@ def pytest_sessionstart():
             print("Starting:", file=testtxt)
 
 
+WORKER = os.environ.get("PYTEST_XDIST_WORKER", "only")
+
 def write_test_name(prefix):
     """For tracking where and when tests are running."""
     if TRACK_TESTS:     # pragma: debugging
         with open(TEST_TXT, "a") as testtxt:
-            worker = os.environ.get('PYTEST_XDIST_WORKER', 'none')
             test = os.environ.get("PYTEST_CURRENT_TEST", "unknown")
-            print(f"{prefix} {worker}: {test}", file=testtxt, flush=True)
+            print(f"{prefix} {WORKER}: {test}", file=testtxt, flush=True)
 
 
 @pytest.hookimpl(hookwrapper=True)
@@ -144,3 +147,47 @@ def pytest_collection_modifyitems(items):
     if TRACK_TESTS:     # pragma: debugging
         with open("/tmp/items.txt", "w") as f:
             print("\n".join(i.nodeid for i in items), file=f)
+
+
+def possible_pth_dirs():
+    """Produce a sequence of directories for trying to write .pth files."""
+    # First look through sys.path, and if we find a .pth file, then it's a good
+    # place to put ours.
+    for pth_dir in sys.path:                        # pragma: part covered
+        pth_files = glob.glob(os.path.join(pth_dir, "*.pth"))
+        if pth_files:
+            yield pth_dir
+
+    # If we're still looking, then try the Python library directory.
+    # https://github.com/nedbat/coveragepy/issues/339
+    yield sysconfig.get_path("purelib")             # pragma: cant happen
+
+
+def find_writable_pth_directory():
+    """Find a place to write a .pth file."""
+    for pth_dir in possible_pth_dirs():             # pragma: part covered
+        try_it = os.path.join(pth_dir, f"touch_{WORKER}.it")
+        try:
+            with open(try_it, "w") as f:
+                f.write("foo")
+        except OSError:                             # pragma: cant happen
+            continue
+
+        os.remove(try_it)
+        return pth_dir
+
+    return None                                     # pragma: cant happen
+
+
+@pytest.fixture(scope="session", autouse=True)
+def create_pth_file():
+    """Create a .pth file for measuring subprocess coverage."""
+    pth_dir = find_writable_pth_directory()
+    assert pth_dir
+    pth_path = os.path.join(pth_dir, f"subcover_{WORKER}.pth")
+    with open(pth_path, "w") as pth:
+        pth.write("import coverage; coverage.process_startup()\n")
+
+    yield
+
+    os.remove(pth_path)