test: don't try to make pth files when invoked from pth #2011

When we're invoked by a .pth file, site.py hasn't fully run, so we try
to make a .pth file in the system Python instead of the virtualenv.
System Pythons are often not writable.

Also try to create this kind of not-writable arrangement in CI to find
problems like this in the future.

Author: nedbat

.github/workflows/testsuite.yml

@@ -132,6 +132,12 @@ jobs:
           # For extreme debugging:
           # python -c "import urllib.request as r; exec(r.urlopen('https://bit.ly/pydoctor').read())"
           env | sort
+          # Ideally, the system Python wouldn't have writable sitepackages so
+          # try to make it not writable.  We can't always always change the
+          # permissions (Ubuntu & Windows yes, Mac no), so be ready for it to fail.
+          chmod u-w $(python -c "import site; print(site.getsitepackages()[0])") || echo "Couldn't lock down site-packages"
+          ls -ld $(python -c "import site; print(site.getsitepackages()[0])")
+          whoami
 
       - name: "Install dependencies"
         run: |

coverage/control.py

@@ -269,6 +269,9 @@ def __init__(                       # pylint: disable=too-many-arguments
         self._no_warn_slugs: set[str] = set()
         self._messages = messages
 
+        # If we're invoked from a .pth file, we shouldn't try to make another one.
+        self._make_pth_file = True
+
         # A record of all the warnings that have been issued.
         self._warnings: list[str] = []
 
@@ -678,7 +681,7 @@ def start(self) -> None:
         if self._auto_load:
             self.load()
 
-        apply_patches(self, self.config)
+        apply_patches(self, self.config, make_pth_file=self._make_pth_file)
 
         self._collector.start()
         self._started = True
@@ -1432,6 +1435,7 @@ def process_startup() -> Coverage | None:
     cov._warn_unimported_source = False
     cov._warn_preimported_source = False
     cov._auto_save = True
+    cov._make_pth_file = False
     cov.start()
 
     return cov

coverage/patch.py

@@ -19,7 +19,7 @@
     from coverage.config import CoverageConfig
 
 
-def apply_patches(cov: Coverage, config: CoverageConfig) -> None:
+def apply_patches(cov: Coverage, config: CoverageConfig, *, make_pth_file: bool=True) -> None:
     """Apply invasive patches requested by `[run] patch=`."""
 
     for patch in sorted(set(config.patch)):
@@ -78,9 +78,10 @@ def coverage_execv_patch(*args: Any, **kwargs: Any) -> Any:
             os.execve = make_execv_patch("execve", os.execve)
 
         elif patch == "subprocess":
-            pth_file = create_pth_file()
-            assert pth_file is not None
-            atexit.register(pth_file.unlink, missing_ok=True)
+            if make_pth_file:
+                pth_file = create_pth_file()
+                assert pth_file is not None
+                atexit.register(pth_file.unlink, missing_ok=True)
             assert config.config_file is not None
             os.environ["COVERAGE_PROCESS_START"] = config.config_file