wip: debug logging for patches

Author: nedbat

coverage/control.py

@@ -683,7 +683,7 @@ def start(self) -> None:
         if self._auto_load:
             self.load()
 
-        apply_patches(self, self.config, make_pth_file=self._make_pth_file)
+        apply_patches(self, self.config, self._debug, make_pth_file=self._make_pth_file)
 
         self._collector.start()
         self._started = True

coverage/patch.py

@@ -19,16 +19,26 @@
     from coverage.config import CoverageConfig
 
 
-def apply_patches(cov: Coverage, config: CoverageConfig, *, make_pth_file: bool=True) -> None:
+def apply_patches(
+    cov: Coverage,
+    config: CoverageConfig,
+    debug: TDebugCtl,
+    *,
+    make_pth_file: bool = True,
+) -> None:
     """Apply invasive patches requested by `[run] patch=`."""
 
     for patch in sorted(set(config.patch)):
         if patch == "_exit":
+            if debug.should("patch"):
+                debug.write("Patching _exit")
 
             def make_exit_patch(
                 old_exit: Callable[[int], NoReturn],
             ) -> Callable[[int], NoReturn]:
                 def coverage_os_exit_patch(status: int) -> NoReturn:
+                    if debug.should("patch"):
+                        debug.write("Using _exit patch")
                     try:
                         cov.save()
                     except:  # pylint: disable=bare-except
@@ -43,8 +53,13 @@ def coverage_os_exit_patch(status: int) -> NoReturn:
             if env.WINDOWS:
                 raise CoverageException("patch=execv isn't supported yet on Windows.")
 
+            if debug.should("patch"):
+                debug.write("Patching execv")
+
             def make_execv_patch(fname: str, old_execv: Any) -> Any:
                 def coverage_execv_patch(*args: Any, **kwargs: Any) -> Any:
+                    if debug.should("patch"):
+                        debug.write(f"Using execv patch for {fname}")
                     try:
                         cov.save()
                     except:  # pylint: disable=bare-except
@@ -62,9 +77,7 @@ def coverage_execv_patch(*args: Any, **kwargs: Any) -> Any:
                             # When testing locally, we need to honor the pyc file location
                             # or they get written to the .tox directories and pollute the
                             # next run with a different core.
-                            if (
-                                cache_prefix := os.getenv("PYTHONPYCACHEPREFIX")
-                            ) is not None:
+                            if (cache_prefix := os.getenv("PYTHONPYCACHEPREFIX")) is not None:
                                 new_env["PYTHONPYCACHEPREFIX"] = cache_prefix
 
                             # Without this, it fails on PyPy and Ubuntu.
@@ -78,6 +91,9 @@ def coverage_execv_patch(*args: Any, **kwargs: Any) -> Any:
             os.execve = make_execv_patch("execve", os.execve)
 
         elif patch == "subprocess":
+            if debug.should("patch"):
+                debug.write("Patching subprocess")
+
             if make_pth_file:
                 pth_file = create_pth_file()
                 assert pth_file is not None