Fixed to_process.run_sync() sometimes failing to initialize on Windows (

#744)

Fixes #696.

docs/versionhistory.rst

@@ -9,6 +9,9 @@ This library adheres to `Semantic Versioning 2.0 <http:https://semver.org/>`_.
  in ``anyio.Path``, newly added in Python 3.13
 - Changed the ``ResourceWarning`` from an unclosed memory object stream to include its
  address for easier identification
+- Fixed ``to_process.run_sync()`` failing to initialize if ``__main__.__file__`` pointed
+ to a file in a nonexistent directory
+ (`#696 <https://github.com/agronholm/anyio/issues/696>`_)
 
 **4.4.0**
 

src/anyio/to_process.py

@@ -223,7 +223,7 @@ def process_worker() -> None:
  main_module_path: str | None
  sys.path, main_module_path = args
  del sys.modules["__main__"]
- if main_module_path:
+ if main_module_path and os.path.isfile(main_module_path):
  # Load the parent's main module but as __mp_main__ instead of
  # __main__ (like multiprocessing does) to avoid infinite recursion
  try:
@@ -234,7 +234,6 @@ def process_worker() -> None:
  sys.modules["__main__"] = main
  except BaseException as exc:
  exception = exc
-
  try:
  if exception is not None:
  status = b"EXCEPTION"

tests/test_to_process.py

@@ -4,9 +4,11 @@
 import sys
 import time
 from functools import partial
+from pathlib import Path
 from unittest.mock import Mock
 
 import pytest
+from pytest import MonkeyPatch
 
 from anyio import (
  CancelScope,
@@ -113,3 +115,17 @@ async def test_exec_while_pruning() -> None:
  assert idle_workers[0][0] is real_worker
  finally:
  workers.discard(fake_idle_process)
+
+
+async def test_nonexistent_main_module(
+ monkeypatch: MonkeyPatch, tmp_path: Path
+) -> None:
+ """
+ Test that worker process creation won't fail if the detected path to the `__main__`
+ module doesn't exist. Regression test for #696.
+ """
+
+ script_path = tmp_path / "badscript"
+ script_path.touch()
+ monkeypatch.setattr("__main__.__file__", str(script_path / "__main__.py"))
+ await to_process.run_sync(os.getpid)