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

Fixes #696.

Author: agronholm

docs/versionhistory.rst

@@ -7,6 +7,9 @@ This library adheres to `Semantic Versioning 2.0 <http://semver.org/>`_.
 
 - Added support for the ``from_uri()``, ``full_match()``, ``parser`` methods/properties
   in ``anyio.Path``, newly added in Python 3.13
+- Fixed ``to_process.run_sync()`` failing to initialize on Windows if the script pointed
+  to by ``__main__.__file__`` did not exist
+  (`#696 <https://github.com/agronholm/anyio/issues/696>`_)
 
 **4.4.0**
 

src/anyio/to_process.py

@@ -223,18 +223,24 @@ def process_worker() -> None:
                 main_module_path: str | None
                 sys.path, main_module_path = args
                 del sys.modules["__main__"]
-                if main_module_path:
-                    # Load the parent's main module but as __mp_main__ instead of
-                    # __main__ (like multiprocessing does) to avoid infinite recursion
-                    try:
-                        spec = spec_from_file_location("__mp_main__", main_module_path)
-                        if spec and spec.loader:
-                            main = module_from_spec(spec)
-                            spec.loader.exec_module(main)
-                            sys.modules["__main__"] = main
-                    except BaseException as exc:
-                        exception = exc
-
+                with open("/tmp/output", "w") as f:
+                    print("main module path:", main_module_path, file=f)
+                    if main_module_path:
+                        # Load the parent's main module but as __mp_main__ instead of
+                        # __main__ (like multiprocessing does) to avoid infinite recursion
+                        try:
+                            spec = spec_from_file_location(
+                                "__mp_main__", main_module_path
+                            )
+                            print(f"{spec=}", file=f)
+                            if spec and spec.loader:
+                                print(f"{spec.loader=}", file=f)
+                                main = module_from_spec(spec)
+                                spec.loader.exec_module(main)
+                                sys.modules["__main__"] = main
+                                print("loaded", main_module_path, file=f)
+                        except BaseException as exc:
+                            exception = exc
         try:
             if exception is not None:
                 status = b"EXCEPTION"

tests/test_to_process.py

@@ -1,12 +1,15 @@
 from __future__ import annotations
 
 import os
+import platform
 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 +116,19 @@ async def test_exec_while_pruning() -> None:
         assert idle_workers[0][0] is real_worker
     finally:
         workers.discard(fake_idle_process)
+
+
+@pytest.mark.skipif(
+    platform.system() != "Windows", reason="This test fails only on Windows"
+)
+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 / "nonexistent"
+    monkeypatch.setattr("__main__.__file__", str(script_path))
+    await to_process.run_sync(os.getpid)