fix: improve setproctitle workaround

Author: nicklafleur

README.rst

@@ -208,6 +208,25 @@ to failing tests.
     debug=true
 
 
+Disable setproctitle (macOS)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Mutmut uses ``setproctitle`` to show the current mutant name in the process
+list, which is helpful for monitoring long runs. However, ``setproctitle``
+uses CoreFoundation APIs on macOS that are not fork-safe, causing segfaults
+in child processes.
+
+By default, mutmut automatically disables ``setproctitle`` on macOS and
+enables it on other platforms. If you need to override this (e.g. to enable it on
+macOS at your own risk, or to disable it on other platforms), set ``use_setproctitle``:
+
+.. code-block:: toml
+
+    # pyproject.toml
+    [tool.mutmut]
+    use_setproctitle = false
+
+
 Whitelisting
 ~~~~~~~~~~~~
 

src/mutmut/configuration.py

@@ -2,6 +2,7 @@
 
 import fnmatch
 import os
+import platform
 import sys
 from collections.abc import Callable
 from configparser import ConfigParser
@@ -110,6 +111,9 @@ def _load_config() -> Config:
         pytest_add_cli_args=s("pytest_add_cli_args", []),
         pytest_add_cli_args_test_selection=s("pytest_add_cli_args_test_selection", []),
         type_check_command=s("type_check_command", []),
+        use_setproctitle=s(
+            "use_setproctitle", not platform.system() == "Darwin"
+        ),  # False on Mac, true otherwise as default (https://github.com/boxed/mutmut/pull/450#issuecomment-4002571055)
     )
 
 
@@ -128,6 +132,7 @@ class Config:
     tests_dir: list[str]
     mutate_only_covered_lines: bool
     type_check_command: list[str]
+    use_setproctitle: bool
 
     def should_ignore_for_mutation(self, path: Path | str) -> bool:
         path_str = str(path)

src/mutmut/utils/safe_setproctitle.py

@@ -6,16 +6,14 @@
 This module provides a safe_setproctitle() function that:
 - Works normally on Linux
 - Is a no-op on macOS to avoid crashes after fork()
-"""
-
-import platform
-import sys
 
-from setproctitle import setproctitle as _setproctitle
+Related: https://github.com/boxed/mutmut/pull/450#issuecomment-4002571055
+"""
 
-_use_setproctitle = not (sys.version_info >= (3, 10) and platform.system() == "Darwin")
+from mutmut.configuration import Config
 
-if _use_setproctitle:
+if Config.get().use_setproctitle:
+    from setproctitle import setproctitle as _setproctitle
 
     def safe_setproctitle(title: str) -> None:
         """Set the process title."""

tests/test_configuration.py

@@ -65,6 +65,7 @@ def test_ignores_non_python_files(self):
             tests_dir=[],
             mutate_only_covered_lines=False,
             type_check_command=[],
+            use_setproctitle=False,
         )
         assert config.should_ignore_for_mutation("foo.txt") is True
         assert config.should_ignore_for_mutation("foo.js") is True
@@ -82,6 +83,7 @@ def test_does_not_ignore_python_files(self):
             tests_dir=[],
             mutate_only_covered_lines=False,
             type_check_command=[],
+            use_setproctitle=False,
         )
         assert config.should_ignore_for_mutation("foo.py") is False
         assert config.should_ignore_for_mutation("src/foo.py") is False
@@ -98,6 +100,7 @@ def test_respects_do_not_mutate_exact_match(self):
             tests_dir=[],
             mutate_only_covered_lines=False,
             type_check_command=[],
+            use_setproctitle=False,
         )
         assert config.should_ignore_for_mutation("foo.py") is True
         assert config.should_ignore_for_mutation("bar.py") is False
@@ -114,6 +117,7 @@ def test_respects_do_not_mutate_glob_pattern(self):
             tests_dir=[],
             mutate_only_covered_lines=False,
             type_check_command=[],
+            use_setproctitle=False,
         )
         assert config.should_ignore_for_mutation("tests/test_foo.py") is True
         assert config.should_ignore_for_mutation("src/ignore_me.py") is True
@@ -131,6 +135,7 @@ def test_accepts_path_objects(self):
             tests_dir=[],
             mutate_only_covered_lines=False,
             type_check_command=[],
+            use_setproctitle=False,
         )
         assert config.should_ignore_for_mutation(Path("foo.py")) is True
         assert config.should_ignore_for_mutation(Path("bar.py")) is False

tests/utils/test_safe_setproctitle.py

@@ -8,18 +8,17 @@
 import os
 import platform
 import signal
-import sys
 
 import pytest
 from setproctitle import setproctitle
 
 from mutmut.utils.safe_setproctitle import safe_setproctitle
 
-# Only run this test on macOS with Python 3.14+
-IS_MACOS_314 = sys.version_info >= (3, 14) and platform.system() == "Darwin"
+# Only run this test on macOS
+IS_MACOS = platform.system() == "Darwin"
 
 
-@pytest.mark.skipif(not IS_MACOS_314, reason="setproctitle only crashes after fork on macOS Python 3.14+")
+@pytest.mark.skipif(not IS_MACOS, reason="setproctitle only crashes after fork on macOS")
 def test_setproctitle_crashes_after_fork_with_corefoundation_loaded():
     """Verify setproctitle segfaults after fork when CoreFoundation is loaded.
 
@@ -63,7 +62,7 @@ def test_setproctitle_crashes_after_fork_with_corefoundation_loaded():
             )
 
 
-@pytest.mark.skipif(not IS_MACOS_314, reason="safe_setproctitle workaround only applies to macOS Python 3.14+")
+@pytest.mark.skipif(not IS_MACOS, reason="safe_setproctitle workaround only applies to macOS")
 def test_safe_setproctitle_does_not_crash_after_fork():
     """Verify our safe_setproctitle wrapper doesn't crash after fork."""
     pid = os.fork()