🐛 fix(config): ignore a config file that fails to parse instead of crashing

docs/changelog/3230.bugfix.rst

@@ -0,0 +1,4 @@
+A ``virtualenv.ini`` that fails to parse no longer crashes every invocation with an ``AttributeError``; it is skipped
+with the error logged and ``--help`` reports it as ``failed to parse``. A UTF-8 byte order mark, which PowerShell 5 and
+older versions of Notepad write, is now tolerated in the config file instead of being treated as a missing section
+header - by :user:`darrenhuai`.

src/virtualenv/config/ini.py

@@ -38,6 +38,7 @@ def __init__(self, env: Mapping[str, str] | None = None) -> None:
 
         exception = None
         self.has_config_file = None
+        self.has_virtualenv_section = False
         try:
             self.has_config_file = self.config_file.exists()
         except OSError as exc:
@@ -50,12 +51,13 @@ def __init__(self, env: Mapping[str, str] | None = None) -> None:
                     self._load()
                     self.has_virtualenv_section = self.config_parser.has_section(self.section)
                 except Exception as exc:  # ruff:ignore[blind-except]
+                    self.has_config_file = None  # mark it failed to parse, so the config is ignored
                     exception = exc
         if exception is not None:
             LOGGER.error("failed to read config file %s because %r", config_file, exception)
 
     def _load(self) -> None:
-        with self.config_file.open("rt", encoding="utf-8") as file_handler:
+        with self.config_file.open("rt", encoding="utf-8-sig") as file_handler:
             return self.config_parser.read_file(file_handler)
 
     def get(self, key: str, as_type: TypeData) -> tuple[Any, str] | None:

tests/unit/config/test_ini.py

@@ -1,10 +1,13 @@
 from __future__ import annotations
 
+import codecs
+import logging
 import sys
 from textwrap import dedent
 
 import pytest
 
+from virtualenv.config.ini import IniConfig
 from virtualenv.info import IS_PYPY, IS_WIN, fs_supports_symlink
 from virtualenv.run import session_via_cli
 
@@ -32,3 +35,41 @@ def test_ini_can_be_overwritten_by_flag(tmp_path, monkeypatch) -> None:
 
     symlinks = result.creator.symlinks
     assert symlinks is True
+
+
+def test_ini_that_fails_to_parse_is_ignored(tmp_path, caplog) -> None:
+    bad_ini = tmp_path / "conf.ini"
+    bad_ini.write_text("this line has no section header\n", encoding="utf-8")
+
+    config = IniConfig(env={"VIRTUALENV_CONFIG_FILE": str(bad_ini)})
+
+    assert bool(config) is False
+    assert config.has_virtualenv_section is False
+    assert "failed to parse" in config.epilog
+    assert any("failed to read config file" in r.message for r in caplog.records if r.levelno == logging.ERROR)
+
+
+def test_ini_that_fails_to_parse_does_not_break_the_cli(tmp_path, monkeypatch) -> None:
+    bad_ini = tmp_path / "conf.ini"
+    bad_ini.write_text("this line has no section header\n", encoding="utf-8")
+    monkeypatch.setenv("VIRTUALENV_CONFIG_FILE", str(bad_ini))
+
+    result = session_via_cli(["venv"])  # the config is ignored, defaults apply
+
+    assert result.creator.clear is False
+
+
+def test_ini_with_utf8_bom_is_read(tmp_path, monkeypatch) -> None:
+    custom_ini = tmp_path / "conf.ini"
+    content = dedent(
+        """
+        [virtualenv]
+        clear = True
+        """,
+    )
+    custom_ini.write_bytes(codecs.BOM_UTF8 + content.encode("utf-8"))  # how Notepad and PowerShell 5 write UTF-8
+    monkeypatch.setenv("VIRTUALENV_CONFIG_FILE", str(custom_ini))
+
+    result = session_via_cli(["venv"])
+
+    assert result.creator.clear is True