Simplify/clarify bash.exe check for hook tests; do it only once

- Make the design of the enum simpler.
- Move some informaton to the check method's docstring.
- Call the method once instead of in each decoration.

Doing the check only once makes things a little faster, but the
more important reason is that the checks can become stale very
quickly in some situations. This is unlikely to be an issue on CI,
but locally WSL may fail (or not fail) to start up when bash.exe
is used in a check, then not fail (or fail) when actaully needed
in the test, if the reason it fails is due to resource usage, such
as most RAM being used on the system.

Although this seems like a reason to do the check multiple times,
doing it multiple times in the decorations still does it ahead of
when any of the tests is actually run. In contrast, with the change
here, we may get outdated results by the time the tests run, but
the xfail effects of the check are always consistent with each
other and are no longer written in a way that suggests they are
more current than they really are.

test/test_index.py

@@ -39,32 +39,11 @@
 log = logging.getLogger(__name__)
 
 
-class _WinBashMeta(enum.EnumMeta):
-    """Metaclass allowing :class:`_WinBash` custom behavior when called."""
-
-    def __call__(cls):
-        return cls._check()
-
-
 @enum.unique
-class _WinBash(enum.Enum, metaclass=_WinBashMeta):
+class _WinBashStatus(enum.Enum):
     """Status of bash.exe for native Windows. Affects which commit hook tests can pass.
 
-    Call ``_WinBash()`` to check the status.
-
-    This can't be reliably discovered using :func:`shutil.which`, as that approximates
-    how a shell is expected to search for an executable. On Windows, there are major
-    differences between how executables are found by a shell and otherwise. (This is the
-    cmd.exe Windows shell and should not be confused with bash.exe.) Our run_commit_hook
-    function in GitPython uses subprocess.Popen, including to run bash.exe on Windows.
-    It does not pass shell=True (and should not). Popen calls CreateProcessW, which
-    searches several locations prior to using the PATH environment variable. It is
-    expected to search the System32 directory, even if another directory containing the
-    executable precedes it in PATH. (There are other differences, less relevant here.)
-    When WSL is installed, even if no WSL *systems* are installed, bash.exe exists in
-    System32, and Popen finds it even if another bash.exe precedes it in PATH, as
-    happens on CI. If WSL is absent, System32 may still have bash.exe, as Windows users
-    and administrators occasionally copy executables there in lieu of extending PATH.
+    Call :meth:`check` to check the status.
     """
 
     INAPPLICABLE = enum.auto()
@@ -74,13 +53,13 @@ class _WinBash(enum.Enum, metaclass=_WinBashMeta):
     """No command for ``bash.exe`` is found on the system."""
 
     NATIVE = enum.auto()
-    """Running ``bash.exe`` operates outside any WSL environment (as with Git Bash)."""
+    """Running ``bash.exe`` operates outside any WSL distribution (as with Git Bash)."""
 
     WSL = enum.auto()
-    """Running ``bash.exe`` runs bash on a WSL system."""
+    """Running ``bash.exe`` calls ``bash`` in a WSL distribution."""
 
     WSL_NO_DISTRO = enum.auto()
-    """Running ``bash.exe` tries to run bash on a WSL system, but none exists."""
+    """Running ``bash.exe` tries to run bash on a WSL distribution, but none exists."""
 
     ERROR_WHILE_CHECKING = enum.auto()
     """Could not determine the status.
@@ -92,13 +71,34 @@ class _WinBash(enum.Enum, metaclass=_WinBashMeta):
     """
 
     @classmethod
-    def _check(cls):
+    def check(cls):
+        """Check the status of the ``bash.exe`` :func:`index.fun.run_commit_hook` uses.
+
+        This uses EAFP, attempting to run a command via ``bash.exe``. Which ``bash.exe``
+        is used can't be reliably discovered by :func:`shutil.which`, which approximates
+        how a shell is expected to search for an executable. On Windows, there are major
+        differences between how executables are found by a shell and otherwise. (This is
+        the cmd.exe Windows shell, and shouldn't be confused with bash.exe itself. That
+        the command being looked up also happens to be an interpreter is not relevant.)
+
+        :func:`index.fun.run_commit_hook` uses :class:`subprocess.Popen`, including when
+        it runs ``bash.exe`` on Windows. It doesn't pass ``shell=True`` (and shouldn't).
+        On Windows, `Popen` calls ``CreateProcessW``, which searches several locations
+        prior to using the ``PATH`` environment variable. It is expected to search the
+        ``System32`` directory, even if another directory containing the executable
+        precedes it in ``PATH``. (Other differences are less relevant here.) When WSL is
+        installed, even with no distributions, ``bash.exe`` exists in ``System32``, and
+        `Popen` finds it even if another ``bash.exe`` precedes it in ``PATH``, as on CI.
+        If WSL is absent, ``System32`` may still have ``bash.exe``, as Windows users and
+        administrators occasionally put executables there in lieu of extending ``PATH``.
+        """
         if os.name != "nt":
             return cls.INAPPLICABLE
 
         try:
             # Print rather than returning the test command's exit status so that if a
-            # failure occurs before we even get to this point, we will detect it.
+            # failure occurs before we even get to this point, we will detect it. See
+            # https://superuser.com/a/1749811 for information on ways to check for WSL.
             script = 'test -e /proc/sys/fs/binfmt_misc/WSLInterop; echo "$?"'
             command = ["bash.exe", "-c", script]
             proc = subprocess.run(command, capture_output=True, check=True, text=True)
@@ -129,6 +129,9 @@ def _error(cls, error_or_process):
         return cls.ERROR_WHILE_CHECKING
 
 
+_win_bash_status = _WinBashStatus.check()
+
+
 def _make_hook(git_dir, name, content, make_exec=True):
     """A helper to create a hook"""
     hp = hook_path(name, git_dir)
@@ -998,7 +1001,7 @@ class Mocked:
         self.assertEqual(rel, os.path.relpath(path, root))
 
     @pytest.mark.xfail(
-        _WinBash() is _WinBash.WSL_NO_DISTRO,
+        _win_bash_status is _WinBashStatus.WSL_NO_DISTRO,
         reason="Currently uses the bash.exe for WSL even with no WSL distro installed",
         raises=HookExecutionError,
     )
@@ -1009,7 +1012,7 @@ def test_pre_commit_hook_success(self, rw_repo):
         index.commit("This should not fail")
 
     @pytest.mark.xfail(
-        _WinBash() is _WinBash.WSL_NO_DISTRO,
+        _win_bash_status is _WinBashStatus.WSL_NO_DISTRO,
         reason="Currently uses the bash.exe for WSL even with no WSL distro installed",
         raises=AssertionError,
     )
@@ -1020,7 +1023,7 @@ def test_pre_commit_hook_fail(self, rw_repo):
         try:
             index.commit("This should fail")
         except HookExecutionError as err:
-            if _WinBash() is _WinBash.ABSENT:
+            if _win_bash_status is _WinBashStatus.ABSENT:
                 self.assertIsInstance(err.status, OSError)
                 self.assertEqual(err.command, [hp])
                 self.assertEqual(err.stdout, "")
@@ -1036,12 +1039,12 @@ def test_pre_commit_hook_fail(self, rw_repo):
             raise AssertionError("Should have caught a HookExecutionError")
 
     @pytest.mark.xfail(
-        _WinBash() in {_WinBash.ABSENT, _WinBash.WSL},
+        _win_bash_status in {_WinBashStatus.ABSENT, _WinBashStatus.WSL},
         reason="Specifically seems to fail on WSL bash (in spite of #1399)",
         raises=AssertionError,
     )
     @pytest.mark.xfail(
-        _WinBash() is _WinBash.WSL_NO_DISTRO,
+        _win_bash_status is _WinBashStatus.WSL_NO_DISTRO,
         reason="Currently uses the bash.exe for WSL even with no WSL distro installed",
         raises=HookExecutionError,
     )
@@ -1059,7 +1062,7 @@ def test_commit_msg_hook_success(self, rw_repo):
         self.assertEqual(new_commit.message, "{} {}".format(commit_message, from_hook_message))
 
     @pytest.mark.xfail(
-        _WinBash() is _WinBash.WSL_NO_DISTRO,
+        _win_bash_status is _WinBashStatus.WSL_NO_DISTRO,
         reason="Currently uses the bash.exe for WSL even with no WSL distro installed",
         raises=AssertionError,
     )
@@ -1070,7 +1073,7 @@ def test_commit_msg_hook_fail(self, rw_repo):
         try:
             index.commit("This should fail")
         except HookExecutionError as err:
-            if _WinBash() is _WinBash.ABSENT:
+            if _win_bash_status is _WinBashStatus.ABSENT:
                 self.assertIsInstance(err.status, OSError)
                 self.assertEqual(err.command, [hp])
                 self.assertEqual(err.stdout, "")