Add a check that fixture directories are usable by git

Many tests rely on git accepting their fixture directories -- the
GitPython working tree and the vendored gitdb and smmap submodule
working trees. When git rejects one for "dubious ownership", typically
because a CI workflow's safe.directory list is missing an entry,
downstream tests fail in opaque ways: IndexError in
test_docs::Tutorials::test_submodules, AssertionError in
test_repo::TestRepo::test_submodules and
test_submodule::TestSubmodule::test_root_module, plus BrokenPipeError
or ValueError elsewhere depending on a race between Python's flush and
pipe-close on the cached "git cat-file --batch-check" subprocess.

test/test_fixture_health.py runs `git rev-parse --show-toplevel` in
each fixture directory and asserts both that git is willing to operate
there and that it reports the directory as its own toplevel. The
failure message names safe.directory and ownership as the likely
causes, so a misconfigured environment is recognizable directly from
the test output rather than only inferable from the cascade of
downstream failures.

This commit adds the test only. On the Cygwin CI workflow as
configured at this commit, the test fails for gitdb and smmap,
demonstrating the bug. Subsequent commits in this chain reproduce the
bug at scale and apply the fix.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: EliahKagan

test/test_fixture_health.py

@@ -0,0 +1,87 @@
+# This module is part of GitPython and is released under the
+# 3-Clause BSD License: https://opensource.org/license/bsd-3-clause/
+
+"""Verify that fixture directories are usable by git.
+
+If a directory the test suite relies on is rejected by git -- typically for
+"dubious ownership" because the CI workflow or local clone is missing a
+``safe.directory`` entry, or because directory ownership doesn't match the
+running user -- many tests downstream fail in confusing ways. The checks here
+name the root cause clearly so a misconfigured environment is recognizable
+from the test output.
+
+These tests do not exercise GitPython's production code. They verify the
+conditions under which production code is exercised are valid.
+
+A check is skipped, rather than failed, if a fixture directory is missing or
+has no ``.git`` marker, since that condition is more naturally diagnosed as
+"``init-tests-after-clone.sh`` hasn't been run" than as a problem with
+``safe.directory``.
+"""
+
+import subprocess
+from pathlib import Path
+
+import pytest
+
+REPO_ROOT = Path(__file__).resolve().parent.parent
+
+# Directories git must trust for the test suite to operate normally. The
+# current set is the GitPython working tree plus the working trees of the
+# vendored submodules (gitdb and the nested smmap inside it). New entries
+# should be added here whenever the test suite gains a dependency on git
+# accepting another directory.
+FIXTURE_DIRS = [
+    pytest.param(REPO_ROOT, id="repo_root"),
+    pytest.param(REPO_ROOT / "git" / "ext" / "gitdb", id="gitdb"),
+    pytest.param(
+        REPO_ROOT / "git" / "ext" / "gitdb" / "gitdb" / "ext" / "smmap",
+        id="smmap",
+    ),
+]
+
+
+@pytest.mark.parametrize("fixture_dir", FIXTURE_DIRS)
+def test_fixture_dir_is_trusted_by_git(fixture_dir: Path) -> None:
+    """git accepts ``fixture_dir`` as its own repository owned by a trusted user.
+
+    Run ``git -C <fixture_dir> rev-parse --show-toplevel`` and assert it
+    succeeds and reports ``fixture_dir`` itself as the toplevel. Failure
+    typically means a missing ``safe.directory`` entry in the CI workflow or
+    in the developer's local git config, or a clone whose on-disk ownership
+    doesn't match the running user.
+    """
+    if not fixture_dir.exists():
+        pytest.skip(
+            f"{fixture_dir} not present "
+            "(run ./init-tests-after-clone.sh from the repo root)"
+        )
+    if not (fixture_dir / ".git").exists():
+        pytest.skip(
+            f"{fixture_dir} has no .git marker "
+            "(submodule not initialized; run ./init-tests-after-clone.sh "
+            "from the repo root)"
+        )
+    try:
+        result = subprocess.run(
+            ["git", "-C", str(fixture_dir), "rev-parse", "--show-toplevel"],
+            capture_output=True,
+            text=True,
+            check=False,
+        )
+    except FileNotFoundError:
+        pytest.skip("git is not installed or not on PATH")
+    assert result.returncode == 0, (
+        f"git refuses to operate in {fixture_dir}.\n"
+        f"stderr: {result.stderr.strip()}\n"
+        "This usually means the CI workflow or local environment is missing "
+        "a `safe.directory` entry for this path, or that the directory's "
+        "ownership doesn't match the running user."
+    )
+    reported = Path(result.stdout.strip())
+    assert reported.samefile(fixture_dir), (
+        f"git reports the toplevel as {reported}, "
+        f"not as {fixture_dir} itself. "
+        "This usually means the directory is not an initialized git "
+        "repository (its `.git` marker may be stale or pointing elsewhere)."
+    )