Add a test that required submodules are initialized

This adds the new test, `test_required_submodule_is_initialized`, to
`test_fixture_health.py`. For the gitdb and smmap submodules, the
test asserts that the working tree directory exists and contains a
`.git` marker (file or directory). The failure message reminds the
developer to run `git submodule update --init --recursive`.

This is a regression test for a different contract than the preexisting
`test_fixture_dir_is_trusted_by_git` in `test_fixture_health.py`. That
test verifies git's willingness to operate in each fixture directory
(the `safe.directory` / dubious-ownership contract). This one verifies
that the directories are populated at all.

A bug where they were not populated affected Arch Linux in gitpython-developers#1713 when
`init-tests-after-clone.sh` stopped running `git submodule update` on
CI. The visible failure was a cascade of test failures across
`test_docs.py`, `test_repo.py`, and `test_submodule.py` -- tests that
don't set out to check submodule presence but fail when the submodules
aren't there. This test checks for that explicitly, with a single clear
assertion.

The test skips when the source tree is not a git clone (no `.git` at
`REPO_ROOT`). This is to accommodate setups that run pytest from an
extracted release tarball and prepare submodules in a separately-pointed
tree via `GIT_PYTHON_TEST_GIT_REPO_BASE` (e.g. OpenIndiana's package
build): in such setups, `git submodule update` cannot operate on the
source tree, and the submodule paths checked here would never be
populated there -- but the test suite still works because `rorepo`
points elsewhere.

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

Author: EliahKagan

test/test_fixture_health.py

@@ -45,6 +45,19 @@
     ),
 ]
 
+# Submodule working trees that must be present and initialized for the
+# test suite to operate normally: gitdb at `git/ext/gitdb`, and smmap
+# nested inside gitdb at `git/ext/gitdb/gitdb/ext/smmap`. The paths
+# below are anchored at REPO_ROOT (the GitPython source tree), not at
+# any rorepo redirection target.
+SUBMODULE_DIRS = [
+    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:
@@ -92,3 +105,40 @@ def test_fixture_dir_is_trusted_by_git(fixture_dir: Path) -> None:
         "This usually means the directory is not an initialized git "
         "repository (its `.git` marker may be stale or pointing elsewhere)."
     )
+
+
+@pytest.mark.parametrize("submodule_dir", SUBMODULE_DIRS)
+def test_required_submodule_is_initialized(submodule_dir: Path) -> None:
+    """The submodule's working tree is present and initialized.
+
+    Failure means the source tree is a git clone but the submodule's
+    working tree hasn't been populated. Skipped when the source tree
+    itself isn't a git clone (e.g. an extracted release tarball), since
+    ``git submodule update`` cannot operate there; setups that handle
+    submodules in a separately-prepared tree (via
+    ``GIT_PYTHON_TEST_GIT_REPO_BASE``) are exempted from this check.
+    """
+    if not (REPO_ROOT / ".git").exists():
+        pytest.skip(
+            "Source tree is not a git clone (no .git in REPO_ROOT); submodules "
+            "cannot be initialized via `git submodule update` here. Setups "
+            "that prepare submodules in a separately-pointed tree (via "
+            "GIT_PYTHON_TEST_GIT_REPO_BASE) are exempted from this check."
+        )
+    # The assertion messages below recommend `git submodule update --init
+    # --recursive` rather than `init-tests-after-clone.sh`, even though the
+    # latter is the documented entry point for first-time test setup. Two
+    # reasons: the script performs `git reset --hard` operations that can
+    # destroy local work, and #1713 showed the script itself can carry
+    # submodule-init regressions, in which case recommending it would lead
+    # developers in a circle. The direct git command is a safe minimal fix
+    # for this test's specific failure mode and bypasses any such regression.
+    assert submodule_dir.is_dir(), (
+        f"Submodule working tree missing: {submodule_dir}.\n"
+        "Run `git submodule update --init --recursive` from the repo root."
+    )
+    assert (submodule_dir / ".git").exists(), (
+        f"Submodule directory exists but has no .git marker: {submodule_dir}.\n"
+        "The submodule hasn't been initialized. "
+        "Run `git submodule update --init --recursive` from the repo root."
+    )