[core] Stop parent git repos from breaking ESP-IDF/PlatformIO builds (#16994)

tests/unit_tests/test_helpers.py

@@ -196,6 +196,33 @@ def test_is_ha_addon(monkeypatch, value, expected):
     assert actual == expected
 
 
+def test_add_git_ceiling_directory_sets_when_unset():
+    """An empty env gets GIT_CEILING_DIRECTORIES set to the directory."""
+    env: dict[str, str] = {}
+    directory = Path("/home/user/config")
+    helpers.add_git_ceiling_directory(env, directory)
+    assert env["GIT_CEILING_DIRECTORIES"] == str(directory)
+
+
+def test_add_git_ceiling_directory_appends_to_existing():
+    """An existing value is preserved and the new directory is appended."""
+    env = {"GIT_CEILING_DIRECTORIES": str(Path("/some/ceiling"))}
+    directory = Path("/home/user/config")
+    helpers.add_git_ceiling_directory(env, directory)
+    assert env["GIT_CEILING_DIRECTORIES"].split(os.pathsep) == [
+        str(Path("/some/ceiling")),
+        str(directory),
+    ]
+
+
+def test_add_git_ceiling_directory_skips_duplicate():
+    """A directory already in the list is not appended again."""
+    directory = Path("/home/user/config")
+    env = {"GIT_CEILING_DIRECTORIES": str(directory)}
+    helpers.add_git_ceiling_directory(env, directory)
+    assert env["GIT_CEILING_DIRECTORIES"] == str(directory)
+
+
 def test_walk_files(fixture_path):
     path = fixture_path / "helpers"