tests: Set HOME to a temporary directory when running tests.

When running the tests in my environment, tests that derived from `test_wrapper.GitCheckoutTestCase` would fail on commit or tag due to incomplete or incorrect gpg config. Ideally, the tests should not be dependent on the user's git config. This change ensures $HOME (or Windows equivalent) is replaced for the session.

Bug: 302797407

Change-Id: Ib42b712dd7b6602fee6e18329a8c6d52fb9458b9
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/388235
Tested-by: Jason R. Coombs <jaraco@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Commit-Queue: Jason R. Coombs <jaraco@google.com>

tests/conftest.py

@@ -14,8 +14,11 @@
 
 """Common fixtures for pytests."""
 
+import pathlib
+
 import pytest
 
+import platform_utils
 import repo_trace
 
 
@@ -23,3 +26,49 @@ import repo_trace
 def disable_repo_trace(tmp_path):
     """Set an environment marker to relax certain strict checks for test code."""  # noqa: E501
     repo_trace._TRACE_FILE = str(tmp_path / "TRACE_FILE_from_test")
+
+
+# adapted from pytest-home 0.5.1
+def _set_home(monkeypatch, path: pathlib.Path):
+    """
+    Set the home dir using a pytest monkeypatch context.
+    """
+    win = platform_utils.isWindows()
+    vars = ["HOME"] + win * ["USERPROFILE"]
+    for var in vars:
+        monkeypatch.setenv(var, str(path))
+    return path
+
+
+# copied from
+# https://github.com/pytest-dev/pytest/issues/363#issuecomment-1335631998
+@pytest.fixture(scope="session")
+def monkeysession():
+    with pytest.MonkeyPatch.context() as mp:
+        yield mp
+
+
+@pytest.fixture(autouse=True, scope="session")
+def session_tmp_home_dir(tmp_path_factory, monkeysession):
+    """Set HOME to a temporary directory, avoiding user's .gitconfig.
+
+    b/302797407
+
+    Set home at session scope to take effect prior to
+    ``test_wrapper.GitCheckoutTestCase.setUpClass``.
+    """
+    return _set_home(monkeysession, tmp_path_factory.mktemp("home"))
+
+
+# adapted from pytest-home 0.5.1
+@pytest.fixture(autouse=True)
+def tmp_home_dir(monkeypatch, tmp_path_factory):
+    """Set HOME to a temporary directory.
+
+    Ensures that state doesn't accumulate in $HOME across tests.
+
+    Note that in conjunction with session_tmp_homedir, the HOME
+    dir is patched twice, once at session scope, and then again at
+    the function scope.
+    """
+    return _set_home(monkeypatch, tmp_path_factory.mktemp("home"))