git_refs: read refs via git plumbing for files/reftable

Replace direct `packed-refs` file parsing with `git for-each-ref`
plumbing to support both `files` and `reftable` backends.

Bug: 476209856
Change-Id: I2ad8ff8f3382426600f15370c997f9bc17165485
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/550401
Commit-Queue: Gavin Mak <gavinmak@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Gavin Mak <gavinmak@google.com>

tests/test_git_refs.py

@@ -0,0 +1,99 @@
+# Copyright (C) 2026 The Android Open Source Project
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Unittests for the git_refs.py module."""
+
+import os
+from pathlib import Path
+import subprocess
+
+import pytest
+import utils_for_test
+
+import git_refs
+
+
+def _run(repo, *args):
+    return subprocess.run(
+        ["git", "-C", repo, *args],
+        stdout=subprocess.PIPE,
+        stderr=subprocess.PIPE,
+        encoding="utf-8",
+        check=True,
+    ).stdout.strip()
+
+
+def _init_repo(tmp_path, reftable=False):
+    repo = os.path.join(tmp_path, "repo")
+    ref_format = "reftable" if reftable else "files"
+    utils_for_test.init_git_tree(repo, ref_format=ref_format)
+
+    Path(os.path.join(repo, "a")).write_text("1")
+    _run(repo, "add", "a")
+    _run(repo, "commit", "-q", "-m", "init")
+    return repo
+
+
+@pytest.mark.parametrize("reftable", [False, True])
+def test_reads_refs(tmp_path, reftable):
+    if reftable and not utils_for_test.supports_reftable():
+        pytest.skip("reftable not supported")
+
+    repo = _init_repo(tmp_path, reftable=reftable)
+    gitdir = os.path.join(repo, ".git")
+    refs = git_refs.GitRefs(gitdir)
+
+    branch = _run(repo, "symbolic-ref", "--short", "HEAD")
+    head = _run(repo, "rev-parse", "HEAD")
+    assert refs.symref("HEAD") == f"refs/heads/{branch}"
+    assert refs.get("HEAD") == head
+    assert refs.get(f"refs/heads/{branch}") == head
+
+
+@pytest.mark.parametrize("reftable", [False, True])
+def test_updates_when_refs_change(tmp_path, reftable):
+    if reftable and not utils_for_test.supports_reftable():
+        pytest.skip("reftable not supported")
+
+    repo = _init_repo(tmp_path, reftable=reftable)
+    gitdir = os.path.join(repo, ".git")
+    refs = git_refs.GitRefs(gitdir)
+
+    head = _run(repo, "rev-parse", "HEAD")
+    assert refs.get("refs/heads/topic") == ""
+    _run(repo, "branch", "topic")
+    assert refs.get("refs/heads/topic") == head
+    _run(repo, "branch", "-D", "topic")
+    assert refs.get("refs/heads/topic") == ""
+
+
+@pytest.mark.skipif(
+    not utils_for_test.supports_refs_migrate(),
+    reason="git refs migrate reftable support is required for this test",
+)
+def test_updates_when_storage_backend_toggles(tmp_path):
+    repo = _init_repo(tmp_path, reftable=False)
+    gitdir = os.path.join(repo, ".git")
+    refs = git_refs.GitRefs(gitdir)
+
+    head = _run(repo, "rev-parse", "HEAD")
+    assert refs.get("refs/heads/reftable-branch") == ""
+    _run(repo, "refs", "migrate", "--ref-format=reftable")
+    _run(repo, "branch", "reftable-branch")
+    assert refs.get("refs/heads/reftable-branch") == head
+
+    assert refs.get("refs/heads/files-branch") == ""
+    _run(repo, "refs", "migrate", "--ref-format=files")
+    _run(repo, "branch", "files-branch")
+    assert refs.get("refs/heads/files-branch") == head

tests/utils_for_test.py

@@ -18,20 +18,28 @@ If you want to write a per-test fixture, see conftest.py instead.
 """
 
 import contextlib
+import functools
 from pathlib import Path
 import subprocess
 import tempfile
-from typing import Union
+from typing import Optional, Union
 
 import git_command
 
 
-def init_git_tree(path: Union[str, Path]) -> None:
+def init_git_tree(
+    path: Union[str, Path],
+    ref_format: Optional[str] = None,
+) -> None:
     """Initialize `path` as a new git repo."""
     with contextlib.ExitStack() as stack:
         # Tests need to assume, that main is default branch at init,
         # which is not supported in config until 2.28.
-        cmd = ["git", "init"]
+        cmd = ["git"]
+        if ref_format:
+            cmd += ["-c", f"init.defaultRefFormat={ref_format}"]
+        cmd += ["init"]
+
         if git_command.git_require((2, 28, 0)):
             cmd += ["--initial-branch=main"]
         else:
@@ -51,3 +59,41 @@ def TempGitTree():
     with tempfile.TemporaryDirectory(prefix="repo-tests") as tempdir:
         init_git_tree(tempdir)
         yield tempdir
+
+
+@functools.lru_cache(maxsize=None)
+def supports_reftable() -> bool:
+    """Check if git supports reftable."""
+    with tempfile.TemporaryDirectory(prefix="repo-tests") as tempdir:
+        proc = subprocess.run(
+            ["git", "-c", "init.defaultRefFormat=reftable", "init"],
+            cwd=tempdir,
+            stdout=subprocess.PIPE,
+            stderr=subprocess.PIPE,
+            check=False,
+        )
+    return proc.returncode == 0
+
+
+@functools.lru_cache(maxsize=None)
+def supports_refs_migrate() -> bool:
+    """Check if git supports refs migrate."""
+    with tempfile.TemporaryDirectory(prefix="repo-tests") as tempdir:
+        subprocess.check_call(
+            ["git", "-c", "init.defaultRefFormat=files", "init"],
+            cwd=tempdir,
+        )
+        proc = subprocess.run(
+            [
+                "git",
+                "refs",
+                "migrate",
+                "--ref-format=reftable",
+                "--dry-run",
+            ],
+            cwd=tempdir,
+            stdout=subprocess.PIPE,
+            stderr=subprocess.PIPE,
+            check=False,
+        )
+    return proc.returncode == 0