git_refs: load HEAD with the ref snapshot on Git 2.45

Use for-each-ref --include-root-refs to resolve attached and detached
HEAD in the same process as refs/* on Git 2.45 and newer. Limit the
patterns to HEAD and refs so volatile pseudo-refs do not enter the
cache, and retain the existing fallback for older Git and unborn HEADs.

Bug: 543851900
Bug: 553599402
Change-Id: Ie091ca0757f21ec6b1e19d49e09d160e055ec639
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/622981
Tested-by: Gavin Mak <gavinmak@google.com>
Reviewed-by: Brian Gan <brgan@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>

git_refs.py

@@ -14,6 +14,7 @@
 
 import os
 
+from git_command import git_require
 from git_command import GitCommand
 import platform_utils
 from repo_trace import Trace
@@ -41,6 +42,12 @@ class GitRefs:
         self._EnsureLoaded()
         return self._phyref
 
+    @property
+    def head(self) -> str:
+        """Return HEAD's symbolic target or detached object ID."""
+        self._EnsureLoaded()
+        return self._symref.get(HEAD) or self._phyref.get(HEAD, "")
+
     def get(self, name):
         try:
             return self.all[name]
@@ -87,8 +94,12 @@ class GitRefs:
             self._symref = {}
             self._mtime = {}
 
-            self._ReadRefs()
-            self._ReadSymbolicRef(HEAD)
+            root_refs_loaded = self._ReadRefs()
+            if not root_refs_loaded or (
+                HEAD not in self._phyref and HEAD not in self._symref
+            ):
+                # --include-root-refs does not report an unborn HEAD.
+                self._ReadSymbolicRef(HEAD)
 
             scan = self._symref
             attempts = 0
@@ -113,18 +124,32 @@ class GitRefs:
         """Check if a ref_id is a null object ID."""
         return ref_id and all(ch == "0" for ch in ref_id)
 
-    def _ReadRefs(self) -> None:
-        """Read all references using git for-each-ref."""
+    def _ReadRefs(self) -> bool:
+        """Read all references using git for-each-ref.
+
+        Returns:
+            Whether root refs, including HEAD when it exists, were loaded.
+        """
+        include_root_refs = git_require((2, 45, 0))
+        cmd = [
+            "for-each-ref",
+            "--format=%(objectname)%00%(refname)%00%(symref)",
+        ]
+        if include_root_refs:
+            cmd.insert(1, "--include-root-refs")
+            # Avoid caching volatile root refs such as ORIG_HEAD.  HEAD and
+            # refs/* are the only namespaces GitRefs exposes to callers.
+            cmd.extend([HEAD, "refs"])
         p = GitCommand(
             None,
-            ["for-each-ref", "--format=%(objectname)%00%(refname)%00%(symref)"],
+            cmd,
             capture_stdout=True,
             capture_stderr=True,
             bare=True,
             gitdir=self._gitdir,
         )
         if p.Wait() != 0:
-            return
+            return False
 
         for line in p.stdout.splitlines():
             ref_id, name, symref = line.split("\0")
@@ -132,6 +157,7 @@ class GitRefs:
                 self._symref[name] = symref
             elif ref_id and not self._IsNullRef(ref_id):
                 self._phyref[name] = ref_id
+        return include_root_refs
 
     def _ReadSymbolicRef(self, name: str) -> None:
         """Read a symbolic reference."""

tests/test_git_refs.py

@@ -17,6 +17,8 @@
 import os
 from pathlib import Path
 import subprocess
+from typing import Any, List
+from unittest import mock
 
 import pytest
 import utils_for_test
@@ -24,7 +26,7 @@ import utils_for_test
 import git_refs
 
 
-def _run(repo, *args):
+def _run(repo: str, *args: str) -> str:
     return subprocess.run(
         ["git", "-C", repo, *args],
         stdout=subprocess.PIPE,
@@ -34,7 +36,7 @@ def _run(repo, *args):
     ).stdout.strip()
 
 
-def _init_repo(tmp_path, reftable=False):
+def _init_repo(tmp_path: Path, reftable: bool = False) -> str:
     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)
@@ -61,6 +63,149 @@ def test_reads_refs(tmp_path, reftable):
     assert refs.get(f"refs/heads/{branch}") == head
 
 
+@pytest.mark.parametrize("reftable", [False, True])
+def test_reads_detached_head(tmp_path: Path, reftable: bool) -> None:
+    if reftable and not utils_for_test.supports_reftable():
+        pytest.skip("reftable not supported")
+
+    repo = _init_repo(tmp_path, reftable=reftable)
+    head = _run(repo, "rev-parse", "HEAD")
+    _run(repo, "checkout", "--detach", head)
+    refs = git_refs.GitRefs(os.path.join(repo, ".git"))
+
+    assert refs.symref("HEAD") == ""
+    assert refs.head == head
+    assert refs.get("HEAD") == head
+
+
+def test_reads_head_with_root_refs_in_one_command(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """Git 2.45 and newer include HEAD in the ref snapshot."""
+    head = "1" * 40
+    commands = []
+
+    class FakeGitCommand:
+        def __init__(
+            self, _project: Any, cmdv: List[str], **_kwargs: Any
+        ) -> None:
+            commands.append(cmdv)
+            self.stdout = (
+                f"{head}\0HEAD\0refs/heads/main\n"
+                f"{head}\0refs/heads/main\0\n"
+            )
+
+        def Wait(self) -> int:
+            return 0
+
+    monkeypatch.setattr(git_refs, "GitCommand", FakeGitCommand)
+    monkeypatch.setattr(git_refs, "git_require", lambda _version: True)
+    refs = git_refs.GitRefs("/nonexistent")
+    with mock.patch.object(refs, "_ReadSymbolicRef") as read_head:
+        assert refs.get("HEAD") == head
+
+    assert commands == [
+        [
+            "for-each-ref",
+            "--include-root-refs",
+            "--format=%(objectname)%00%(refname)%00%(symref)",
+            "HEAD",
+            "refs",
+        ]
+    ]
+    read_head.assert_not_called()
+
+
+def test_root_ref_snapshot_falls_back_for_unborn_head(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """An unborn HEAD still uses symbolic-ref after the ref snapshot."""
+
+    class FakeGitCommand:
+        def __init__(
+            self, _project: Any, _cmdv: List[str], **_kwargs: Any
+        ) -> None:
+            self.stdout = ""
+
+        def Wait(self) -> int:
+            return 0
+
+    monkeypatch.setattr(git_refs, "GitCommand", FakeGitCommand)
+    monkeypatch.setattr(git_refs, "git_require", lambda _version: True)
+    refs = git_refs.GitRefs("/nonexistent")
+
+    def read_head(name: str) -> None:
+        assert name == "HEAD"
+        refs._symref[name] = "refs/heads/main"
+
+    monkeypatch.setattr(refs, "_ReadSymbolicRef", read_head)
+
+    assert refs.symref("HEAD") == "refs/heads/main"
+
+
+def test_old_git_keeps_separate_head_fallback(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """Git before 2.45 uses the original for-each-ref and HEAD calls."""
+    head = "1" * 40
+    commands = []
+
+    class FakeGitCommand:
+        def __init__(
+            self, _project: Any, cmdv: List[str], **_kwargs: Any
+        ) -> None:
+            commands.append(cmdv)
+            self.stdout = f"{head}\0refs/heads/main\0\n"
+
+        def Wait(self) -> int:
+            return 0
+
+    monkeypatch.setattr(git_refs, "GitCommand", FakeGitCommand)
+    monkeypatch.setattr(git_refs, "git_require", lambda _version: False)
+    refs = git_refs.GitRefs("/nonexistent")
+
+    def read_head(name: str) -> None:
+        assert name == "HEAD"
+        refs._symref[name] = "refs/heads/main"
+
+    monkeypatch.setattr(refs, "_ReadSymbolicRef", read_head)
+
+    assert refs.get("HEAD") == head
+    assert commands == [
+        [
+            "for-each-ref",
+            "--format=%(objectname)%00%(refname)%00%(symref)",
+        ]
+    ]
+
+
+def test_for_each_ref_failure_falls_back_to_symbolic_ref(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """When for-each-ref fails, HEAD is still resolved via symbolic-ref."""
+
+    class FakeGitCommand:
+        def __init__(
+            self, _project: Any, _cmdv: List[str], **_kwargs: Any
+        ) -> None:
+            self.stdout = ""
+
+        def Wait(self) -> int:
+            return 1
+
+    monkeypatch.setattr(git_refs, "GitCommand", FakeGitCommand)
+    monkeypatch.setattr(git_refs, "git_require", lambda _version: True)
+    refs = git_refs.GitRefs("/nonexistent")
+
+    def read_head(name: str) -> None:
+        assert name == "HEAD"
+        refs._symref[name] = "refs/heads/main"
+
+    monkeypatch.setattr(refs, "_ReadSymbolicRef", read_head)
+
+    assert refs.symref("HEAD") == "refs/heads/main"
+
+
 @pytest.mark.parametrize("reftable", [False, True])
 def test_updates_when_refs_change(tmp_path, reftable):
     if reftable and not utils_for_test.supports_reftable():