cherry_pick: resolve and read commits in one process

Send a typed revision expression to cat-file --batch over stdin and
parse its object header and exact byte length. This safely resolves the
commit OID and retrieves its raw message in one Git process instead of
separate rev-parse and cat-file calls.

Bug: 553599402
Change-Id: I764894323ec28e134f2523bd1d5694738b6b52d0
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/623903
Reviewed-by: Brian Gan <brgan@google.com>
Tested-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>

subcmds/cherry_pick.py

@@ -14,6 +14,7 @@
 
 import re
 import sys
+from typing import Tuple
 
 from command import Command
 from error import GitError
@@ -43,36 +44,8 @@ change id will be added.
 
     def Execute(self, opt, args):
         reference = args[0]
-
-        p = GitCommand(
-            None,
-            ["rev-parse", "--verify", reference],
-            capture_stdout=True,
-            capture_stderr=True,
-            verify_command=True,
-        )
-        try:
-            p.Wait()
-        except GitError:
-            logger.error(p.stderr)
-            raise
-
-        sha1 = p.stdout.strip()
-
-        p = GitCommand(
-            None,
-            ["cat-file", "commit", sha1],
-            capture_stdout=True,
-            verify_command=True,
-        )
-
-        try:
-            p.Wait()
-        except GitError:
-            logger.error("error: Failed to retrieve old commit message")
-            raise
-
-        old_msg = self._StripHeader(p.stdout)
+        sha1, commit = self._ResolveReference(reference)
+        old_msg = self._StripHeader(commit)
 
         p = GitCommand(
             None,
@@ -117,6 +90,48 @@ change id will be added.
             logger.error("error: Failed to update commit message")
             raise
 
+    def _ResolveReference(self, reference: str) -> Tuple[str, str]:
+        """Resolve a commit and read it through one cat-file batch request."""
+        expression = f"{reference}^{{commit}}"
+        p = GitCommand(
+            None,
+            ["cat-file", "--batch"],
+            input=expression + "\n",
+            capture_stdout=True,
+            capture_stderr=True,
+            verify_command=True,
+        )
+        try:
+            p.Wait()
+            header, separator, output = p.stdout.partition("\n")
+            if not separator:
+                raise ValueError("missing cat-file header")
+            if header.endswith(" missing") or header.endswith(" ambiguous"):
+                raise GitError(f"commit {reference} not found")
+
+            parts = header.split(" ", 2)
+            if len(parts) != 3 or parts[1] != "commit":
+                raise ValueError(
+                    f"unexpected object type {parts[1]!r}"
+                    if len(parts) >= 2
+                    else "invalid header"
+                )
+            sha1, _object_type, _size = parts
+
+            if not output.endswith("\n"):
+                raise ValueError("truncated cat-file object")
+
+            commit = output[:-1]
+        except (GitError, ValueError) as e:
+            logger.error(
+                "error: Failed to resolve or read commit %s", reference
+            )
+            if isinstance(e, GitError):
+                raise
+            raise GitError(str(e)) from e
+
+        return sha1, commit
+
     def _IsChangeId(self, line):
         return CHANGE_ID_RE.match(line)
 

tests/test_subcmds_cherry_pick.py

@@ -0,0 +1,85 @@
+# 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 subcmds/cherry_pick.py."""
+
+from unittest import mock
+
+import pytest
+
+from error import GitError
+from git_command import GitCommand
+from subcmds import cherry_pick
+
+
+def test_resolve_reference_uses_one_typed_batch_request(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    oid = "1" * 40
+    commit = "tree " + "2" * 40 + "\n\nSubject 🚀\n\nBody\n"
+    output = oid + " commit " + str(len(commit.encode("utf-8"))) + "\n"
+    output += commit + "\n"
+    command = mock.create_autospec(GitCommand, instance=True)
+    command.stdout = output
+    command.stderr = ""
+    command.Wait.return_value = 0
+    run_git = mock.create_autospec(GitCommand, return_value=command)
+    monkeypatch.setattr(cherry_pick, "GitCommand", run_git)
+
+    resolved, contents = cherry_pick.CherryPick()._ResolveReference("topic")
+
+    assert resolved == oid
+    assert contents == commit
+    run_git.assert_called_once_with(
+        None,
+        ["cat-file", "--batch"],
+        input="topic^{commit}\n",
+        capture_stdout=True,
+        capture_stderr=True,
+        verify_command=True,
+    )
+
+
+def test_resolve_reference_rejects_missing_object(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    command = mock.create_autospec(GitCommand, instance=True)
+    command.stdout = "topic^{commit} missing\n"
+    command.stderr = ""
+    command.Wait.return_value = 0
+    monkeypatch.setattr(
+        cherry_pick,
+        "GitCommand",
+        mock.create_autospec(GitCommand, return_value=command),
+    )
+
+    with pytest.raises(GitError, match="commit topic not found"):
+        cherry_pick.CherryPick()._ResolveReference("topic")
+
+
+def test_resolve_reference_rejects_ambiguous_object(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    command = mock.create_autospec(GitCommand, instance=True)
+    command.stdout = "topic^{commit} ambiguous\n"
+    command.stderr = ""
+    command.Wait.return_value = 0
+    monkeypatch.setattr(
+        cherry_pick,
+        "GitCommand",
+        mock.create_autospec(GitCommand, return_value=command),
+    )
+
+    with pytest.raises(GitError, match="commit topic not found"):
+        cherry_pick.CherryPick()._ResolveReference("topic")