chore(release): address PR comments on sync PR

- Use is_news_file helper to avoid accidental deletion of non-news files.
- Make branch checkout robust by resetting existing branch to main.
- Specify UTF-8 encoding when writing patch files.

Author: rickeylev

tests/tools/private/release/process_backports_test.py

@@ -134,6 +134,115 @@ def mock_resolve(items):
         self.assertEqual(call_args[0], 123)
         self.assertIn("- [x] #124 | status=done rc=rc0 commit= 12345678", call_args[1])
 
+    @patch("tools.private.release.process_backports.datetime")
+    def test_process_backports_sync_branch_exists(self, mock_datetime):
+        mock_datetime.date.today.return_value = datetime.date(2026, 7, 1)
+        args = argparse.Namespace(
+            issue=123, remote="origin", dry_run=False, add=None, triggering_comment=None
+        )
+        self.mock_gh.get_issue_title.return_value = "Release 2.0.0"
+        self.mock_gh.get_issue_body.return_value = """
+## Checklist
+- [ ] Prepare Release
+- [ ] Create Release branch
+
+## Backports
+- [ ] #124 | status=pending
+"""
+        self.mock_git.get_remote_tags.return_value = []
+
+        def mock_resolve(items):
+            for item in items:
+                if item.pr_ref == "#124":
+                    item.commit = "abcdef12"
+                    item.status = "done"
+            return items
+
+        self.mock_gh.get_merge_commits_for_prs.side_effect = mock_resolve
+
+        self.mock_git.sort_commits_chronologically.return_value = ["abcdef12"]
+        self.mock_git.get_commit_sha.side_effect = ["12345678", "12345678", "main_sha"]
+        self.mock_git.get_commit_message.return_value = 'Cherry-pick "fix bug"'
+        self.mock_git.get_modified_files.return_value = ["news/124.fixed.md"]
+        self.mock_git.diff.return_value = "version diff for 124"
+        self.mock_git.apply_check.return_value = True
+        self.mock_gh.create_pr.return_value = "https://github.com/foo/bar/pull/999"
+
+        # Configure branch to exist
+        self.mock_git.branch_exists.return_value = True
+
+        result = ProcessBackports(args, self.mock_git, self.mock_gh).run()
+
+        self.assertEqual(result, 0)
+        self.mock_git.fetch.assert_has_calls(
+            [
+                call("origin", tags=True, force=True),
+                call("origin"),
+                call("origin", refspec="main"),
+            ]
+        )
+        self.mock_git.checkout.assert_has_calls(
+            [
+                call("release/2.0", track_remote="origin"),
+                call("main", track_remote="origin"),
+                # Called without create_branch=True
+                call("prepare-2.0.0-backports-6affdae"),
+                call("release/2.0"),
+            ]
+        )
+        # Verify reset_hard was called to reset the existing branch to main
+        self.mock_git.reset_hard.assert_has_calls(
+            [
+                call("main"),
+            ]
+        )
+        self.mock_git.cherry_pick.assert_called_once_with("abcdef12")
+        self.mock_git.diff.assert_called_once()
+        self.mock_git.apply_check.assert_called_once_with(unittest.mock.ANY)
+        self.mock_git.apply.assert_called_once_with(unittest.mock.ANY)
+        self.mock_changelog_news.update_changelog.assert_has_calls(
+            [
+                call("2.0.0", "2026-07-01"),
+                call(
+                    "2.0.0",
+                    "2026-07-01",
+                    news_files=["news/124.fixed.md"],
+                    delete_news=True,
+                ),
+            ]
+        )
+        self.assertEqual(self.mock_git.add_modified_and_deleted.call_count, 2)
+        self.mock_replace_version_next.assert_called_once_with("2.0.0")
+        self.mock_git.commit.assert_has_calls(
+            [
+                call('Cherry-pick "fix bug"\n\nWork towards #123', amend=True),
+                call("chore(release): sync changelog for v2.0.0 backports"),
+            ]
+        )
+        self.mock_git.push.assert_has_calls(
+            [
+                call("origin", "release/2.0"),
+                call(
+                    "origin",
+                    "prepare-2.0.0-backports-6affdae",
+                    set_upstream=True,
+                    force=True,
+                ),
+            ]
+        )
+
+        self.mock_gh.create_pr.assert_called_once_with(
+            title="chore(release): sync changelog for v2.0.0 backports",
+            body="Updates CHANGELOG.md and removes news files for backports:\n- #124\n\nWork towards #123",
+            base="main",
+        )
+        self.mock_gh.enable_auto_merge.assert_called_once_with(999)
+
+        self.mock_gh.update_issue_body.assert_called_once()
+        call_args = self.mock_gh.update_issue_body.call_args[0]
+        self.assertEqual(call_args[0], 123)
+        self.assertIn("- [x] #124 | status=done rc=rc0 commit= 12345678", call_args[1])
+
     @patch("tools.private.release.process_backports.datetime")
     def test_process_backports_dry_run(self, mock_datetime):
         mock_datetime.date.today.return_value = datetime.date(2026, 7, 1)

tools/private/release/process_backports.py

@@ -116,11 +116,7 @@ def _cherry_pick_and_update_prs(
                 # Collect news files before they are deleted by update_changelog
                 modified_files = self.git.get_modified_files("HEAD")
                 for f in modified_files:
-                    if (
-                        f.startswith("news/")
-                        and f.endswith(".md")
-                        and f != "news/BUILD.bazel"
-                    ):
+                    if changelog_news.is_news_file(f):
                         collected_news_files.append(f)
 
                 # Replace version markers FIRST to isolate diff
@@ -258,7 +254,11 @@ def _sync_changelog_to_main(
                     f"[DRY RUN] Would create and checkout branch {backport_branch} from {main_branch}"
                 )
             else:
-                self.git.checkout(backport_branch, create_branch=True)
+                if self.git.branch_exists(backport_branch):
+                    self.git.checkout(backport_branch)
+                    self.git.reset_hard(main_branch)
+                else:
+                    self.git.checkout(backport_branch, create_branch=True)
 
             print(
                 f"Updating CHANGELOG.md and removing news files on {backport_branch}..."
@@ -347,7 +347,7 @@ def _apply_version_marker_diffs(
                     )
 
                 patch_filepath = os.path.join(temp_dir, f"{pr_num}.patch")
-                with open(patch_filepath, "w") as f:
+                with open(patch_filepath, "w", encoding="utf-8") as f:
                     f.write(diff_content)
 
                 if self.git.apply_check(patch_filepath):