[tools] Add a roll_merge using the Gerrit API

roll_merge.py manually checks out V8 into a temporary directory, locally
builds a cherrypick, and uploads this to Gerrit. However, Gerrit has its
own REST API which allows cherrypicking. Using this API directly has two
advantages:

  1) We don't need to perform any local checkouts, so it's much faster,
     and
  2) The cherry-picked commit is marked as a cherry-pick by Gerrit,
     which means Rubber-Stamper-Bot will treat it as a cherry-pick.

The implementation for now is very simple, and doesn't support things
like cherry-picking multiple revisions or applying an additional local
patch. It does, however, increment the patch value in v8-version.h, and
tries to set Owners-Override +1.

Bug: v8:12849
Change-Id: Ie242dbec6b3d24f5118d601e9d326465d190a8f0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3644609
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81072}

tools/release/roll_merge_gerrit.py

+#!/usr/bin/env python3
+# Copyright 2022 the V8 project authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import argparse
+import logging
+import os
+import re
+import sys
+
+# Add depot tools to the sys path, for gerrit_util
+sys.path.append(
+    os.path.abspath(
+        os.path.join(
+            os.path.dirname(os.path.abspath(__file__)),
+            '../../third_party/depot_tools')))
+
+import gerrit_util
+
+from common_includes import VERSION_FILE
+
+GERRIT_HOST = 'chromium-review.googlesource.com'
+
+
+def ExtractVersion(include_file_text):
+  version = {}
+  for line in include_file_text.split('\n'):
+
+    def ReadAndPersist(var_name, def_name):
+      match = re.match(r"^#define %s\s+(\d*)" % def_name, line)
+      if match:
+        value = match.group(1)
+        version[var_name] = int(value)
+
+    for (var_name, def_name) in [("major", "V8_MAJOR_VERSION"),
+                                 ("minor", "V8_MINOR_VERSION"),
+                                 ("build", "V8_BUILD_NUMBER"),
+                                 ("patch", "V8_PATCH_LEVEL")]:
+      ReadAndPersist(var_name, def_name)
+  return version
+
+
+def main():
+  parser = argparse.ArgumentParser(
+      description="Use the gerrit API to cherry-pick a revision")
+  parser.add_argument(
+      "-a",
+      "--author",
+      default="",
+      help="The author email used for code review.")
+
+  group = parser.add_mutually_exclusive_group(required=True)
+  group.add_argument("--branch", help="The branch to merge to (e.g. 10.3.171)")
+  # TODO(leszeks): Add support for more than one revision. This will need to
+  # cherry pick one of them using the gerrit API, then somehow applying the rest
+  # onto it as additional patches.
+  parser.add_argument("revision", nargs=1, help="The revision to merge.")
+
+  options = parser.parse_args()
+
+  # Get the original commit.
+  revision = options.revision[0]
+  print("Cherry-picking %s onto %s" % (revision, options.branch))
+
+  # Create a cherry pick commit from the original commit.
+  cherry_pick = gerrit_util.CherryPick(GERRIT_HOST, revision, options.branch)
+  cherry_pick_id = cherry_pick['id']
+  print("Created cherry-pick: https://%s/c/%s" %
+        (GERRIT_HOST, cherry_pick['_number']))
+
+  print("Extracting target version...")
+  # Get the version out of the cherry-picked commit's v8-version.h
+  include_file_text = gerrit_util.GetFileContents(GERRIT_HOST, cherry_pick_id,
+                                                  VERSION_FILE).decode('utf-8')
+  version = ExtractVersion(include_file_text)
+  print("... version is %s" % str(version))
+
+  new_patch = version['patch'] + 1
+
+  # Add the 'roll_merge' hashtag so that Rubberstamper knows that this is a
+  # benign cherry pick.
+  print("Setting 'roll-merge' hashtag...")
+  gerrit_util.CallGerritApi(
+      GERRIT_HOST,
+      'changes/%s/hashtags' % cherry_pick_id,
+      reqtype='POST',
+      body={'add': ["roll-merge"]})
+
+  # Increment the patch number in v8-version.h
+  print("Updating %s to patch level %d..." % (VERSION_FILE, new_patch))
+  include_file_text = re.sub(
+      r"(?<=#define V8_PATCH_LEVEL)(?P<space>\s+)\d*$",
+      r"\g<space>%d" % new_patch,
+      include_file_text,
+      flags=re.MULTILINE)
+  gerrit_util.ChangeEdit(GERRIT_HOST, cherry_pick_id, VERSION_FILE,
+                         include_file_text)
+
+  # Create the commit message, using the new version and information about the
+  # original commit.
+  print("Updating commit message...")
+  original_commit = gerrit_util.GetChangeCommit(GERRIT_HOST, revision)
+  commit_msg = "\n".join([
+      "Version %d.%d.%d.%d (cherry-pick)" %
+      (version["major"], version["minor"], version["build"], new_patch),  #
+      "",  #
+      "Merged %s" % original_commit['commit'],  #
+      "",  #
+      "%s" % original_commit['subject'],  #
+  ])
+  gerrit_util.SetChangeEditMessage(GERRIT_HOST, cherry_pick_id, commit_msg)
+
+  # Publish the change edit with the v8-version.h and commit message changes.
+  print("Publishing changes...")
+  gerrit_util.PublishChangeEdit(GERRIT_HOST, cherry_pick_id)
+
+  # Set Owners-Override +1
+  print("Setting 'Owners-Override +1'...")
+  try:
+    gerrit_util.SetReview(
+        GERRIT_HOST, cherry_pick_id, labels={"Owners-Override": 1})
+  except:
+    logging.WARNING("Could not set Owners-Override +1")
+
+  print("Adding Rubber Stamper as a reviewer...")
+  gerrit_util.AddReviewers(
+      GERRIT_HOST,
+      cherry_pick_id,
+      reviewers=['rubber-stamper@appspot.gserviceaccount.com'])
+
+  print("Cherry-pick created successfully: https://%s/c/%s" %
+        (GERRIT_HOST, cherry_pick['_number']))
+
+
+if __name__ == "__main__":  # pragma: no cover
+  sys.exit(main())