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diff --git a/llvm_tools/revert_checker.py b/llvm_tools/revert_checker.py
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+#!/usr/bin/env python3
+# -*- coding: utf-8 -*-
+# Copyright 2020 The Chromium OS Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+"""Checks for reverts of commits across a given git commit.
+
+To clarify the meaning of 'across' with an example, if we had the following
+commit history (where `a -> b` notes that `b` is a direct child of `a`):
+
+123abc -> 223abc -> 323abc -> 423abc -> 523abc
+
+And where 423abc is a revert of 223abc, this revert is considered to be 'across'
+323abc. More generally, a revert A of a parent commit B is considered to be
+'across' a commit C if C is a parent of A and B is a parent of C.
+
+Please note that revert detection in general is really difficult, since merge
+conflicts/etc always introduce _some_ amount of fuzziness. This script just
+uses a bundle of heuristics, and is bound to ignore / incorrectly flag some
+reverts. The hope is that it'll easily catch the vast majority (>90%) of them,
+though.
+"""
+
+# pylint: disable=cros-logging-import
+
+from __future__ import print_function
+
+import argparse
+import collections
+import logging
+import re
+import subprocess
+import sys
+import typing as t
+
+# People are creative with their reverts, and heuristics are a bit difficult.
+# Like 90% of of reverts have "This reverts commit ${full_sha}".
+# Some lack that entirely, while others have many of them specified in ad-hoc
+# ways, while others use short SHAs and whatever.
+#
+# The 90% case is trivial to handle (and 100% free + automatic). The extra 10%
+# starts involving human intervention, which is probably not worth it for now.
+
+
+def _try_parse_reverts_from_commit_message(commit_message: str) -> t.List[str]:
+  if not commit_message:
+    return []
+
+  results = re.findall(r'This reverts commit ([a-f0-9]{40})\b', commit_message)
+
+  first_line = commit_message.splitlines()[0]
+  initial_revert = re.match(r'Revert ([a-f0-9]{6,}) "', first_line)
+  if initial_revert:
+    results.append(initial_revert.group(1))
+  return results
+
+
+def _stream_stdout(command: t.List[str]) -> t.Generator[str, None, None]:
+  with subprocess.Popen(
+      command, stdout=subprocess.PIPE, encoding='utf-8', errors='replace') as p:
+    yield from p.stdout
+
+
+def _resolve_sha(git_dir: str, sha: str) -> str:
+  if len(sha) == 40:
+    return sha
+
+  return subprocess.check_output(
+      ['git', '-C', git_dir, 'rev-parse', sha],
+      encoding='utf-8',
+      stderr=subprocess.DEVNULL,
+  ).strip()
+
+
+_LogEntry = t.NamedTuple('_LogEntry', [
+    ('sha', str),
+    ('commit_message', t.List[str]),
+])
+
+
+def _log_stream(git_dir: str, root_sha: str,
+                end_at_sha: str) -> t.Iterable[_LogEntry]:
+  sep = 50 * '<>'
+  log_command = [
+      'git',
+      '-C',
+      git_dir,
+      'log',
+      '^' + end_at_sha,
+      root_sha,
+      '--format=' + sep + '%n%H%n%B%n',
+  ]
+
+  stdout_stream = iter(_stream_stdout(log_command))
+
+  # Find the next separator line. If there's nothing to log, it may not exist.
+  # It might not be the first line if git feels complainy.
+  found_commit_header = False
+  for line in stdout_stream:
+    if line.rstrip() == sep:
+      found_commit_header = True
+      break
+
+  while found_commit_header:
+    # crbug.com/1041148
+    # pylint: disable=stop-iteration-return
+    sha = next(stdout_stream, None)
+    assert sha is not None, 'git died?'
+    sha = sha.rstrip()
+
+    commit_message = []
+
+    found_commit_header = False
+    for line in stdout_stream:
+      line = line.rstrip()
+      if line.rstrip() == sep:
+        found_commit_header = True
+        break
+      commit_message.append(line)
+
+    yield _LogEntry(sha, '\n'.join(commit_message).rstrip())
+
+
+def _shas_between(git_dir: str, base_ref: str,
+                  head_ref: str) -> t.Iterable[str]:
+  rev_list = [
+      'git',
+      '-C',
+      git_dir,
+      'rev-list',
+      '--first-parent',
+      '%s..%s' % (base_ref, head_ref),
+  ]
+  return (x.strip() for x in _stream_stdout(rev_list))
+
+
+def _rev_parse(git_dir: str, ref: str) -> str:
+  result = subprocess.check_output(
+      ['git', '-C', git_dir, 'rev-parse', ref],
+      encoding='utf-8',
+  ).strip()
+  return t.cast(str, result)
+
+
+Revert = t.NamedTuple('Revert', [
+    ('sha', str),
+    ('reverted_sha', str),
+])
+
+
+def find_common_parent_commit(git_dir: str, ref_a: str, ref_b: str) -> str:
+  return subprocess.check_output(
+      ['git', '-C', git_dir, 'merge-base', ref_a, ref_b],
+      encoding='utf-8',
+  ).strip()
+
+
+def find_reverts(git_dir: str, across_ref: str, root: str) -> t.List[Revert]:
+  """Finds reverts across `across_ref` in `git_dir`, starting from `root`."""
+  across_sha = _rev_parse(git_dir, across_ref)
+  root_sha = _rev_parse(git_dir, root)
+
+  common_ancestor = find_common_parent_commit(git_dir, across_sha, root_sha)
+  if common_ancestor != across_sha:
+    raise ValueError("%s isn't an ancestor of %s (common ancestor: %s)" %
+                     (across_sha, root_sha, common_ancestor))
+
+  intermediate_commits = set(_shas_between(git_dir, across_sha, root_sha))
+  assert across_ref not in intermediate_commits
+
+  logging.debug('%d commits appear between %s and %s',
+                len(intermediate_commits), across_sha, root_sha)
+
+  all_reverts = []
+  for sha, commit_message in _log_stream(git_dir, root_sha, across_sha):
+    reverts = _try_parse_reverts_from_commit_message(commit_message)
+    if not reverts:
+      continue
+
+    resolved_reverts = sorted(set(_resolve_sha(git_dir, x) for x in reverts))
+    for reverted_sha in resolved_reverts:
+      if reverted_sha in intermediate_commits:
+        logging.debug('Commit %s reverts %s, which happened after %s', sha,
+                      reverted_sha, across_sha)
+        continue
+
+      try:
+        object_type = subprocess.check_output(
+            ['git', '-C', git_dir, 'cat-file', '-t', reverted_sha],
+            encoding='utf-8',
+            stderr=subprocess.DEVNULL,
+        ).strip()
+      except subprocess.CalledProcessError:
+        logging.warning(
+            'Failed to resolve reverted object %s (claimed to be reverted '
+            'by sha %s)', reverted_sha, sha)
+        continue
+
+      if object_type == 'commit':
+        all_reverts.append(Revert(sha, reverted_sha))
+        continue
+
+      logging.error("%s claims to revert %s -- which isn't a commit -- %s", sha,
+                    object_type, reverted_sha)
+
+  return all_reverts
+
+
+def main(args: t.List[str]) -> int:
+  parser = argparse.ArgumentParser(
+      description=__doc__, formatter_class=argparse.RawDescriptionHelpFormatter)
+  parser.add_argument(
+      'base_ref', help='Git ref or sha to check for reverts around.')
+  parser.add_argument(
+      '-C', '--git_dir', default='.', help='Git directory to use.')
+  parser.add_argument(
+      'root', nargs='+', help='Root(s) to search for commits from.')
+  parser.add_argument('--debug', action='store_true')
+  opts = parser.parse_args(args)
+
+  logging.basicConfig(
+      format='%(asctime)s: %(levelname)s: %(filename)s:%(lineno)d: %(message)s',
+      level=logging.DEBUG if opts.debug else logging.INFO,
+  )
+
+  # `root`s can have related history, so we want to filter duplicate commits
+  # out. The overwhelmingly common case is also to have one root, and it's way
+  # easier to reason about output that comes in an order that's meaningful to
+  # git.
+  all_reverts = collections.OrderedDict()
+  for root in opts.root:
+    for revert in find_reverts(opts.git_dir, opts.base_ref, root):
+      all_reverts[revert] = None
+
+  for revert in all_reverts.keys():
+    print('%s claims to revert %s' % (revert.sha, revert.reverted_sha))
+
+
+if __name__ == '__main__':
+  sys.exit(main(sys.argv[1:]))