Update the merge_to_master.py script for Git.

Changes the merge_to_master.py script to deal with the Chromium's
switch to Git.
Furhermore, this change introduces pedantic checks when merging a given
master-chromium snapshot (using a repo.prop file), verifying that the
projects SHAs in the archive match the SHAs in the repo.prop.
Shouldn't this happen, it analyzes the history of the mismatching
projects and prints out a detailed error message. In case of a mismatch,
the merge can be forced using the --force option.

Change-Id: I8909d064709921220afbed1cc55cbcc7c48ececa
(cherry picked from commit 18e8e49dbb79d50be03314b30f997cfa8561c347)

3 files changed, 331 insertions, 157 deletions

diff --git a/chromium/tools/merge_common.py b/chromium/tools/merge_common.py
index f8621e6..634e19c 100644
--- a/chromium/tools/merge_common.py
+++ b/chromium/tools/merge_common.py
@@ -73,7 +73,8 @@ THIRD_PARTY_PROJECTS = (THIRD_PARTY_PROJECTS_WITH_FLAT_HISTORY +
                         THIRD_PARTY_PROJECTS_WITH_FULL_HISTORY)
 
 ALL_PROJECTS = ['.'] + THIRD_PARTY_PROJECTS
-
+assert(set(ALL_PROJECTS) ==
+       set(PROJECTS_WITH_FLAT_HISTORY + PROJECTS_WITH_FULL_HISTORY))
 
 # Directories to be removed when flattening history.
 PRUNE_WHEN_FLATTENING = {
@@ -112,6 +113,11 @@ class TemporaryMergeError(MergeError):
   """A merge error that can potentially be resolved by trying again later."""
 
 
+def Abbrev(commitish):
+  """Returns the abbrev commitish for a given Git SHA."""
+  return commitish[:12]
+
+
 def GetCommandStdout(args, cwd=REPOSITORY_ROOT, ignore_errors=False):
   """Gets stdout from runnng the specified shell command.
 
@@ -123,15 +129,16 @@ def GetCommandStdout(args, cwd=REPOSITORY_ROOT, ignore_errors=False):
     cwd: The working directory to use. Defaults to REPOSITORY_ROOT.
     ignore_errors: Ignore the command's return code and stderr.
   Returns:
-    stdout from running the command.
+    A concatenation of stdout + stderr from running the command.
   Raises:
     CommandError: if the command exited with a nonzero status.
   """
   p = subprocess.Popen(args=args, cwd=cwd, stdout=subprocess.PIPE,
                        stderr=subprocess.PIPE)
   stdout, stderr = p.communicate()
+  output = stdout + ('\n===STDERR===\n' if stderr and stdout else '') + stderr
   if p.returncode == 0 or ignore_errors:
-    return stdout
+    return output
   else:
     raise CommandError(p.returncode, ' '.join(args), cwd, stdout, stderr)
 
diff --git a/chromium/tools/merge_from_chromium.py b/chromium/tools/merge_from_chromium.py
index 0ab20f9..027b909 100755
--- a/chromium/tools/merge_from_chromium.py
+++ b/chromium/tools/merge_from_chromium.py
@@ -16,13 +16,11 @@
 
 """Merge Chromium into the Android tree."""
 
-import contextlib
 import logging
 import optparse
 import os
 import re
 import sys
-import urllib2
 
 import merge_common
 
@@ -34,7 +32,7 @@ webview_licenses = None
 
 AUTOGEN_MESSAGE = 'This commit was generated by merge_from_chromium.py.'
 SRC_GIT_BRANCH = 'refs/remotes/history/upstream-master'
-GIT_ABBREV_LENGTH = 12
+
 
 def _ReadGitFile(sha1, path, git_url=None, git_branch=None):
   """Reads a file from a (possibly remote) git project at a specific revision.
@@ -71,6 +69,7 @@ def _ParseDEPS(deps_content):
       return 'From("%s")' % self.module_name
 
   class _VarImpl(object):
+
     def __init__(self, custom_vars, local_scope):
       self._custom_vars = custom_vars
       self._local_scope = local_scope
@@ -86,7 +85,7 @@ def _ParseDEPS(deps_content):
   tmp_locals = {}
   var = _VarImpl({}, tmp_locals)
   tmp_globals = {'From': FromImpl, 'Var': var.Lookup, 'deps_os': {}}
-  exec(deps_content) in tmp_globals, tmp_locals
+  exec(deps_content) in tmp_globals, tmp_locals  # pylint: disable=W0122
   return tmp_locals
 
 
@@ -109,7 +108,7 @@ def _GetProjectMergeInfo(projects, deps_vars):
   result = {}
   for path in projects:
     for deps in deps_fallback_order:
-      if len(path) > 0:
+      if path:
         upstream_path = os.path.join('src', path)
       else:
         upstream_path = 'src'
@@ -157,8 +156,8 @@ def _MergeProjects(version, root_sha1, target, unattended, buildspec_url):
   if root_sha1:
     deps_content = _ReadGitFile(root_sha1, 'DEPS')
   else:
-    # TODO: At some point the release branches will use DEPS as well, instead of
-    # .DEPS.git. Rename below when that day will come.
+    # TODO(primiano): At some point the release branches will use DEPS as well,
+    # instead of .DEPS.git. Rename below when that day will come.
     deps_content = _ReadGitFile('FETCH_HEAD',
                                 'releases/' + version + '/.DEPS.git',
                                 buildspec_url,
@@ -212,7 +211,6 @@ def _MergeProjects(version, root_sha1, target, unattended, buildspec_url):
   else:
     merge_msg_version = root_sha1
 
-
   logging.debug('Merging Chromium at %s ...', root_sha1)
   # Merge conflicts make git merge return 1, so ignore errors
   merge_common.GetCommandStdout(['git', 'merge', '--no-commit', root_sha1],
@@ -229,9 +227,9 @@ def _MergeProjects(version, root_sha1, target, unattended, buildspec_url):
   sys.path.append(os.path.join(merge_common.REPOSITORY_ROOT, 'android_webview',
                                'tools'))
   sys.dont_write_bytecode = True
-  global webview_licenses
-  import webview_licenses
-  import known_issues
+  global webview_licenses  # pylint: disable=W0602
+  import webview_licenses  # pylint: disable=W0621,W0612,C6204
+  import known_issues  # pylint: disable=C6204
 
   for path, exclude_list in known_issues.KNOWN_INCOMPATIBLE.iteritems():
     logging.debug('  %s', '\n  '.join(os.path.join(path, x) for x in
@@ -350,7 +348,7 @@ def _GenerateLastChange(version, root_sha1):
   logging.debug('Updating LASTCHANGE ...')
   with open(os.path.join(merge_common.REPOSITORY_ROOT, 'build/util/LASTCHANGE'),
             'w') as f:
-    f.write('LASTCHANGE=%s\n' % root_sha1[:GIT_ABBREV_LENGTH])
+    f.write('LASTCHANGE=%s\n' % merge_common.Abbrev(root_sha1))
   merge_common.GetCommandStdout(['git', 'add', '-f', 'build/util/LASTCHANGE'])
   logging.debug('Updating LASTCHANGE.blink ...')
   with open(os.path.join(merge_common.REPOSITORY_ROOT,
@@ -380,8 +378,9 @@ def _ParseSvnRevisionFromGitCommitMessage(commit_message):
 
 
 def _GetGitAbbrevSHA1(git_branch, revision):
-  assert(revision)
-  logging.debug('Getting Git revision for %s ...' % revision)
+  """Returns an abbrev. SHA for the given revision (or branch, if HEAD)."""
+  assert revision
+  logging.debug('Getting Git revision for %s ...', revision)
 
   upstream = git_branch if revision == 'HEAD' else revision
 
@@ -393,16 +392,15 @@ def _GetGitAbbrevSHA1(git_branch, revision):
     raise merge_common.TemporaryMergeError(
         'Upstream object (%s) not reachable from %s' % (upstream, git_branch))
 
-  abbrev_sha = merge_common.GetCommandStdout(['git', 'rev-list',
-      '--abbrev-commit', '--abbrev=%d' % GIT_ABBREV_LENGTH,
-      '--max-count=1', upstream])
-  return abbrev_sha.split()[0]
+  abbrev_sha = merge_common.Abbrev(merge_common.GetCommandStdout(
+      ['git', 'rev-list', '--max-count=1', upstream]).split()[0])
+  return abbrev_sha
 
 
 def _GetBlinkRevision():
-  # TODO: Switch to Git as soon as Blink gets migrated as well.
-  commit = merge_common.GetCommandStdout([
-      'git', 'log', '-n1', '--grep=git-svn-id:', '--format=%H%n%b'],
+  # TODO(primiano): Switch to Git as soon as Blink gets migrated as well.
+  commit = merge_common.GetCommandStdout(
+      ['git', 'log', '-n1', '--grep=git-svn-id:', '--format=%H%n%b'],
       cwd=os.path.join(merge_common.REPOSITORY_ROOT, 'third_party', 'WebKit'))
   return _ParseSvnRevisionFromGitCommitMessage(commit)
 
@@ -431,11 +429,11 @@ def Snapshot(root_sha1, release, target, unattended, buildspec_url):
     root_sha1 = _GetGitAbbrevSHA1(SRC_GIT_BRANCH, root_sha1)
     version = root_sha1
 
-  assert((root_sha1 is not None and len(root_sha1) > 6) or version == release)
+  assert (root_sha1 is not None and len(root_sha1) > 6) or version == release
 
   if root_sha1 and not merge_common.GetCommandStdout(
       ['git', 'rev-list', '-1', 'HEAD..' + root_sha1]):
-    logging.info('No new commits to merge at %s (%s)' % (version, root_sha1))
+    logging.info('No new commits to merge at %s (%s)', version, root_sha1)
     return False
 
   logging.info('Snapshotting Chromium at %s (%s)', version, root_sha1)
@@ -470,7 +468,7 @@ def Push(version, target):
   if target == 'master-chromium':
     refspecs.insert(0, '+%s:master-chromium-merge' % src)
   for refspec in refspecs:
-    logging.debug('Pushing to server (%s) ...' % refspec)
+    logging.debug('Pushing to server (%s) ...', refspec)
     for path in merge_common.ALL_PROJECTS:
       if path in merge_common.PROJECTS_WITH_FLAT_HISTORY:
         remote = 'history'
@@ -550,7 +548,7 @@ def main():
       return 1
   else:
     if not Snapshot(options.sha1, options.release, options.target,
-        options.unattended, options.buildspec_url):
+                    options.unattended, options.buildspec_url):
       return options.no_changes_exit
 
   return 0
diff --git a/chromium/tools/merge_to_master.py b/chromium/tools/merge_to_master.py
index 9f8e07a..9a2e78d 100755
--- a/chromium/tools/merge_to_master.py
+++ b/chromium/tools/merge_to_master.py
@@ -21,153 +21,318 @@ import optparse
 import os
 import re
 import shutil
+import subprocess
 import sys
 
 import merge_common
 
 
 AUTOGEN_MESSAGE = 'This commit was generated by merge_to_master.py.'
+WEBVIEW_PROJECT = 'frameworks/webview'
 
 
-def _MergeProjects(svn_revision, target):
-  """Merges the Chromium projects from master-chromium to target.
+def _GetAbsPath(project):
+  """Returns the full path to a given project (either Chromium or Android)."""
+  if project in merge_common.ALL_PROJECTS:
+    abs_path = os.path.join(merge_common.REPOSITORY_ROOT, project)
+  else:
+    abs_path = os.path.join(os.environ['ANDROID_BUILD_TOP'], project)
+  if not os.path.exists(abs_path):
+    raise merge_common.MergeError('Cannot find path ' + abs_path)
+  return abs_path
 
-  The larger projects' histories are flattened in the process.
+
+def _CheckoutSingleProject(project, target_branch):
+  """Checks out the tip of the target_branch into a local branch (merge-to-XXX).
 
   Args:
-    svn_revision: The SVN revision for the main Chromium repository
+    project: a Chromium project (., third_party/foo) or frameworks/webview.
+    target_branch: name of the target branch (in the goog remote).
   """
-  for path in merge_common.PROJECTS_WITH_FLAT_HISTORY:
-    dest_dir = os.path.join(merge_common.REPOSITORY_ROOT, path)
-    merge_common.GetCommandStdout(['git', 'remote', 'update',
-                                   'goog', 'history'], cwd=dest_dir)
-    merge_common.GetCommandStdout(['git', 'checkout',
-                                   '-b', 'merge-to-' + target,
-                                   '-t', 'goog/' + target], cwd=dest_dir)
-    merge_common.GetCommandStdout(['git', 'fetch', 'history',
-                                   'refs/archive/chromium-%s' % svn_revision],
-                                  cwd=dest_dir)
-    merge_sha1 = merge_common.GetCommandStdout(['git', 'rev-parse',
-                                                'FETCH_HEAD'],
-                                               cwd=dest_dir).strip()
-    old_sha1 = merge_common.GetCommandStdout(['git', 'rev-parse', 'HEAD'],
-                                             cwd=dest_dir).strip()
+  dest_dir = _GetAbsPath(project)
+  tracking_branch = 'goog/' + target_branch
+  logging.debug('Check out %-45s at %-16s', project, tracking_branch)
+  merge_common.GetCommandStdout(['git', 'remote', 'update', 'goog'],
+                                cwd=dest_dir)
+  merge_common.GetCommandStdout(['git', 'checkout',
+                                 '-b', 'merge-to-' + target_branch,
+                                 '-t', tracking_branch], cwd=dest_dir)
+
+
+def _FetchSingleProject(project, remote, remote_ref):
+  """Fetches a remote ref for the given project and returns the fetched SHA.
+
+  Args:
+    project: a Chromium project (., third_party/foo) or frameworks/webview.
+    remote: Git remote name (goog for most projects, history for squashed ones).
+    remote_ref: the remote ref to fetch (e.g., refs/archive/chromium-XXX).
+
+  Returns:
+    The SHA1 of the FETCH_HEAD.
+  """
+  dest_dir = _GetAbsPath(project)
+  logging.debug('Fetch     %-45s %s:%s', project, remote, remote_ref)
+  merge_common.GetCommandStdout(['git', 'fetch', remote, remote_ref],
+                                cwd=dest_dir)
+  return merge_common.GetCommandStdout(['git', 'rev-parse', 'FETCH_HEAD'],
+                                       cwd=dest_dir).strip()
+
+
+def _MergeSingleProject(project, merge_sha, revision, target_branch, flatten):
+  """Merges a single project at a given SHA.
+
+  Args:
+    project: a Chromium project (., third_party/foo) or frameworks/webview.
+    merge_sha: the SHA to merge.
+    revision: Abbrev. commitish in the main Chromium repository.
+    target_branch: name of the target branch.
+    flatten: True: squash history while merging; False: perform a normal merge.
+  """
+  dest_dir = _GetAbsPath(project)
+  if flatten:
     # Make the previous merges into grafts so we can do a correct merge.
+    old_sha = merge_common.GetCommandStdout(['git', 'rev-parse', 'HEAD'],
+                                            cwd=dest_dir).strip()
     merge_log = os.path.join(dest_dir, '.merged-revisions')
     if os.path.exists(merge_log):
       shutil.copyfile(merge_log,
                       os.path.join(dest_dir, '.git', 'info', 'grafts'))
-    if merge_common.GetCommandStdout(['git', 'rev-list', '-1',
-                                      'HEAD..' + merge_sha1], cwd=dest_dir):
-      logging.debug('Merging project %s ...', path)
-      # Merge conflicts cause 'git merge' to return 1, so ignore errors
-      merge_common.GetCommandStdout(['git', 'merge', '--no-commit', '--squash',
-                                     merge_sha1],
-                                    cwd=dest_dir, ignore_errors=True)
-      dirs_to_prune = merge_common.PRUNE_WHEN_FLATTENING.get(path, [])
-      if dirs_to_prune:
-        merge_common.GetCommandStdout(['git', 'rm', '--ignore-unmatch', '-rf'] +
-                                      dirs_to_prune, cwd=dest_dir)
-      merge_common.CheckNoConflictsAndCommitMerge(
-          'Merge from Chromium at DEPS revision %s\n\n%s' %
-          (svn_revision, AUTOGEN_MESSAGE), cwd=dest_dir)
-      new_sha1 = merge_common.GetCommandStdout(['git', 'rev-parse', 'HEAD'],
-                                               cwd=dest_dir).strip()
-      with open(merge_log, 'a+') as f:
-        f.write('%s %s %s\n' % (new_sha1, old_sha1, merge_sha1))
-      merge_common.GetCommandStdout(['git', 'add', '.merged-revisions'],
-                                    cwd=dest_dir)
-      merge_common.GetCommandStdout(
-          ['git', 'commit', '-m',
-           'Record Chromium merge at DEPS revision %s\n\n%s' %
-           (svn_revision, AUTOGEN_MESSAGE)], cwd=dest_dir)
-    else:
-      logging.debug('No new commits to merge in project %s', path)
-
-  for path in merge_common.PROJECTS_WITH_FULL_HISTORY:
-    dest_dir = os.path.join(merge_common.REPOSITORY_ROOT, path)
-    merge_common.GetCommandStdout(['git', 'remote', 'update', 'goog'],
-                                  cwd=dest_dir)
-    merge_common.GetCommandStdout(['git', 'checkout',
-                                   '-b', 'merge-to-' + target,
-                                   '-t', 'goog/' + target], cwd=dest_dir)
-    merge_common.GetCommandStdout(['git', 'fetch', 'goog',
-                                   'refs/archive/chromium-%s' % svn_revision],
+
+  # Early out if there is nothing to merge.
+  if not merge_common.GetCommandStdout(['git', 'rev-list', '-1',
+                                        'HEAD..' + merge_sha], cwd=dest_dir):
+    logging.debug('No new commits to merge in project %s', project)
+    return
+
+  logging.debug('Merging project %s (flatten: %s)...', project, flatten)
+  merge_cmd = ['git', 'merge', '--no-commit']
+  merge_cmd += ['--squash'] if flatten else ['--no-ff']
+  merge_cmd += [merge_sha]
+  # Merge conflicts cause 'git merge' to return 1, so ignore errors
+  merge_common.GetCommandStdout(merge_cmd, cwd=dest_dir, ignore_errors=True)
+
+  if flatten:
+    dirs_to_prune = merge_common.PRUNE_WHEN_FLATTENING.get(project, [])
+    if dirs_to_prune:
+      merge_common.GetCommandStdout(['git', 'rm', '--ignore-unmatch', '-rf'] +
+                                    dirs_to_prune, cwd=dest_dir)
+
+  if project in merge_common.ALL_PROJECTS:
+    commit_msg = 'Merge from Chromium at DEPS revision %s' % revision
+  else:
+    commit_msg = 'Merge master-chromium into %s at %s' % (target_branch,
+                                                          revision)
+  commit_msg += '\n\n' + AUTOGEN_MESSAGE
+  merge_common.CheckNoConflictsAndCommitMerge(commit_msg, cwd=dest_dir)
+
+  if flatten:
+    # Generate the new grafts file and commit it on top of the merge.
+    new_sha = merge_common.GetCommandStdout(['git', 'rev-parse', 'HEAD'],
+                                            cwd=dest_dir).strip()
+    with open(merge_log, 'a+') as f:
+      f.write('%s %s %s\n' % (new_sha, old_sha, merge_sha))
+    merge_common.GetCommandStdout(['git', 'add', '.merged-revisions'],
                                   cwd=dest_dir)
-    if merge_common.GetCommandStdout(['git', 'rev-list', '-1',
-                                      'HEAD..FETCH_HEAD'],
-                                     cwd=dest_dir):
-      logging.debug('Merging project %s ...', path)
-      # Merge conflicts cause 'git merge' to return 1, so ignore errors
-      merge_common.GetCommandStdout(['git', 'merge', '--no-commit', '--no-ff',
-                                     'FETCH_HEAD'],
-                                    cwd=dest_dir, ignore_errors=True)
-      merge_common.CheckNoConflictsAndCommitMerge(
-          'Merge from Chromium at DEPS revision %s\n\n%s' %
-          (svn_revision, AUTOGEN_MESSAGE), cwd=dest_dir)
-    else:
-      logging.debug('No new commits to merge in project %s', path)
-
-
-def _GetSVNRevision(commitish='history/master-chromium'):
-  logging.debug('Getting SVN revision ...')
-  commit = merge_common.GetCommandStdout([
-      'git', 'log', '-n1', '--grep=git-svn-id:', '--format=%H%n%b', commitish])
-  svn_revision = re.search(r'^git-svn-id: .*@([0-9]+)', commit,
-                           flags=re.MULTILINE).group(1)
-  return svn_revision
-
-
-def _MergeWithRepoProp(repo_prop_file, target):
+    merge_common.GetCommandStdout(
+        ['git', 'commit', '-m',
+         'Record Chromium merge at DEPS revision %s\n\n%s' %
+         (revision, AUTOGEN_MESSAGE)], cwd=dest_dir)
+
+
+def _IsAncestor(ref1, ref2, cwd):
+  """Checks whether ref1 is a ancestor of ref2 in the given Git repo."""
+  cmd = ['git', 'merge-base', '--is-ancestor', ref1, ref2]
+  ret = subprocess.call(cmd, cwd=cwd)
+  if ret == 0:
+    return True
+  elif ret == 1:
+    return False
+  else:
+    raise merge_common.CommandError(ret, ' '.join(cmd), cwd, 'N/A', 'N/A')
+
+
+def _MergeChromiumProjects(revision, target_branch, repo_shas=None,
+                           force=False):
+  """Merges the Chromium projects from master-chromium to target_branch.
+
+  The larger projects' histories are flattened in the process.
+  When repo_shas != None, it checks that the SHAs of the projects in the
+  archive match exactly the SHAs of the projects in repo.prop.
+
+  Args:
+    revision: Abbrev. commitish in the main Chromium repository.
+    target_branch: target branch name to merge and push to.
+    repo_shas: optional dict. of expected revisions (only for --repo-prop).
+    force: True: merge anyways using the SHAs from repo.prop; False: bail out if
+                 projects mismatch (archive vs repo.prop).
+  """
+  # Sync and checkout ToT for all projects (creating the merge-to-XXX branch)
+  # and fetch the archive snapshot.
+  fetched_shas = {}
+  remote_ref = 'refs/archive/chromium-%s' % revision
+  for project in merge_common.PROJECTS_WITH_FLAT_HISTORY:
+    _CheckoutSingleProject(project, target_branch)
+    fetched_shas[project] = _FetchSingleProject(project, 'history', remote_ref)
+  for project in merge_common.PROJECTS_WITH_FULL_HISTORY:
+    _CheckoutSingleProject(project, target_branch)
+    fetched_shas[project] = _FetchSingleProject(project, 'goog', remote_ref)
+
+  if repo_shas:
+    project_shas_mismatch = False
+    for project, merge_sha in fetched_shas.items():  # the dict can be modified.
+      expected_sha = repo_shas.get(project)
+      if expected_sha != merge_sha:
+        logging.warn('The SHA for project %s specified in the repo.prop (%s) '
+                     'and the one in the archive (%s) differ.',
+                     project, expected_sha, merge_sha)
+        dest_dir = _GetAbsPath(project)
+        if expected_sha is None:
+          reason = 'cannot find a SHA in the repo.pro for %s' % project
+        elif _IsAncestor(merge_sha, expected_sha, cwd=dest_dir):
+          reason = 'the SHA in repo.prop is ahead of the SHA in the archive. '
+          log_cmd = ['git', 'log', '--oneline', '--graph', '--max-count=10',
+                     '%s..%s' % (merge_sha, expected_sha)]
+          log_cmd_output = merge_common.GetCommandStdout(log_cmd, cwd=dest_dir)
+          reason += 'showing partial log (%s): \n %s' % (' '.join(log_cmd),
+                                                         log_cmd_output)
+        elif _IsAncestor(expected_sha, merge_sha, cwd=dest_dir):
+          reason = 'The SHA is already merged in the archive'
+        else:
+          reason = 'The project history diverged. Consult your Git historian.'
+
+        project_shas_mismatch = True
+        if force:
+          logging.debug('Merging the SHA in repo.prop anyways (due to --force)')
+          fetched_shas[project] = expected_sha
+        else:
+          logging.debug('Reason: %s', reason)
+    if not force and project_shas_mismatch:
+      raise merge_common.MergeError(
+          'The revision of some projects in the archive is different from the '
+          'one provided in build.prop. See the log for more details. Re-run '
+          'with --force to continue.')
+
+  for project in merge_common.PROJECTS_WITH_FLAT_HISTORY:
+    _MergeSingleProject(project, fetched_shas[project], revision, target_branch,
+                        flatten=True)
+  for project in merge_common.PROJECTS_WITH_FULL_HISTORY:
+    _MergeSingleProject(project, fetched_shas[project], revision, target_branch,
+                        flatten=False)
+
+
+def _GetNearestUpstreamAbbrevSHA(reference='history/master-chromium'):
+  """Returns the abbrev. upstream SHA which closest to the given reference."""
+  logging.debug('Getting upstream SHA for %s...', reference)
+  merge_common.GetCommandStdout(['git', 'remote', 'update', 'history'])
+  upstream_commit = merge_common.Abbrev(merge_common.GetCommandStdout([
+      'git', 'merge-base', 'history/upstream-master', reference]))
+
+  # Pedantic check: look for the existence of a merge commit which contains the
+  # |upstream_commit| in its message and is its children.
+  merge_parents = merge_common.GetCommandStdout([
+      'git', 'rev-list', reference, '--grep', upstream_commit, '--merges',
+      '--parents', '-1'])
+  if upstream_commit not in merge_parents:
+    raise merge_common.MergeError(
+        'Found upstream commit %s, but the merge child (%s) could not be found '
+        'or is not a parent of the upstream SHA')
+  logging.debug('Found nearest Chromium revision %s', upstream_commit)
+  return upstream_commit
+
+
+def _MergeWithRepoProp(repo_prop_file, target_branch, force):
+  """Performs a merge using a repo.prop file (from Android build waterfall).
+
+  This does NOT merge (unless forced with force=True) the pinned
+  revisions in repo.prop, as a repo.prop can snapshot an intermediate state
+  (between two automerger cycles). Instead, this looks up the archived snapshot
+  (generated by the chromium->master-chromium auto-merger) which is closest to
+  the given repo.prop (following the main Chromium project) and merges that one.
+  If the projects revisions don't match, it fails with detailed error messages.
+
+  Args:
+    repo_prop_file: Path to a downloaded repo.prop file.
+    target_branch: name of the target branch to merget to.
+    force: ignores the aforementioned check and merged anyways.
+  """
   chromium_sha = None
   webview_sha = None
+  repo_shas = {}  # 'project/path' -> 'sha'
   with open(repo_prop_file) as prop:
     for line in prop:
-      project, sha = line.split()
-      if project == 'platform/external/chromium_org-history':
-        chromium_sha = sha
-      elif project == 'platform/frameworks/webview':
-        webview_sha = sha
+      repo, sha = line.split()
+      # Translate the Android repo paths into the relative project paths used in
+      # merge_common (e.g., platform/external/chromium_org/foo -> foo).
+      m = (
+          re.match(r'^platform/(frameworks/.+)$', repo) or
+          re.match(r'^platform/external/chromium_org/?(.*?)(-history)?$', repo))
+      if m:
+        project = m.group(1) if m.group(1) else '.'  # '.' = Main project.
+        repo_shas[project] = sha
+
+  chromium_sha = repo_shas.get('.')
+  webview_sha = repo_shas.get(WEBVIEW_PROJECT)
   if not chromium_sha or not webview_sha:
-    logging.error('SHA1s for projects not found; invalid build.prop?')
-    return 1
-  chromium_revision = _GetSVNRevision(chromium_sha)
-  logging.info('Merging Chromium at r%s and WebView at %s', chromium_revision,
-               webview_sha)
-  _MergeProjects(chromium_revision, target)
+    raise merge_common.MergeError('SHAs for projects not found; '
+                                  'invalid build.prop?')
 
-  dest_dir = os.path.join(os.environ['ANDROID_BUILD_TOP'], 'frameworks/webview')
-  merge_common.GetCommandStdout(['git', 'remote', 'update', 'goog'],
-                                cwd=dest_dir)
-  merge_common.GetCommandStdout(['git', 'checkout',
-                                 '-b', 'merge-to-' + target,
-                                 '-t', 'goog/' + target], cwd=dest_dir)
-  if merge_common.GetCommandStdout(['git', 'rev-list', '-1',
-                                    'HEAD..' + webview_sha], cwd=dest_dir):
-    logging.debug('Creating merge for framework...')
-    # Merge conflicts cause 'git merge' to return 1, so ignore errors
-    merge_common.GetCommandStdout(['git', 'merge', '--no-commit', '--no-ff',
-                                   webview_sha], cwd=dest_dir,
-                                  ignore_errors=True)
-    merge_common.CheckNoConflictsAndCommitMerge(
-        'Merge master-chromium into %s at r%s\n\n%s' %
-        (target, chromium_revision, AUTOGEN_MESSAGE), cwd=dest_dir)
-    upload = merge_common.GetCommandStdout(['git', 'push', 'goog',
-                                            'HEAD:refs/for/' + target],
-                                           cwd=dest_dir)
-    logging.info(upload)
-  else:
-    logging.debug('No new commits to merge in framework')
-  return 0
+  # Check that the revisions in repo.prop and the on in the archive match.
+  archived_chromium_revision = _GetNearestUpstreamAbbrevSHA(chromium_sha)
+  logging.info('Merging Chromium at %s and WebView at %s',
+               archived_chromium_revision, webview_sha)
+  _MergeChromiumProjects(archived_chromium_revision, target_branch, repo_shas,
+                         force)
+
+  _CheckoutSingleProject(WEBVIEW_PROJECT, target_branch)
+  _MergeSingleProject(WEBVIEW_PROJECT, webview_sha,
+                      archived_chromium_revision, target_branch, flatten=False)
 
 
-def Push(target):
-  """Push the finished snapshot to the Android repository."""
-  logging.debug('Pushing to server ...')
-  refspec = 'merge-to-%s:%s' % (target, target)
+def Push(target_branch):
+  """Push the finished snapshot to the Android repository.
+
+  Creates first a CL for frameworks/webview (if the merge-to-XXX branch exists)
+  then wait for user confirmation and pushes the Chromium merges. This is to
+  give an opportunity to get a +2 for  frameworks/webview and then push both
+  frameworks/webview and the Chromium projects atomically(ish).
+
+  Args:
+    target_branch: name of the target branch (in the goog remote).
+  """
+  merge_branch = 'merge-to-%s' % target_branch
+
+  # Create a Gerrit CL for the frameworks/webview project (if needed).
+  dest_dir = _GetAbsPath(WEBVIEW_PROJECT)
+  did_upload_webview_cl = False
+  if merge_common.GetCommandStdout(['git', 'branch', '--list', merge_branch],
+                                   cwd=dest_dir):
+    # Check that there was actually something to merge.
+    merge_range = 'goog/%s..%s' % (target_branch, merge_branch)
+    if merge_common.GetCommandStdout(['git', 'rev-list', '-1', merge_range],
+                                     cwd=dest_dir):
+      logging.info('Uploading a merge CL for %s...', WEBVIEW_PROJECT)
+      refspec = '%s:refs/for/%s' % (merge_branch, target_branch)
+      upload = merge_common.GetCommandStdout(['git', 'push', 'goog', refspec],
+                                             cwd=dest_dir)
+      logging.info(upload)
+      did_upload_webview_cl = True
+
+  prompt_msg = 'About push the Chromium projects merge. '
+  if not did_upload_webview_cl:
+    logging.info('No merge CL needed for %s.', WEBVIEW_PROJECT)
+  else:
+    prompt_msg += ('At this point you should have the CL +2-ed and merge it '
+                   'together with this push.')
+  prompt_msg += '\nPress "y" to continue: '
+  if raw_input(prompt_msg) != 'y':
+    logging.warn('Push aborted by the user!')
+    return
+
+  logging.debug('Pushing Chromium projects to %s ...', target_branch)
+  refspec = '%s:%s' % (merge_branch, target_branch)
   for path in merge_common.ALL_PROJECTS:
     logging.debug('Pushing %s', path)
-    dest_dir = os.path.join(merge_common.REPOSITORY_ROOT, path)
+    dest_dir = _GetAbsPath(path)
     # Delete the graft before pushing otherwise git will attempt to push all the
     # grafted-in objects to the server as well as the ones we want.
     graftfile = os.path.join(dest_dir, '.git', 'info', 'grafts')
@@ -177,22 +342,26 @@ def Push(target):
                                   cwd=dest_dir)
 
 
-
 def main():
   parser = optparse.OptionParser(usage='%prog [options]')
   parser.epilog = ('Takes the current master-chromium branch of the Chromium '
                    'projects in Android and merges them into master to publish '
                    'them.')
   parser.add_option(
-      '', '--svn_revision', '--release',
+      '', '--revision',
       default=None,
-      help=('Merge to the specified archived master-chromium SVN revision,'
-            'rather than using HEAD.'))
+      help=('Merge to the specified archived master-chromium revision (abbrev. '
+            'SHA or release version) rather than using HEAD. e.g., '
+            '--revision=a1b2c3d4e5f6 or --revision=38.0.2125.24'))
   parser.add_option(
       '', '--repo-prop',
       default=None, metavar='FILE',
       help=('Merge to the revisions specified in this repo.prop file.'))
   parser.add_option(
+      '', '--force',
+      default=False, action='store_true',
+      help=('Skip history checks and merged anyways (only for --repo-prop).'))
+  parser.add_option(
       '', '--push',
       default=False, action='store_true',
       help=('Push the result of a previous merge to the server.'))
@@ -211,13 +380,13 @@ def main():
   if options.push:
     Push(options.target)
   elif options.repo_prop:
-    return _MergeWithRepoProp(os.path.expanduser(options.repo_prop),
-                              options.target)
-  elif options.svn_revision:
-    _MergeProjects(options.svn_revision, options.target)
+    _MergeWithRepoProp(os.path.expanduser(options.repo_prop),
+                       options.target, options.force)
+  elif options.revision:
+    _MergeChromiumProjects(options.revision, options.target)
   else:
-    svn_revision = _GetSVNRevision()
-    _MergeProjects(svn_revision, options.target)
+    first_upstream_sha = _GetNearestUpstreamAbbrevSHA()
+    _MergeChromiumProjects(first_upstream_sha, options.target)
 
   return 0