Add git number footer generation to git cl land.

git_cl.py

Index: git_cl.py
diff --git a/git_cl.py b/git_cl.py
index b669bc3c8311b69a54d72e0cf2800113dd78209d..47d7f2260cdb1264e50c7d55ffeb531801b9e0d1 100755
--- a/git_cl.py
+++ b/git_cl.py
@@ -4186,7 +4186,16 @@ def SendUpstream(parser, args, cmd):
       mirror = settings.GetGitMirror(remote)
       pushurl = mirror.url if mirror else remote
       pending_prefix = settings.GetPendingRefPrefix()
-      if not pending_prefix or branch.startswith(pending_prefix):
+      git_number_footer = settings.GetHasGitNumberFooter()
+      # NOTE: that pending_prefix takes precendence over git_number_footer.
+      #   If pending_prefix is set, it means gnumbd service is still working
+      #   for this repo OR deve's checkout is stale, in which case push to
+      #   refs/pending/ below fail anyway.
+      if not pending_prefix and git_number_footer:
+        # git_number_footer is used only if pending_prefix is not used.
+        retcode, output = PushToGitWithGitNumberFooter(pushurl, branch)
+        pushed_to_pending = False
+      elif not pending_prefix or branch.startswith(pending_prefix):
         # If not using refs/pending/heads/* at all, or target ref is already set
         # to pending, then push to the target ref directly.
         retcode, output = RunGitWithCode(
@@ -4310,6 +4319,7 @@ def PushToGitPending(remote, pending_ref):
   Returns:
     (retcode of last operation, output log of last operation).
   """
+  # TODO(tandrii): delete this with gnumbd service, see http://crbug.com/644915.
   assert pending_ref.startswith('refs/'), pending_ref
   local_pending_ref = 'refs/git-cl/' + pending_ref[len('refs/'):]
   cherry = RunGit(['rev-parse', 'HEAD']).strip()
@@ -4365,6 +4375,71 @@ def PushToGitPending(remote, pending_ref):
   return code, out
 
 
+def PushToGitWithGitNumberFooter(remote, upstream_ref):
+  """Fetches remote ref, cherry-picks current HEAD on top of it, updates
+  position footers, pushes.
+
+  Returns:
+    (retcode of last operation, output log of last operation).
+  """
+  local_upstream_ref = 'refs/git-cl/' + upstream_ref[len('refs/'):]
+  cherry = RunGit(['rev-parse', 'HEAD']).strip()
+  code = 0
+  out = ''
+  max_attempts = 3
+  attempts_left = max_attempts
+  while attempts_left:
+    if attempts_left != max_attempts:
+      print('Retrying, %d attempts left...' % (attempts_left - 1,))
+    attempts_left -= 1
+
+    # Fetch. Retry fetch errors.
+    print('Fetching pending ref %s...' % upstream_ref)
+    code, out = RunGitWithCode(
+        ['retry', 'fetch', remote, '+%s:%s' % (upstream_ref, local_upstream_ref)])
+    if code:
+      print('Fetch failed with exit code %d.' % code)
+      if out.strip():
+        print(out.strip())
+      continue
+
+    # Try to cherry pick. Abort on merge conflicts.
+    print('Cherry-picking commit on top of pending ref...')
+    RunGitWithCode(['checkout', local_upstream_ref], suppress_stderr=True)
+    code, out = RunGitWithCode(['cherry-pick', cherry])
+    if code:
+      print('Your patch doesn\'t apply cleanly to ref \'%s\', '
+            'the following files have merge conflicts:' % upstream_ref)
+      print(RunGit(['diff', '--name-status', '--diff-filter=U']).strip())
+      print('Please rebase your patch and try again.')
+      RunGitWithCode(['cherry-pick', '--abort'])
+      return code, out
+
+    # Applied cleanly, amend commit message with position footers.
+
+    # TODO(tandrii): amend footers.
+
+    # Try to push now. Retry on error (flake or non-ff push).
+    print('Pushing commit to %s... It can take a while.' % upstream_ref)
+    code, out = RunGitWithCode(
+        ['retry', 'push', '--porcelain', remote, 'HEAD:%s' % upstream_ref])
+    if code == 0:
+      # Success.
+      print('Commit pushed to pending ref successfully!')
+      return code, out
+
+    print('Push failed with exit code %d.' % code)
+    if out.strip():
+      print(out.strip())
+    if IsFatalPushFailure(out):
+      print('Fatal push error. Make sure your .netrc credentials and git '
+            'user.email are correct and you have push access to the repo.')
+      return code, out
+
+  print('All attempts to push to pending ref failed.')
+  return code, out
+
+
 def IsFatalPushFailure(push_stdout):
   """True if retrying push won't help."""
   return '(prohibited by Gerrit)' in push_stdout