github: Fix release automation /branch command with new repo

[tstellar]

Summary

We started using the llvm/llvm-project-release-prs repo for
backport pull requests, but since this repo is not a fork of
llvm/llvm-project it will reject pull requests from other repos. In
order to fix this, when ever someone uses the /branch command to request
a branch be merged into the release branch, we first copy the branch to
the llvm-project-release-prs repo and then create the pull request.

[thieta]

This looks good - only a comment about error handling, but I think we can also address that later if we see it's a large problem.

llvm/utils/git/github-automation.py

233

Maybe there should be some error handling here? this block can be retried if fetch or push fails - network errors happens fairly often.

[nikic]

llvm/utils/git/github-automation.py

234

Should we include owner in the branch name, to avoid clashing when multiple repos use the same branch name?

[tstellar]

Address some review comments.

[tstellar]

llvm/utils/git/github-automation.py

233

Is there a concise way to do retries in python? I tried implementing this, but I thought it made the script too complicated.

[thieta]

llvm/utils/git/github-automation.py

233

maybe:

for i in range(5):
  try:
    local_repo.git.fetch(...)
    local_repo.git.push(...)
  except:
    continue

Or use https://pypi.org/project/retrying/ which is very nice and looks much neater in the code.

[tstellar]

Add retries for copying branches to llvm-project-release-prs repo

[tstellar]

llvm/utils/git/github-automation.py

233

That module looks nice, but I didn't want to add another dependency, so I just implemented the retry manually.

[thieta]

LGTM with the caveat that I think we should print the error.

llvm/utils/git/github-automation.py

243

I think maybe we should log the exception here - otherwise we will swallow the error completely and it will be hard to troubleshoot

[tstellar]

Print exception message when any fetch attempt fails.

llvm/utils/git/github-automation.py

 #!/usr/bin/env python3
 #
 # ======- github-automation - LLVM GitHub Automation Routines--*- python -*--==#
 #
 # Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 # See https://llvm.org/LICENSE.txt for license information.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 #
 # ==-------------------------------------------------------------------------==#
 
 import argparse
 from git import Repo # type: ignore
 import github
 import os
 import re
 import sys
+import time
 from typing import *
 
 class IssueSubscriber:
 
     @property
     def team_name(self) -> str:
         return self._team_name
 
def create_pull_request(self, owner:str, branch:str) -> bool:
         https://docs.github.com/en/get-started/quickstart/github-glossary#compare-branch
         """
         repo = github.Github(self.token).get_repo(self.branch_repo_name)
         issue_ref = '{}#{}'.format(self.repo_name, self.issue_number)
         pull = None
         release_branch_for_issue = self.release_branch_for_issue
         if release_branch_for_issue is None:
             return False
-        head = f"{owner}:{branch}"
+        head_branch = branch
+        if not repo.fork:
+            # If the target repo is not a fork of llvm-project, we need to copy
+            # the branch into the target repo.  GitHub only supports cross-repo pull
+            # requests on forked repos.
+            head_branch = f'{owner}-{branch}'
+            local_repo = Repo(self.llvm_project_dir)
+            push_done = False
+            for i in range(0,5):
+                try:
+                    local_repo.git.fetch(f'https://github.com/{owner}/llvm-project', f'{branch}:{branch}')
+                    local_repo.git.push(self.push_url, f'{branch}:{head_branch}', force=True)
+                    push_done = True
+                    break
+                except Exception as e:
+                    print(e)
+                    time.sleep(30)
+                    continue
+            if not push_done:
+                raise Exception("Failed to mirror branch into {}".format(self.push_url))
+            owner = repo.owner.login
+
+        head = f"{owner}:{head_branch}"
         if self.check_if_pull_request_exists(repo, head):
             print("PR already exists...")
             return True
         try:
             pull = repo.create_pull(title=f"PR for {issue_ref}",
                                     body='resolves {}'.format(issue_ref),
                                     base=release_branch_for_issue,
                                     head=head,