github: Add a helpful message for issues without milestones

[tstellar]

Summary

The /branch and /cherry-pick commands only work when an isssue has
a milestone, so give the user a helpful error message when they
try these commands on issue without a milestone.

[thieta]

Lgtm

[kwk]

Looks good except for what I think is a typo and some suggestion for improving the readability and how things are connected with the workflow. Afterall, this is triggered by a comment, that produces another comment that we want to ignore or skip. Hm, maybe skip is better than ignore?

llvm/utils/git/github-automation.py

153–155

<!--IGNORE--> seems like a special comment that gets parsed here: https://github.com/llvm/llvm-project/blob/7e5f75aa37801b9e4e9f0e3fb9f0358cbafb6f63/.github/workflows/issue-release-workflow.yml#L35. I suggest to create a dedicated function for this to make this more obvious where this gets ignored. Also I think the > before the {} placeholder was not intended, right?

164

[tstellar]

Address review comments.

[tstellar]

llvm/utils/git/github-automation.py

153–155

<!--IGNORE--> seems like a special comment that gets parsed here: https://github.com/llvm/llvm-project/blob/7e5f75aa37801b9e4e9f0e3fb9f0358cbafb6f63/.github/workflows/issue-release-workflow.yml#L35. I suggest to create a dedicated function for this to make this more obvious where this gets ignored. Also I think the > before the {} placeholder was not intended, right?

The '>' character is to quote the original comment.

[kwk]

LGTM now. Thank you for addressing the comments.

llvm/utils/git/github-automation.py

163

Not sure if this was in here before but this is a great improvement in case the comment spanned more than one line!

llvm/utils/git/github-automation.py

def print_release_branch(self) -> None:
         print(self.release_branch_for_issue)
 
     def issue_notify_branch(self) -> None:
         self.issue.create_comment('/branch {}/{}'.format(self.branch_repo_name, self.branch_name))
 
     def issue_notify_pull_request(self, pull:github.PullRequest.PullRequest) -> None:
         self.issue.create_comment('/pull-request {}#{}'.format(self.branch_repo_name, pull.number))
 
+    def issue_notify_no_milestone(self, comment) -> None:
+        message = "<!--IGNORE-->\n>{}\n\nError: Command failed due to missing milestone.".format(comment)
+        self.issue.create_comment(message)
+
     @property
     def action_url(self) -> str:
         if os.getenv('CI'):
             return 'https://github.com/{}/actions/runs/{}'.format(os.getenv('GITHUB_REPOSITORY'), os.getenv('GITHUB_RUN_ID'))
         return ""
 
     def issue_notify_cherry_pick_failure(self, commit:str) -> github.IssueComment.IssueComment:
         message = "<!--IGNORE-->\nFailed to cherry-pick: {}\n\n".format(commit)
         action_url = self.action_url
         if action_url:
             message += action_url + "\n\n"
         message += "Please manually backport the fix and push it to your github fork.  Once this is done, please add a comment like this:\n\n`/branch <user>/<repo>/<branch>`"
         issue = self.issue
         comment = issue.create_comment(message)
         issue.add_to_labels(self.CHERRY_PICK_FAILED_LABEL)
         return comment

 
 if args.command == 'issue-subscriber':
     issue_subscriber = IssueSubscriber(args.token, args.repo, args.issue_number, args.label_name)
     issue_subscriber.run()
 elif args.command == 'release-workflow':
     release_workflow = ReleaseWorkflow(args.token, args.repo, args.issue_number,
                                        args.branch_repo, args.branch_repo_token,
                                        args.llvm_project_dir)
+    if not release_workflow.release_branch_for_issue:
+        release_workflow.issue_notify_no_milestone(''.join(sys.stdin.readlines()))
+        sys.exit(1)
     if args.sub_command == 'print-release-branch':
         release_workflow.print_release_branch()
     else:
         if not release_workflow.execute_command():
             sys.exit(1)
 elif args.command == 'setup-llvmbot-git':
     setup_llvmbot_git()
 elif args.command == 'request-cherry-pick':