[GitHub][workflows] Add buildbot information comment to first merged PR from a new contributor

.github/workflows/merged-prs.yml

@@ -0,0 +1,41 @@
+name: "Add buildbot information to first PRs from new contributors"
+
+permissions:
+  contents: read
+
+on:
+  # It's safe to use pull_request_target here, because we aren't checking out
+  # code from the pull request branch.
+  # See https://securitylab.github.com/research/github-actions-preventing-pwn-requests/
+  pull_request_target:
+    types:
+      - closed
+
+jobs:
+  buildbot_comment:
+    runs-on: ubuntu-latest
+    permissions:
+      pull-requests: write
+    if: >-
+      (github.repository == 'llvm/llvm-project') &&
+      (github.event.pull_request.merged == true)
+    steps:
+      - name: Checkout Automation Script
+        uses: actions/checkout@v4
+        with:
+          sparse-checkout: llvm/utils/git/
+          ref: main
+
+      - name: Setup Automation Script
+        working-directory: ./llvm/utils/git/
+        run: |
+          pip install -r requirements.txt
+
+      - name: Add Buildbot information comment
+        working-directory: ./llvm/utils/git/
+        run: |
+          python3 ./github-automation.py \
+            --token '${{ secrets.GITHUB_TOKEN }}' \
+            pr-buildbot-information \
+            --issue-number "${{ github.event.pull_request.number }}" \
+            --author "${{ github.event.pull_request.user.login }}"

llvm/utils/git/github-automation.py

@@ -208,6 +208,8 @@ def _get_curent_team(self) -> Optional[github.Team.Team]:
 
 
 class PRGreeter:
+    COMMENT_TAG = "<!--LLVM NEW CONTRIBUTOR COMMENT-->\n"
+
     def __init__(self, token: str, repo: str, pr_number: int):
         repo = github.Github(token).get_repo(repo)
         self.pr = repo.get_issue(pr_number).as_pull_request()
@@ -217,7 +219,9 @@ def run(self) -> bool:
         # by a user new to LLVM and/or GitHub itself.
 
         # This text is using Markdown formatting.
+
         comment = f"""\
+{PRGreeter.COMMENT_TAG}
 Thank you for submitting a Pull Request (PR) to the LLVM Project!
 
 This PR will be automatically labeled and the relevant teams will be
@@ -240,6 +244,61 @@ def run(self) -> bool:
         return True
 
 
+class PRBuildbotInformation:
+    COMMENT_TAG = "<!--LLVM BUILDBOT INFORMATION COMMENT-->\n"
+
+    def __init__(self, token: str, repo: str, pr_number: int, author: str):
+        repo = github.Github(token).get_repo(repo)
+        self.pr = repo.get_issue(pr_number).as_pull_request()
+        self.author = author
+
+    def should_comment(self) -> bool:
+        # As soon as a new contributor has a PR merged, they are no longer a new contributor.
+        # We can tell that they were a new contributor previously because we would have
+        # added a new contributor greeting comment when they opened the PR.
+        found_greeting = False
+        for comment in self.pr.as_issue().get_comments():
+            if PRGreeter.COMMENT_TAG in comment.body:
+                found_greeting = True
+            elif PRBuildbotInformation.COMMENT_TAG in comment.body:
+                # When an issue is reopened, then closed as merged again, we should not
+                # add a second comment. This event will be rare in practice as it seems
+                # like it's only possible when the main branch is still at the exact
+                # revision that the PR was merged on to, beyond that it's closed forever.
+                return False
+        return found_greeting
+
+    def run(self) -> bool:
+        if not self.should_comment():
+            return
+
+        # This text is using Markdown formatting. Some of the lines are longer
+        # than others so that the final text is some reasonable looking paragraphs
+        # after the long URLs are rendered.
+        comment = f"""\
+{PRBuildbotInformation.COMMENT_TAG}
+@{self.author} Congratulations on having your first Pull Request (PR) merged into the LLVM Project!
+
+Your changes will be combined with recent changes from other authors, then tested
+by our [build bots](https://lab.llvm.org/buildbot/). If there is a problem with a build,
+you may recieve a report in an email or a comment on this PR.
+
+Please check whether problems have been caused by your change specifically, as
+the builds can include changes from many authors. It is not uncommon for your
+change to be included in a build that fails due to someone else's changes, or
+infrastructure issues.
+
+How to do this, and the rest of the post-merge process, is covered in detail [here](https://llvm.org/docs/MyFirstTypoFix.html#myfirsttypofix-issues-after-landing-your-pr).
+
+If your change does cause a problem, it may be reverted, or you can revert it yourself.
+This is a normal part of [LLVM development](https://llvm.org/docs/DeveloperPolicy.html#patch-reversion-policy). You can fix your changes and open a new PR to merge them again.
+
+If you don't get any reports, no action is required from you. Your changes are working as expected, well done!
+"""
+        self.pr.as_issue().create_comment(comment)
+        return True
+
+
 def setup_llvmbot_git(git_dir="."):
     """
     Configure the git repo in `git_dir` with the llvmbot account so
@@ -585,6 +644,10 @@ def execute_command(self) -> bool:
 pr_greeter_parser = subparsers.add_parser("pr-greeter")
 pr_greeter_parser.add_argument("--issue-number", type=int, required=True)
 
+pr_buildbot_information_parser = subparsers.add_parser("pr-buildbot-information")
+pr_buildbot_information_parser.add_argument("--issue-number", type=int, required=True)
+pr_buildbot_information_parser.add_argument("--author", type=str, required=True)
+
 release_workflow_parser = subparsers.add_parser("release-workflow")
 release_workflow_parser.add_argument(
     "--llvm-project-dir",
@@ -633,6 +696,11 @@ def execute_command(self) -> bool:
 elif args.command == "pr-greeter":
     pr_greeter = PRGreeter(args.token, args.repo, args.issue_number)
     pr_greeter.run()
+elif args.command == "pr-buildbot-information":
+    pr_buildbot_information = PRBuildbotInformation(
+        args.token, args.repo, args.issue_number, args.author
+    )
+    pr_buildbot_information.run()
 elif args.command == "release-workflow":
     release_workflow = ReleaseWorkflow(
         args.token,