[GitHub][workflows] Ask reviewers to merge PRs when author cannot (#81142)

This uses
https://pygithub.readthedocs.io/en/stable/github_objects/Repository.html?highlight=get_collaborator_permission#github.Repository.Repository.get_collaborator_permission.

Which does
https://docs.github.com/en/rest/collaborators/collaborators?apiVersion=2022-11-28#get-repository-permissions-for-a-user
and returns the top level "permission" key.

This is less detailed than the user/permissions key but should be fine
for this
use case.

When a review is submitted we check:
* If it's an approval.
* Whether we have already left a merge on behalf comment (by looking for
a hidden HTML comment).
* Whether the author has permissions to merge their own PR. 
* Whether the reviewer has permissions to merge.

If needed we leave a comment tagging the reviewer. If the reviewer also
doesn't have merge permission, then it asks them to find someone else
who does.

Author: DavidSpickett

.github/workflows/approved-prs.yml

@@ -0,0 +1,39 @@
+name: "Prompt reviewers to merge PRs on behalf of authors"
+
+permissions:
+  contents: read
+
+on:
+  pull_request_review:
+    types:
+      - submitted
+
+jobs:
+  merge-on-behalf-information-comment:
+    runs-on: ubuntu-latest
+    permissions:
+      pull-requests: write
+    if: >-
+      (github.repository == 'llvm/llvm-project') &&
+      (github.event.review.state == 'APPROVED')
+    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 Merge On Behalf Comment
+        working-directory: ./llvm/utils/git/
+        run: |
+          python3 ./github-automation.py \
+            --token '${{ secrets.GITHUB_TOKEN }}' \
+            pr-merge-on-behalf-information \
+            --issue-number "${{ github.event.pull_request.number }}" \
+            --author "${{ github.event.pull_request.user.login }}" \
+            --reviewer "${{ github.event.review.user.login }}"

llvm/utils/git/github-automation.py

@@ -298,6 +298,55 @@ def run(self) -> bool:
         return True
 
 
+class PRMergeOnBehalfInformation:
+    COMMENT_TAG = "<!--LLVM MERGE ON BEHALF INFORMATION COMMENT-->\n"
+
+    def __init__(
+        self, token: str, repo: str, pr_number: int, author: str, reviewer: str
+    ):
+        self.repo = github.Github(token).get_repo(repo)
+        self.pr = self.repo.get_issue(pr_number).as_pull_request()
+        self.author = author
+        self.reviewer = reviewer
+
+    def can_merge(self, user: str) -> bool:
+        try:
+            return self.repo.get_collaborator_permission(user) in ["admin", "write"]
+        # There is a UnknownObjectException for this scenario, but this method
+        # does not use it.
+        except github.GithubException as e:
+            # 404 means the author was not found in the collaborator list, so we
+            # know they don't have push permissions. Anything else is a real API
+            # issue, raise it so it is visible.
+            if e.status != 404:
+                raise e
+            return False
+
+    def run(self) -> bool:
+        # Check this first because it only costs 1 API point.
+        if self.can_merge(self.author):
+            return
+
+        # A review can be approved more than once, only comment the first time.
+        for comment in self.pr.as_issue().get_comments():
+            if self.COMMENT_TAG in comment.body:
+                return
+
+        # This text is using Markdown formatting.
+        if self.can_merge(self.reviewer):
+            comment = f"""\
+{self.COMMENT_TAG}
+@{self.reviewer} the PR author does not have permission to merge their own PRs yet. Please merge on their behalf."""
+        else:
+            comment = f"""\
+{self.COMMENT_TAG}
+@{self.reviewer} the author of this PR does not have permission to merge and neither do you.
+Please find someone who has merge permissions who can merge it on the author's behalf. This could be one of the other reviewers or you can ask on [Discord](https://discord.com/invite/xS7Z362)."""
+
+        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
@@ -665,6 +714,17 @@ def execute_command(self) -> bool:
 pr_buildbot_information_parser.add_argument("--issue-number", type=int, required=True)
 pr_buildbot_information_parser.add_argument("--author", type=str, required=True)
 
+pr_merge_on_behalf_information_parser = subparsers.add_parser(
+    "pr-merge-on-behalf-information"
+)
+pr_merge_on_behalf_information_parser.add_argument(
+    "--issue-number", type=int, required=True
+)
+pr_merge_on_behalf_information_parser.add_argument("--author", type=str, required=True)
+pr_merge_on_behalf_information_parser.add_argument(
+    "--reviewer", type=str, required=True
+)
+
 release_workflow_parser = subparsers.add_parser("release-workflow")
 release_workflow_parser.add_argument(
     "--llvm-project-dir",
@@ -724,6 +784,11 @@ def execute_command(self) -> bool:
         args.token, args.repo, args.issue_number, args.author
     )
     pr_buildbot_information.run()
+elif args.command == "pr-merge-on-behalf-information":
+    pr_merge_on_behalf_information = PRMergeOnBehalfInformation(
+        args.token, args.repo, args.issue_number, args.author, args.reviewer
+    )
+    pr_merge_on_behalf_information.run()
 elif args.command == "release-workflow":
     release_workflow = ReleaseWorkflow(
         args.token,