[Github] Add initial workflow to prune unused user branches

This patch starts implementing the long requested feature of a workflow
to automatically prune user branches that are not tied to a PR. For now
this just consists of a script that finds user branches not attached to
a PR and prints them out. Future patches will add support for dumping
the diff between the branches and main (to persist to artifact storage
so people can recover if they intended to use the branch) and actually
deleting the branches.

Reviewers: cmtice, tstellar, petrhosek, vbvictor

Pull Request: llvm#175693

Author: boomanaiden154

.github/workflows/prune-branches.yml

@@ -0,0 +1,30 @@
+name: Prune Unused Branches
+
+permissions:
+  contents: read
+
+on:
+  pull_request:
+    paths:
+     - .github/workflows/prune-branches.yml
+  schedule:
+    - cron: '0 8 * * *' # Runs daily at 08:00 UTC.
+
+jobs:
+  prune-branches:
+    name: Prune Branches
+    if: github.repository_owner == 'llvm'
+    runs-on: ubuntu-24.04
+    steps:
+      - name: Fetch LLVM sources
+        uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
+        with:
+          fetch-depth: 0
+      - name: Install dependencies
+        run: |
+          pip install --require-hashes -r ./llvm/utils/git/requirements.txt
+      - name: Run Script
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          python3 .github/workflows/prune-unused-branches.py

.github/workflows/prune-unused-branches.py

@@ -0,0 +1,85 @@
+import subprocess
+import os
+
+import github
+
+
+def get_branches() -> list[str]:
+    git_process = subprocess.run(
+        ["git", "branch", "--all"], stdout=subprocess.PIPE, check=True
+    )
+    branches = [
+        branch.strip() for branch in git_process.stdout.decode("utf-8").split("\n")
+    ]
+
+    def branch_filter(branch_name):
+        return "users/" in branch_name or "revert-" in branch_name
+
+    filtered_branches = list(filter(branch_filter, branches))
+    return [branch.replace("remotes/origin/", "") for branch in filtered_branches]
+
+
+def get_branches_from_open_prs(github_token) -> list[str]:
+    gh = github.Github(auth=github.Auth.Token(github_token))
+    query = """
+  query ($after: String) {
+    search(query: "is:pr repo:llvm/llvm-project is:open head:users/", type: ISSUE, first: 100, after: $after) {
+      nodes {
+        ... on PullRequest {
+          baseRefName
+          headRefName
+          isCrossRepository
+          number
+        }
+      }
+      pageInfo {
+        hasNextPage
+        endCursor
+      }
+    }
+  }"""
+    pr_data = []
+    has_next_page = True
+    variables = {"after": None}
+    while has_next_page:
+        _, res_data = gh._Github__requester.graphql_query(query, variables=variables)
+        page_info = res_data["data"]["search"]["pageInfo"]
+        has_next_page = page_info["hasNextPage"]
+        if has_next_page:
+            variables["after"] = page_info["endCursor"]
+        prs = res_data["data"]["search"]["nodes"]
+        pr_data.extend(prs)
+        print(f"Processed {len(prs)} PRs")
+
+    user_branches = []
+    for pr in pr_data:
+        if not pr["isCrossRepository"]:
+            if pr["baseRefName"] != "main":
+                user_branches.append(pr["baseRefName"])
+            user_branches.append(pr["headRefName"])
+    return user_branches
+
+
+def get_user_branches_to_remove(
+    user_branches: list[str], user_branches_from_prs: list[str]
+) -> list[str]:
+    user_branches_to_remove = set(user_branches)
+    for pr_user_branch in set(user_branches_from_prs):
+        user_branches_to_remove.remove(pr_user_branch)
+    return list(user_branches_to_remove)
+
+
+def main(github_token):
+    user_branches = get_branches()
+    user_branches_from_prs = get_branches_from_open_prs(github_token)
+    print(f"Found {len(user_branches)} user branches in the repository")
+    print(f"Found {len(user_branches_from_prs)} user branches associated with PRs")
+    user_branches_to_remove = get_user_branches_to_remove(
+        user_branches, user_branches_from_prs
+    )
+    print(f"Deleting {len(user_branches_to_remove)} user branches.")
+    print(user_branches_to_remove)
+
+
+if __name__ == "__main__":
+    main(os.environ["GITHUB_TOKEN"])