Add automerge script (#13)

This introduces a script to automatically perform the merge of incoming
changes from upstream LLVM which live in the `main` branch into the
downstream `arm-software` branch. It also includes a GitHub Workflow
to execute it after every sync from upstream.

---------

Co-authored-by: Davide <[email protected]>

Author: pratlucas

.github/workflows/automerge.yml

@@ -0,0 +1,26 @@
+# This workflow executes the automerge python script to automatically merge
+# changes from the `main` branch of upstream LLVM into the `arm-software`
+# branch of the arm/arm-toolchain repository.
+name: Automerge
+on:
+  workflow_run:
+    workflows: [Sync from Upstream LLVM]
+    types:
+      - completed
+jobs:
+  Run-Automerge:
+    runs-on: ubuntu-latest
+    env:
+      FROM_BRANCH: main
+      TO_BRANCH: arm-software
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+      - name: Configure Git Identity
+        run: |
+          git config --local user.name "github-actions[bot]"
+          git config --local user.email "41898282+github-actions[bot]@users.noreply.github.com"
+      - name: Run automerge
+        run: python3 arm-software/ci/automerge.py --project-name ${{ env.GITHUB_REPOSITORY }} --from-branch ${{ env.FROM_BRANCH }} --to-branch ${{ env.TO_BRANCH }}
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}

arm-software/ci/.automerge_ignore

@@ -0,0 +1,5 @@
+.github
+arm-software
+CONTRIBUTING.md
+README.md
+LICENSE.TXT

arm-software/ci/automerge.py

@@ -0,0 +1,212 @@
+#!/usr/bin/env python3
+
+"""
+A script to automatically perform the merge of incoming changes from a branch
+in upstream LLVM into a downstream branch.
+"""
+
+import argparse
+import json
+import logging
+import subprocess
+import sys
+from pathlib import Path
+
+logging.basicConfig(level=logging.INFO, format="%(levelname)s: %(message)s")
+logger = logging.getLogger(__name__)
+
+
+MERGE_CONFLICT_LABEL = "automerge_conflict"
+AUTOMERGE_BRANCH = "automerge"
+REMOTE_NAME = "origin"
+MERGE_IGNORE_PATHSPEC_FILE = Path(__file__).parent / ".automerge_ignore"
+
+
+class MergeConflictError(Exception):
+    """
+    An exception representing a failed merge from upstream due to a conflict.
+    """
+
+    def __init__(self, commit_hash: str) -> None:
+        super().__init__()
+        self.commit_hash = commit_hash
+
+
+class Git:
+    """
+    A helper class for running Git commands on a repository that lives in a
+    specific path.
+    """
+
+    def __init__(self, repo_path: Path) -> None:
+        self.repo_path = repo_path
+
+    def run_cmd(self, args: list[str], check: bool = True) -> str:
+        git_cmd = ["git", "-C", str(self.repo_path)] + args
+        git_process = subprocess.run(git_cmd, check=check, capture_output=True, text=True)
+        return git_process.stdout
+
+
+def has_unresolved_conflicts(git_repo: Git) -> bool:
+    diff_output = git_repo.run_cmd(["diff", "--name-only", "--diff-filter=U"])
+    diff_output = diff_output.strip()
+    return bool(diff_output)
+
+
+def prefix_current_commit_message(git_repo: Git) -> None:
+    log_output = git_repo.run_cmd(["log", "HEAD", "--max-count=1", "--pretty=format:%B"])
+    commit_msg = f"Automerge: {log_output}"
+    git_repo.run_cmd(["commit", "--amend", "--message=" + commit_msg])
+
+
+def merge_commit(git_repo: Git, to_branch: str, commit_hash: str, dry_run: bool) -> None:
+    logger.info("Merging commit %s into %s", commit_hash, to_branch)
+    git_repo.run_cmd(["switch", to_branch])
+    git_repo.run_cmd(["merge", commit_hash, "--no-commit", "--no-ff"], check=False)
+    # Ensure all paths that should be ignored stay unchanged
+    git_repo.run_cmd(
+        ["restore", "--ours", "--staged", "--worktree", f"--pathspec-from-file={MERGE_IGNORE_PATHSPEC_FILE}"]
+    )
+    if has_unresolved_conflicts(git_repo):
+        logger.info("Merge failed")
+        git_repo.run_cmd(["merge", "--abort"])
+        raise MergeConflictError(commit_hash)
+    git_repo.run_cmd(["commit", "--reuse-message", commit_hash])
+    prefix_current_commit_message(git_repo)
+    if dry_run:
+        logger.info("Dry run. Skipping push into remote repository.")
+    else:
+        git_repo.run_cmd(["push", REMOTE_NAME, to_branch])
+    logger.info("Merge successful")
+
+
+def create_pull_request(git_repo: Git, to_branch: str) -> None:
+    logger.info("Creating Pull Request")
+    log_output = git_repo.run_cmd(["log", "HEAD", "--max-count=1", "--pretty=format:%s"])
+    pr_title = f"Automerge conflict: {log_output}"
+    subprocess.run(
+        ["gh", "pr", "create", "--head", AUTOMERGE_BRANCH, "--base", to_branch, "--fill", "--title", pr_title],
+        check=True,
+    )
+
+
+def process_conflict(git_repo: Git, commit_hash: str, to_branch: str, dry_run: bool) -> None:
+    logger.info("Processing conflict for %s", commit_hash)
+    git_repo.run_cmd(["switch", "--force-create", AUTOMERGE_BRANCH, commit_hash])
+    if dry_run:
+        logger.info("Dry run, skipping push and creation of PR.")
+        return
+    git_repo.run_cmd(["push", REMOTE_NAME, AUTOMERGE_BRANCH])
+    logger.info("Publishing Pull Request for conflict")
+    create_pull_request(git_repo, to_branch)
+
+
+def get_merge_commit_list(git_repo: Git, from_branch: str, to_branch: str) -> list[str]:
+    logger.info("Calculating list of commits to be merged from %s to %s", from_branch, to_branch)
+    merge_base_output = git_repo.run_cmd(["merge-base", from_branch, to_branch])
+    merge_base_commit = merge_base_output.strip()
+    log_output = git_repo.run_cmd(["log", f"{merge_base_commit}..{from_branch}", "--pretty=format:%H"])
+    commit_list = log_output.strip()
+    if not commit_list:
+        logger.info("No commits to be merged")
+        return []
+    commit_list = commit_list.split("\n")
+    commit_list.reverse()
+    logger.info("Found %d commits to be merged", len(commit_list))
+    return commit_list
+
+
+def ensure_branch_exists(git_repo: Git, branch_name: str) -> None:
+    try:
+        git_repo.run_cmd(["rev-parse", "--verify", branch_name])
+    except subprocess.CalledProcessError:
+        git_repo.run_cmd(["remote", "set-branches", "--add", REMOTE_NAME, branch_name])
+
+
+def fetch_branch(git_repo: Git, branch_name: str) -> None:
+    logger.info("Fetching '%s' branch from remote.", branch_name)
+    ensure_branch_exists(git_repo, branch_name)
+    git_repo.run_cmd(["fetch", REMOTE_NAME, f"{branch_name}:{branch_name}"])
+
+
+def pr_exist_for_label(project_name: str, label: str) -> bool:
+    logger.info("Fetching list of open PRs for label '%s'.", label)
+    gh_process = subprocess.run(
+        ["gh", "pr", "list", "--label", label, "--repo", project_name, "--json", "id"],
+        check=True,
+        capture_output=True,
+        text=True,
+    )
+    return len(json.loads(gh_process.stdout)) > 0
+
+
+def main():
+    arg_parser = argparse.ArgumentParser(
+        prog="automerge",
+        description="A script that automatically merges individual commits from one branch into another.",
+    )
+    arg_parser.add_argument(
+        "--project-name",
+        required=True,
+        metavar="OWNER/REPO",
+        help="The name of the project in GitHub.",
+    )
+    arg_parser.add_argument(
+        "--from-branch",
+        required=True,
+        metavar="BRANCH_NAME",
+        help="The branch where the incoming commits are found.",
+    )
+    arg_parser.add_argument(
+        "--to-branch",
+        required=True,
+        metavar="BRANCH_NAME",
+        help="The target branch for merging incoming commits",
+    )
+    arg_parser.add_argument(
+        "--repo-path",
+        metavar="PATH",
+        default=Path.cwd(),
+        help="The path to the existing local checkout of the repository (default: working directory)",
+    )
+    arg_parser.add_argument(
+        "--dry-run",
+        action="store_true",
+        help="Process changes locally, but don't merge them into the remote repository and don't create PRs",
+    )
+
+    args = arg_parser.parse_args()
+
+    try:
+        if pr_exist_for_label(args.project_name, MERGE_CONFLICT_LABEL):
+            logger.error("There are pending automerge PRs. Cannot continue.")
+            sys.exit(1)
+        logger.info("No pending merge conflicts. Proceeding with automerge.")
+
+        git_repo = Git(args.repo_path)
+
+        fetch_branch(git_repo, args.from_branch)
+        fetch_branch(git_repo, args.to_branch)
+
+        merge_commits = get_merge_commit_list(git_repo, args.from_branch, args.to_branch)
+        for commit_hash in merge_commits:
+            merge_commit(git_repo, args.to_branch, commit_hash, args.dry_run)
+    except MergeConflictError as conflict:
+        process_conflict(
+            git_repo,
+            conflict.commit_hash,
+            args.to_branch,
+            args.dry_run,
+        )
+    except subprocess.CalledProcessError as error:
+        logger.error(
+            'Failed to run command: "%s"\nstdout:\n%s\nstderr:\n%s',
+            " ".join(str(error.cmd)),
+            error.stdout,
+            error.stderr,
+        )
+        sys.exit(1)
+
+
+if __name__ == "__main__":
+    main()