ci: tag-maintainer workflow refactor (#7436)

Break the workflow into multiple scripts to make it easier to test /
maintain. Also fix the remove reviewer process to not review reviews
from people that were manually requested.

Signed-off-by: Austin Horstman <khaneliman12@gmail.com>

lib/python/extract-maintainers.py

@@ -0,0 +1,107 @@
+#!/usr/bin/env python3
+"""
+Extract maintainers from changed Home Manager module files.
+
+This script extracts the maintainer extraction logic from the tag-maintainers workflow
+for easier testing and validation.
+"""
+
+import argparse
+import json
+import logging
+import subprocess
+import sys
+from pathlib import Path
+
+
+class NixEvalError(Exception):
+    """Custom exception for errors during Nix evaluation."""
+    pass
+
+
+def run_nix_eval(nix_file: Path, *args: str) -> str:
+    """Run a Nix evaluation expression and return the result as a string."""
+    command = [
+        "nix-instantiate",
+        "--eval",
+        "--strict",
+        "--json",
+        str(nix_file),
+        *args,
+    ]
+    logging.debug(f"Running command: {' '.join(command)}")
+    try:
+        result = subprocess.run(
+            command,
+            capture_output=True,
+            text=True,
+            check=True,
+        )
+        return result.stdout.strip()
+    except FileNotFoundError:
+        logging.error("'nix-instantiate' command not found. Is Nix installed and in your PATH?")
+        raise NixEvalError("'nix-instantiate' not found")
+    except subprocess.CalledProcessError as e:
+        logging.error(f"Nix evaluation failed with exit code {e.returncode}")
+        logging.error(f"Stderr: {e.stderr.strip()}")
+        raise NixEvalError("Nix evaluation failed") from e
+
+
+def extract_maintainers(changed_files: list[str], pr_author: str) -> list[str]:
+    """Extract and filter maintainers from a list of changed module files."""
+    if not changed_files:
+        logging.info("No module files changed; no maintainers to tag.")
+        return []
+
+    logging.info("Finding maintainers for changed files...")
+    nix_file = Path(__file__).parent.parent / "nix" / "extract-maintainers.nix"
+    changed_files_json = json.dumps(changed_files)
+
+    try:
+        result_json = run_nix_eval(nix_file, "--argstr", "changedFilesJson", changed_files_json)
+        maintainers = set(json.loads(result_json))
+    except NixEvalError:
+        # Error is already logged by run_nix_eval
+        return []
+    except json.JSONDecodeError as e:
+        logging.error(f"Error parsing JSON output from Nix: {e}")
+        return []
+
+    filtered_maintainers = sorted(list(maintainers - {pr_author}))
+
+    if not filtered_maintainers:
+        logging.info("No maintainers found (or only the PR author is a maintainer).")
+        return []
+
+    logging.info(f"Found maintainers to notify: {' '.join(filtered_maintainers)}")
+    return filtered_maintainers
+
+
+def main() -> None:
+    """Parse arguments and run the maintainer extraction."""
+    logging.basicConfig(level=logging.INFO, format="%(message)s", stream=sys.stderr)
+
+    parser = argparse.ArgumentParser(
+        description="Extract maintainers from changed Home Manager module files."
+    )
+    parser.add_argument(
+        "--changed-files",
+        help="Newline-separated list of changed files",
+        default="",
+    )
+    parser.add_argument(
+        "--pr-author",
+        required=True,
+        help="GitHub username of the PR author",
+    )
+    args = parser.parse_args()
+
+    changed_files = [f.strip() for f in args.changed_files.splitlines() if f.strip()]
+
+    maintainers = extract_maintainers(changed_files, args.pr_author)
+
+    print(" ".join(maintainers))
+
+
+if __name__ == "__main__":
+    main()