build/bazel/scripts/check_ninja_build_plan.py

#!/usr/bin/env python3
# Copyright 2023 The Fuchsia Authors. All rights reserved.
# Use of this source code is governed by a BSD-style license that can be
# found in the LICENSE file.

"""Check very quickly whether the Ninja build plan needs to be rebuilt."""

import argparse
import os
import sys

# NOTE: Do not use pathlib.Path here since benchmarking shows that this makes
# this script _significantly_ slower (e.g. 42ms vs 16ms !) compared to os.path
# operations.


def ninja_plan_is_up_to_date(ninja_build_dir: str) -> bool:
    """Return True if the Ninja build plan is up-to-date.

    This expects the Ninja build plan to be generated by GN, which
    produces a `build_ninja.d` file with the right set of dependencies.

    Args:
      ninja_build_dir: Ninja build output directory.

    Returns:
      True if the build plan is up-to-date, False if the next Ninja call
      would invoke a regen step.
    """
    # This reads the build.ninja.d directly and tries to stat() all
    # dependencies in it directly (around 7000+), which is much
    # faster than Ninja trying to stat all build graph paths!
    build_ninja_d = os.path.join(ninja_build_dir, "build.ninja.d")
    if not os.path.exists(build_ninja_d):
        return False

    with open(build_ninja_d) as f:
        build_ninja_deps = f.read().split(" ")

    assert len(build_ninja_deps) > 1
    # The first item is the top-level manifest file followed by a colon,
    # e.g. 'build.ninja:'
    ninja_stamp = os.path.join(ninja_build_dir, build_ninja_deps[0][:-1])
    ninja_stamp_timestamp = os.stat(ninja_stamp).st_mtime

    try:
        for dep in build_ninja_deps[1:]:
            dep_path = os.path.join(ninja_build_dir, dep)
            dep_timestamp = os.stat(dep_path).st_mtime
            if dep_timestamp > ninja_stamp_timestamp:
                return False
    except FileNotFoundError:
        return False

    return True


def main():
    parser = argparse.ArgumentParser(description=__doc__)
    parser.add_argument("ninja_build_dir", help="Ninja build directory.")
    parser.add_argument(
        "--quiet", action="store_true", help="Do not print anything."
    )
    args = parser.parse_args()
    if ninja_plan_is_up_to_date(args.ninja_build_dir):
        if not args.quiet:
            print("up-to-date!")
        return 0

    if not args.quiet:
        print("regen required!")
    return 1


if __name__ == "__main__":
    sys.exit(main())