Initial version

[git-cache] / git_cache.py

# It would be nice if we could share the nix git cache, but as of the
# time of writing it is transitioning from gitv2 (deprecated) to gitv3
# (not ready yet), and trying to straddle them both is too far into nix
# implementation details for my comfort.  So we re-implement here half of
# nix's builtins.fetchGit.  :(

import hashlib
import logging
import os
import subprocess

from typing import Tuple

Path = str  # eg: "/home/user/.cache/git-cache/v1"
Repo = str  # eg: "https://github.com/NixOS/nixpkgs.git"
Ref = str   # eg: "master" or "v1.0.0"
Rev = str   # eg: "53a27350551844e1ed1a9257690294767389ef0d"


def git_cachedir(repo: Repo) -> Path:
    # Use xdg module when it's less painful to have as a dependency
    XDG_CACHE_HOME = Path(
        os.environ.get('XDG_CACHE_HOME', os.path.expanduser('~/.cache')))

    return Path(os.path.join(
        XDG_CACHE_HOME,
        'git-cache/v1',
        hashlib.sha256(repo.encode()).hexdigest()))


def verify_ancestry(repo: Repo, ref: Ref, rev: Rev) -> None:
    cachedir = git_cachedir(repo)
    logging.debug('Verifying rev %s is an ancestor of ref "%s"', rev, ref)
    subprocess.run(['git', '-C', cachedir, 'merge-base', '--is-ancestor',
                    rev, ref], check=True)


def fetch(repo: Repo, ref: Ref) -> Tuple[Path, Rev]:
    cachedir = git_cachedir(repo)
    if not os.path.exists(cachedir):
        logging.debug("Initializing git repo")
        subprocess.run(['git', 'init', '--bare', cachedir], check=True)

    logging.debug('Fetching ref "%s" from %s', ref, repo)
    # We don't use --force here because we want to abort and freak out if forced
    # updates are happening.
    subprocess.run(['git', '-C', cachedir, 'fetch', repo,
                    '%s:%s' % (ref, ref)], check=True)

    with open(os.path.join(cachedir, 'refs', 'heads', ref)) as rev_file:
        rev = Rev(rev_file.read(999).strip())
    verify_ancestry(repo, ref, rev)

    return cachedir, rev


def ensure_rev_available(repo: Repo, ref: Ref, rev: Rev) -> Path:
    cachedir = git_cachedir(repo)
    if os.path.exists(cachedir):
        logging.debug('Checking if we already have rev %s', rev)
        process = subprocess.run(
            ['git', '-C', cachedir, 'cat-file', '-e', rev], check=False)
        if process.returncode == 0:
            logging.debug('We already have rev %s', rev)
            verify_ancestry(repo, ref, rev)
            return cachedir
        if process.returncode != 1:
            raise Exception(
                'Could not test for presence of rev %s.  Is cache dir "%s" messed up?' %
                (rev, cachedir))

    logging.debug(
        'We do not have rev %s.  We will fetch ref "%s" and hope it appears.',
        rev, ref)
    fetch(repo, ref)
    logging.debug('Verifying that fetch retrieved rev %s', rev)
    subprocess.run(['git', '-C', cachedir, 'cat-file', '-e', rev], check=True)

    return cachedir