git.scottworley.com Git - git-cache/blob - git_cache.py

   1 # It would be nice if we could share the nix git cache, but as of the
   2 # time of writing it is transitioning from gitv2 (deprecated) to gitv3
   3 # (not ready yet), and trying to straddle them both is too far into nix
   4 # implementation details for my comfort.  So we re-implement here half of
   5 # nix's builtins.fetchGit.  :(
   6
   7 import hashlib
   8 import logging
   9 import os
  10 import subprocess
  11 import sys
  12
  13 from typing import Tuple
  14
  15 import backoff
  16
  17 Path = str  # eg: "/home/user/.cache/git-cache/v1"
  18 Repo = str  # eg: "https://github.com/NixOS/nixpkgs.git"
  19 Ref = str   # eg: "master" or "v1.0.0"
  20 Rev = str   # eg: "53a27350551844e1ed1a9257690294767389ef0d"
  21
  22
  23 def git_cachedir(repo: Repo) -> Path:
  24     # Use xdg module when it's less painful to have as a dependency
  25     XDG_CACHE_HOME = Path(
  26         os.environ.get('XDG_CACHE_HOME', os.path.expanduser('~/.cache')))
  27
  28     return Path(os.path.join(
  29         XDG_CACHE_HOME,
  30         'git-cache/v1',
  31         hashlib.sha256(repo.encode()).hexdigest()))
  32
  33
  34 def verify_ancestry(repo: Repo, ref: Ref, rev: Rev) -> None:
  35     cachedir = git_cachedir(repo)
  36     logging.debug('Verifying rev %s is an ancestor of ref "%s"', rev, ref)
  37     subprocess.run(['git', '-C', cachedir, 'merge-base', '--is-ancestor',
  38                     rev, ref], check=True)
  39
  40
  41 @backoff.on_exception(
  42     backoff.expo,
  43     subprocess.CalledProcessError,
  44     max_time=lambda: int(os.environ.get('BACKOFF_MAX_TIME', '30')))
  45 def _git_fetch(cachedir: Path, repo: Repo, ref: Ref) -> None:
  46     # We don't use --force here because we want to abort and freak out if forced
  47     # updates are happening.
  48     subprocess.run(['git', '-C', cachedir, 'fetch', repo,
  49                     '%s:%s' % (ref, ref)], check=True)
  50
  51
  52 def fetch(repo: Repo, ref: Ref) -> Tuple[Path, Rev]:
  53     cachedir = git_cachedir(repo)
  54     if not os.path.exists(cachedir):
  55         logging.debug("Initializing git repo")
  56         subprocess.run(['git', 'init', '--bare', cachedir],
  57                        check=True, stdout=sys.stderr)
  58
  59     logging.debug('Fetching ref "%s" from %s', ref, repo)
  60     _git_fetch(cachedir, repo, ref)
  61
  62     with open(os.path.join(cachedir, 'refs', 'heads', ref)) as rev_file:
  63         rev = Rev(rev_file.read(999).strip())
  64     verify_ancestry(repo, ref, rev)
  65
  66     return cachedir, rev
  67
  68
  69 def ensure_rev_available(repo: Repo, ref: Ref, rev: Rev) -> Path:
  70     cachedir = git_cachedir(repo)
  71     if os.path.exists(cachedir):
  72         logging.debug('Checking if we already have rev %s', rev)
  73         process = subprocess.run(
  74             ['git', '-C', cachedir, 'cat-file', '-e', rev], check=False)
  75         if process.returncode == 0:
  76             logging.debug('We already have rev %s', rev)
  77             verify_ancestry(repo, ref, rev)
  78             return cachedir
  79         if process.returncode != 1:
  80             raise Exception(
  81                 'Could not test for presence of rev %s.  Is cache dir "%s" messed up?' %
  82                 (rev, cachedir))
  83
  84     logging.debug(
  85         'We do not have rev %s.  We will fetch ref "%s" and hope it appears.',
  86         rev, ref)
  87     fetch(repo, ref)
  88     logging.debug('Verifying that fetch retrieved rev %s', rev)
  89     subprocess.run(['git', '-C', cachedir, 'cat-file', '-e', rev], check=True)
  90
  91     return cachedir
  92
  93
  94 def _main() -> None:
  95     if len(sys.argv) == 3:
  96         print('{1} {0}'.format(*fetch(Repo(sys.argv[1]), Ref(sys.argv[2]))))
  97     elif len(sys.argv) == 4:
  98         print(ensure_rev_available(
  99             Repo(sys.argv[1]), Ref(sys.argv[2]), Rev(sys.argv[3])))
 100     else:
 101         usage = '''usage: git-cache repo ref [rev]
 102 example: git-cache https://github.com/NixOS/nixpkgs.git master'''
 103         print(usage, file=sys.stderr)
 104         sys.exit(1)