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[git-cache] / 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], check=True)
57
58 logging.debug('Fetching ref "%s" from %s', ref, repo)
59 _git_fetch(cachedir, repo, ref)
60
61 with open(os.path.join(cachedir, 'refs', 'heads', ref)) as rev_file:
62 rev = Rev(rev_file.read(999).strip())
63 verify_ancestry(repo, ref, rev)
64
65 return cachedir, rev
66
67
68 def ensure_rev_available(repo: Repo, ref: Ref, rev: Rev) -> Path:
69 cachedir = git_cachedir(repo)
70 if os.path.exists(cachedir):
71 logging.debug('Checking if we already have rev %s', rev)
72 process = subprocess.run(
73 ['git', '-C', cachedir, 'cat-file', '-e', rev], check=False)
74 if process.returncode == 0:
75 logging.debug('We already have rev %s', rev)
76 verify_ancestry(repo, ref, rev)
77 return cachedir
78 if process.returncode != 1:
79 raise Exception(
80 'Could not test for presence of rev %s. Is cache dir "%s" messed up?' %
81 (rev, cachedir))
82
83 logging.debug(
84 'We do not have rev %s. We will fetch ref "%s" and hope it appears.',
85 rev, ref)
86 fetch(repo, ref)
87 logging.debug('Verifying that fetch retrieved rev %s', rev)
88 subprocess.run(['git', '-C', cachedir, 'cat-file', '-e', rev], check=True)
89
90 return cachedir
91
92
93 def _main() -> None:
94 if len(sys.argv) == 3:
95 print('{1} {0}'.format(*fetch(Repo(sys.argv[1]), Ref(sys.argv[2]))))
96 elif len(sys.argv) == 4:
97 print(ensure_rev_available(
98 Repo(sys.argv[1]), Ref(sys.argv[2]), Rev(sys.argv[3])))
99 else:
100 usage = '''usage: git-cache repo ref [rev]
101 example: git-cache https://github.com/NixOS/nixpkgs.git master'''
102 print(usage, file=sys.stderr)
103 sys.exit(1)