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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, Union
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 RefOrRev = Union[Ref, Rev]
22
23
24 def git_cachedir(repo: Repo) -> Path:
25 # Use xdg module when it's less painful to have as a dependency
26 XDG_CACHE_HOME = Path(
27 os.environ.get('XDG_CACHE_HOME', os.path.expanduser('~/.cache')))
28
29 return Path(os.path.join(
30 XDG_CACHE_HOME,
31 'git-cache/v1',
32 hashlib.sha256(repo.encode()).hexdigest()))
33
34
35 def is_ancestor(repo: Repo, descendant: RefOrRev, ancestor: RefOrRev) -> bool:
36 cachedir = git_cachedir(repo)
37 logging.debug('Checking if %s is an ancestor of %s', ancestor, descendant)
38 process = subprocess.run(['git',
39 '-C',
40 cachedir,
41 'merge-base',
42 '--is-ancestor',
43 ancestor,
44 descendant],
45 check=False)
46 return process.returncode == 0
47
48
49 def verify_ancestry(
50 repo: Repo,
51 descendant: RefOrRev,
52 ancestor: RefOrRev) -> None:
53 if not is_ancestor(repo, descendant, ancestor):
54 raise Exception('%s is not an ancestor of %s' % (ancestor, descendant))
55
56
57 @backoff.on_exception(
58 backoff.expo,
59 subprocess.CalledProcessError,
60 max_time=lambda: int(os.environ.get('BACKOFF_MAX_TIME', '30')))
61 def _git_fetch(cachedir: Path, repo: Repo, ref: Ref) -> None:
62 # We don't use --force here because we want to abort and freak out if forced
63 # updates are happening.
64 subprocess.run(['git', '-C', cachedir, 'fetch', repo,
65 '%s:%s' % (ref, ref)], check=True)
66
67
68 def fetch(repo: Repo, ref: Ref) -> Tuple[Path, Rev]:
69 cachedir = git_cachedir(repo)
70 if not os.path.exists(cachedir):
71 logging.debug("Initializing git repo")
72 subprocess.run(['git', 'init', '--bare', cachedir],
73 check=True, stdout=sys.stderr)
74
75 logging.debug('Fetching ref "%s" from %s', ref, repo)
76 _git_fetch(cachedir, repo, ref)
77
78 with open(os.path.join(cachedir, 'refs', 'heads', ref)) as rev_file:
79 rev = Rev(rev_file.read(999).strip())
80 verify_ancestry(repo, ref, rev)
81
82 return cachedir, rev
83
84
85 def ensure_rev_available(repo: Repo, ref: Ref, rev: Rev) -> Path:
86 cachedir = git_cachedir(repo)
87 if os.path.exists(cachedir) and is_ancestor(repo, ref, rev):
88 return cachedir
89
90 logging.debug(
91 'We do not have rev %s. We will fetch ref "%s" and hope it appears.',
92 rev, ref)
93 fetch(repo, ref)
94 logging.debug('Verifying that fetch retrieved rev %s', rev)
95 subprocess.run(['git', '-C', cachedir, 'cat-file', '-e', rev], check=True)
96 verify_ancestry(repo, ref, rev)
97
98 return cachedir
99
100
101 def _main() -> None:
102 if len(sys.argv) == 3:
103 print('{1} {0}'.format(*fetch(Repo(sys.argv[1]), Ref(sys.argv[2]))))
104 elif len(sys.argv) == 4:
105 print(ensure_rev_available(
106 Repo(sys.argv[1]), Ref(sys.argv[2]), Rev(sys.argv[3])))
107 else:
108 usage = '''usage: git-cache repo ref [rev]
109 example: git-cache https://github.com/NixOS/nixpkgs.git master'''
110 print(usage, file=sys.stderr)
111 sys.exit(1)