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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 argparse
8 import functools
9 import hashlib
10 import logging
11 import os
12 import subprocess
13 import sys
14 import time
15
16 from typing import Iterator, NamedTuple, Optional, TypeVar, Tuple, Union
17
18 import backoff
19
20 Path = str # eg: "/home/user/.cache/git-cache/v1"
21 Repo = str # eg: "https://github.com/NixOS/nixpkgs.git"
22 Ref = str # eg: "master" or "v1.0.0"
23 Rev = str # eg: "53a27350551844e1ed1a9257690294767389ef0d"
24 RefOrRev = Union[Ref, Rev]
25
26
27 class _LogEntry(NamedTuple):
28 ref: Ref
29 rev: Rev
30
31
32 T = TypeVar('T')
33
34
35 def _repo_hashname(repo: Repo) -> str:
36 return hashlib.sha256(repo.encode()).hexdigest()
37
38
39 def git_cachedir(repo: Repo) -> Path:
40 # Use xdg module when it's less painful to have as a dependency
41 XDG_CACHE_HOME = Path(
42 os.environ.get('XDG_CACHE_HOME', os.path.expanduser('~/.cache')))
43
44 return Path(os.path.join(
45 XDG_CACHE_HOME,
46 'git-cache/v1',
47 _repo_hashname(repo)))
48
49
50 def _log_filename(repo: Repo) -> Path:
51 # Use xdg module when it's less painful to have as a dependency
52 XDG_DATA_HOME = Path(
53 os.environ.get('XDG_DATA_HOME', os.path.expanduser('~/.local/share')))
54
55 return Path(os.path.join(
56 XDG_DATA_HOME,
57 'git-cache/v1',
58 _repo_hashname(repo)))
59
60
61 def is_ancestor(repo: Repo, descendant: RefOrRev, ancestor: RefOrRev) -> bool:
62 cachedir = git_cachedir(repo)
63 logging.debug('Checking if %s is an ancestor of %s', ancestor, descendant)
64 process = subprocess.run(['git',
65 '-C',
66 cachedir,
67 'merge-base',
68 '--is-ancestor',
69 ancestor,
70 descendant],
71 check=False)
72 return process.returncode == 0
73
74
75 def verify_ancestry(
76 repo: Repo,
77 descendant: RefOrRev,
78 ancestor: RefOrRev,
79 force: bool = False) -> None:
80 if not force and not is_ancestor(repo, descendant, ancestor):
81 raise Exception('%s is not an ancestor of %s' % (ancestor, descendant))
82
83
84 def _read_fetch_log(repo: Repo) -> Iterator[_LogEntry]:
85 filename = _log_filename(repo)
86 if not os.path.exists(filename):
87 return
88 with open(filename, 'r') as f:
89 for line in f:
90 _, _, rev, ref = line.strip().split(maxsplit=3)
91 yield _LogEntry(ref, rev)
92
93
94 def _last(it: Iterator[T]) -> Optional[T]:
95 return functools.reduce(lambda a, b: b, it, None)
96
97
98 def _previous_fetched_rev(repo: Repo, ref: Ref) -> Optional[Rev]:
99 return _last(entry.rev for entry in _read_fetch_log(
100 repo) if entry.ref == ref)
101
102
103 def _log_fetch(repo: Repo, ref: Ref, rev: Rev, force: bool = False) -> None:
104 if not force:
105 prev_rev = _previous_fetched_rev(repo, ref)
106 if prev_rev is not None:
107 verify_ancestry(repo, rev, prev_rev)
108 filename = _log_filename(repo)
109 os.makedirs(os.path.dirname(filename), exist_ok=True)
110 with open(filename, 'a') as f:
111 f.write('%s %s %s %s\n' %
112 (time.strftime('%Y-%m%d-%H:%M:%S%z'),
113 ('FORCEDFETCH' if force else 'fetch'), rev, ref))
114
115
116 def _show_force_warning() -> None:
117 print('''
118 **************************************************************************
119 * WARNING: git-cache INVOKED WITH --force! *
120 * *
121 * This mode allows previously-fetched refs to be overwritten to point to *
122 * non-descendants -- commits that don't have the previous version of the *
123 * the ref in their history! *
124 * *
125 * This should only be invoked by a human operator who knows what they're *
126 * doing to correct a specific, known, problem. Care should be taken to *
127 * prevent recurrence. *
128 * *
129 * Press ^C to abort. *
130 * *
131 ''', end='', file=sys.stderr)
132 warn_time_override = os.environ.get('FORCE_WARNING_TIME', None)
133 warn_time: int
134 if warn_time_override is None:
135 warn_time = 15
136 else:
137 warn_time = int(warn_time_override)
138 print(
139 '''* !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! *
140 * !! WARNING DISPLAY TIME OVERRIDDEN !! *
141 * !! !! *
142 * !! This message is intended to be displayed long enough for a !! *
143 * !! human operator to read it and have a chance to abort. An !! *
144 * !! override for the delay time is provided FOR THE UNIT TESTS !! *
145 * !! to avoid delaying software builds unnecessarily. This is !! *
146 * !! INTENDED FOR USE IN UNIT TESTS ONLY; THIS MESSAGE SHOULD !! *
147 * !! NEVER BE SEEN OUTSIDE BUILD LOGS! !! *
148 * !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! *
149 * *
150 ''', end='', file=sys.stderr)
151
152 for i in range(warn_time, 0, -1):
153 msg = '* %-70s *' % ("Continuing in %d seconds..." % i)
154 print(msg, file=sys.stderr)
155 time.sleep(1)
156 print('*' * 74, file=sys.stderr)
157
158
159
160 @backoff.on_exception(
161 backoff.expo,
162 subprocess.CalledProcessError,
163 max_time=lambda: int(os.environ.get('BACKOFF_MAX_TIME', '30')))
164 def _git_fetch(
165 cachedir: Path,
166 repo: Repo,
167 ref: Ref,
168 force: bool = False) -> None:
169 subprocess.run(['git', '-C', cachedir, 'fetch'] +
170 (['--force'] if force else []) +
171 [repo, '%s:%s' % (ref, ref)], check=True)
172
173
174 def fetch(repo: Repo, ref: Ref, force: bool = False) -> Tuple[Path, Rev]:
175 if force:
176 _show_force_warning()
177 cachedir = git_cachedir(repo)
178 if not os.path.exists(cachedir):
179 logging.debug("Initializing git repo")
180 subprocess.run(['git', 'init', '--bare', cachedir],
181 check=True, stdout=sys.stderr)
182
183 logging.debug('Fetching ref "%s" from %s', ref, repo)
184 _git_fetch(cachedir, repo, ref, force=force)
185
186 with open(os.path.join(cachedir, 'refs', 'heads', ref)) as rev_file:
187 rev = Rev(rev_file.read(999).strip())
188 verify_ancestry(repo, ref, rev, force=force)
189 _log_fetch(repo, ref, rev, force=force)
190
191 return cachedir, rev
192
193
194 def ensure_rev_available(
195 repo: Repo,
196 ref: Ref,
197 rev: Rev,
198 force: bool = False) -> Path:
199 cachedir = git_cachedir(repo)
200 if os.path.exists(cachedir) and is_ancestor(repo, ref, rev):
201 return cachedir
202
203 logging.debug(
204 'We do not have rev %s. We will fetch ref "%s" and hope it appears.',
205 rev, ref)
206 fetch(repo, ref, force=force)
207 logging.debug('Verifying that fetch retrieved rev %s', rev)
208 subprocess.run(['git', '-C', cachedir, 'cat-file', '-e', rev], check=True)
209 verify_ancestry(repo, ref, rev, force=force)
210
211 return cachedir
212
213
214 def _main() -> None:
215 parser = argparse.ArgumentParser(
216 description='Cache remote git repositories locally.',
217 epilog='example usage: git-cache https://github.com/NixOS/nixpkgs.git master')
218 parser.add_argument(
219 '--force',
220 action='store_true',
221 help='Recover from a force-push in the remote repo')
222 parser.add_argument(
223 'repo',
224 metavar='Repo',
225 type=Repo,
226 help='Git repository URL')
227 parser.add_argument(
228 'ref',
229 metavar='Ref',
230 type=Ref,
231 help='Ref (branch or tag) in the git repo')
232 parser.add_argument(
233 'rev',
234 metavar='Rev',
235 type=Rev,
236 nargs='?',
237 help='Ensure that this revision is present. ' +
238 'If this revision is already present locally, no network operations are performed.')
239 args = parser.parse_args()
240
241 if args.rev is None:
242 print('{1} {0}'.format(*fetch(args.repo, args.ref, force=args.force)))
243 else:
244 print(
245 ensure_rev_available(
246 args.repo,
247 args.ref,
248 args.rev,
249 force=args.force))