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[git-cache] / git_cache.py
1 # git-cache: Cache git content locally
2 #
3 # This program is free software: you can redistribute it and/or modify it
4 # under the terms of the GNU General Public License as published by the
5 # Free Software Foundation, version 3.
6
7
8 # It would be nice if we could share the nix git cache, but as of the
9 # time of writing it is transitioning from gitv2 (deprecated) to gitv3
10 # (not ready yet), and trying to straddle them both is too far into nix
11 # implementation details for my comfort. So we re-implement here half of
12 # nix's builtins.fetchGit. :(
13
14 import argparse
15 import functools
16 import hashlib
17 import logging
18 import os
19 import subprocess
20 import sys
21 import time
22
23 from typing import Iterator, NamedTuple, Optional, TypeVar, Tuple, Union
24
25 import backoff
26
27
28 class GitCacheError(Exception):
29 pass
30
31
32 Path = str # eg: "/home/user/.cache/git-cache/v1"
33 Repo = str # eg: "https://github.com/NixOS/nixpkgs.git"
34 Ref = str # eg: "master" or "v1.0.0"
35 Rev = str # eg: "53a27350551844e1ed1a9257690294767389ef0d"
36 RefOrRev = Union[Ref, Rev]
37
38
39 class _LogEntry(NamedTuple):
40 ref: Ref
41 rev: Rev
42
43
44 T = TypeVar('T')
45
46
47 def _repo_hashname(repo: Repo) -> str:
48 return hashlib.sha256(repo.encode()).hexdigest()
49
50
51 def git_cachedir(repo: Repo) -> Path:
52 # Use xdg module when it's less painful to have as a dependency
53 XDG_CACHE_HOME = Path(
54 os.environ.get('XDG_CACHE_HOME', os.path.expanduser('~/.cache')))
55
56 return Path(os.path.join(
57 XDG_CACHE_HOME,
58 'git-cache/v1',
59 _repo_hashname(repo)))
60
61
62 def _log_filename(repo: Repo) -> Path:
63 # Use xdg module when it's less painful to have as a dependency
64 XDG_DATA_HOME = Path(
65 os.environ.get('XDG_DATA_HOME', os.path.expanduser('~/.local/share')))
66
67 return Path(os.path.join(
68 XDG_DATA_HOME,
69 'git-cache/v1',
70 _repo_hashname(repo)))
71
72
73 def is_ancestor(repo: Repo, descendant: RefOrRev, ancestor: RefOrRev) -> bool:
74 cachedir = git_cachedir(repo)
75 logging.debug('Checking if %s is an ancestor of %s', ancestor, descendant)
76 process = subprocess.run(['git',
77 '-C',
78 cachedir,
79 'merge-base',
80 '--is-ancestor',
81 ancestor,
82 descendant.removeprefix('tag ')],
83 check=False)
84 return process.returncode == 0
85
86
87 def verify_ancestry(
88 repo: Repo,
89 descendant: RefOrRev,
90 ancestor: RefOrRev,
91 force: bool = False) -> None:
92 if not force and not is_ancestor(repo, descendant, ancestor):
93 raise GitCacheError(f'{ancestor} is not an ancestor of {descendant}')
94
95
96 def _read_fetch_log(repo: Repo) -> Iterator[_LogEntry]:
97 filename = _log_filename(repo)
98 if not os.path.exists(filename):
99 return
100 with open(filename, 'r', encoding='utf-8') as f:
101 for line in f:
102 _, _, rev, ref = line.strip().split(maxsplit=3)
103 yield _LogEntry(ref, rev)
104
105
106 def _last(it: Iterator[T]) -> Optional[T]:
107 return functools.reduce(lambda a, b: b, it, None)
108
109
110 def _previous_fetched_rev(repo: Repo, ref: Ref) -> Optional[Rev]:
111 return _last(entry.rev for entry in _read_fetch_log(
112 repo) if entry.ref == ref)
113
114
115 def _log_fetch(repo: Repo, ref: Ref, rev: Rev, force: bool = False) -> None:
116 if not force:
117 prev_rev = _previous_fetched_rev(repo, ref)
118 if prev_rev is not None:
119 verify_ancestry(repo, rev, prev_rev)
120 filename = _log_filename(repo)
121 os.makedirs(os.path.dirname(filename), exist_ok=True)
122 with open(filename, 'a', encoding='utf-8') as f:
123 f.write(
124 f'{time.strftime("%Y-%m%d-%H:%M:%S%z")} '
125 f'{"FORCEDFETCH" if force else "fetch"} {rev} {ref}\n'
126 )
127
128
129 def _show_force_warning() -> None:
130 print('''
131 **************************************************************************
132 * WARNING: git-cache INVOKED WITH --force! *
133 * *
134 * This mode allows previously-fetched refs to be overwritten to point to *
135 * non-descendants -- commits that don't have the previous version of the *
136 * the ref in their history! *
137 * *
138 * This should only be invoked by a human operator who knows what they're *
139 * doing to correct a specific, known, problem. Care should be taken to *
140 * prevent recurrence. *
141 * *
142 * Press ^C to abort. *
143 * *
144 ''', end='', file=sys.stderr)
145 warn_time_override = os.environ.get('FORCE_WARNING_TIME', None)
146 warn_time: int
147 if warn_time_override is None:
148 warn_time = 15
149 else:
150 warn_time = int(warn_time_override)
151 print(
152 '''* !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! *
153 * !! WARNING DISPLAY TIME OVERRIDDEN !! *
154 * !! !! *
155 * !! This message is intended to be displayed long enough for a !! *
156 * !! human operator to read it and have a chance to abort. An !! *
157 * !! override for the delay time is provided FOR THE UNIT TESTS !! *
158 * !! to avoid delaying software builds unnecessarily. This is !! *
159 * !! INTENDED FOR USE IN UNIT TESTS ONLY; THIS MESSAGE SHOULD !! *
160 * !! NEVER BE SEEN OUTSIDE BUILD LOGS! !! *
161 * !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! *
162 * *
163 ''', end='', file=sys.stderr)
164
165 for i in range(warn_time, 0, -1):
166 msg = f'* {f"Continuing in {i} seconds...":-70s} *'
167 print(msg, file=sys.stderr)
168 time.sleep(1)
169 print('*' * 74, file=sys.stderr)
170
171
172 @backoff.on_exception(
173 backoff.expo,
174 subprocess.CalledProcessError,
175 max_time=lambda: int(os.environ.get('BACKOFF_MAX_TIME', '30')))
176 def _git_fetch(
177 cachedir: Path,
178 repo: Repo,
179 ref: Ref,
180 force: bool = False) -> None:
181 refargs = (['tag', ref.removeprefix('tag ')]
182 if ref.startswith('tag ')
183 else [f'{ref}:{ref}'])
184 subprocess.run(['git', '-C', cachedir, 'fetch'] +
185 (['--force'] if force else []) +
186 [repo] + refargs, check=True)
187
188
189 def fetch(repo: Repo, ref: Ref, force: bool = False) -> Tuple[Path, Rev]:
190 if force:
191 _show_force_warning()
192 cachedir = git_cachedir(repo)
193 if not os.path.exists(cachedir):
194 logging.debug("Initializing git repo")
195 subprocess.run(['git',
196 '-c',
197 'init.defaultBranch=git-cache--no-default-branch',
198 'init',
199 '--bare',
200 cachedir],
201 check=True,
202 stdout=sys.stderr)
203
204 logging.debug('Fetching ref "%s" from %s', ref, repo)
205 _git_fetch(cachedir, repo, ref, force=force)
206
207 rev_path = (['tags', ref.removeprefix('tag ')]
208 if ref.startswith('tag ')
209 else ['heads', ref])
210 with open(os.path.join(cachedir, 'refs', *rev_path), encoding='utf-8') as rev_file:
211 rev = Rev(rev_file.read(999).strip())
212 verify_ancestry(repo, ref, rev, force=force)
213 _log_fetch(repo, ref, rev, force=force)
214
215 return cachedir, rev
216
217
218 def ensure_rev_available(
219 repo: Repo,
220 ref: Ref,
221 rev: Rev,
222 force: bool = False) -> Path:
223 cachedir = git_cachedir(repo)
224 if os.path.exists(cachedir) and is_ancestor(repo, ref, rev):
225 return cachedir
226
227 logging.debug(
228 'We do not have rev %s. We will fetch ref "%s" and hope it appears.',
229 rev, ref)
230 fetch(repo, ref, force=force)
231 logging.debug('Verifying that fetch retrieved rev %s', rev)
232 subprocess.run(['git', '-C', cachedir, 'cat-file', '-e', rev], check=True)
233 verify_ancestry(repo, ref, rev, force=force)
234
235 return cachedir
236
237
238 def _main() -> None:
239 parser = argparse.ArgumentParser(
240 description='Cache remote git repositories locally.',
241 epilog='example usage: git-cache https://github.com/NixOS/nixpkgs.git master')
242 parser.add_argument(
243 '--force',
244 action='store_true',
245 help='Recover from a force-push in the remote repo')
246 parser.add_argument(
247 'repo',
248 metavar='Repo',
249 type=Repo,
250 help='Git repository URL')
251 parser.add_argument(
252 'ref',
253 metavar='Ref',
254 type=Ref,
255 help='Ref (branch or tag) in the git repo')
256 parser.add_argument(
257 'rev',
258 metavar='Rev',
259 type=Rev,
260 nargs='?',
261 help='Ensure that this revision is present. ' +
262 'If this revision is already present locally, no network operations are performed.')
263 args = parser.parse_args()
264
265 if args.rev is None:
266 cachedir, rev = fetch(args.repo, args.ref, force=args.force)
267 print(f'{rev} {cachedir}')
268 else:
269 print(
270 ensure_rev_available(
271 args.repo,
272 args.ref,
273 args.rev,
274 force=args.force))