Parse command line arguments more extensibly

[git-cache] / git_cache.py

diff --git a/git_cache.py b/git_cache.py
index 077406c63275d97d701e1e9498e1efca989045c1..d43df2624fd8e9d9bc92536a4617c21eeb69d249 100644 (file)
--- a/git_cache.py
+++ b/git_cache.py
@@ -4,23 +4,38 @@
 # implementation details for my comfort.  So we re-implement here half of
 # nix's builtins.fetchGit.  :(
 
+import argparse
+import functools
 import hashlib
 import logging
 import os
 import subprocess
 import sys
+import time
 
-from typing import Tuple, Union
+from typing import Iterator, NamedTuple, Optional, TypeVar, Tuple, Union
 
 import backoff
 
-Path = str  # eg: "/home/user/.cache/git-cache/v1"
-Repo = str  # eg: "https://github.com/NixOS/nixpkgs.git"
-Ref = str   # eg: "master" or "v1.0.0"
-Rev = str   # eg: "53a27350551844e1ed1a9257690294767389ef0d"
+Path = str   # eg: "/home/user/.cache/git-cache/v1"
+Repo = str   # eg: "https://github.com/NixOS/nixpkgs.git"
+Ref = str    # eg: "master" or "v1.0.0"
+Rev = str    # eg: "53a27350551844e1ed1a9257690294767389ef0d"
 RefOrRev = Union[Ref, Rev]
 
 
+class _LogEntry(NamedTuple):
+    ref: Ref
+    rev: Rev
+
+
+T = TypeVar('T')
+
+
+def _repo_hashname(repo: Repo) -> str:
+    return hashlib.sha256(repo.encode()).hexdigest()
+
+
 def git_cachedir(repo: Repo) -> Path:
     # Use xdg module when it's less painful to have as a dependency
     XDG_CACHE_HOME = Path(
@@ -29,7 +44,18 @@ def git_cachedir(repo: Repo) -> Path:
     return Path(os.path.join(
         XDG_CACHE_HOME,
         'git-cache/v1',
-        hashlib.sha256(repo.encode()).hexdigest()))
+        _repo_hashname(repo)))
+
+
+def _log_filename(repo: Repo) -> Path:
+    # Use xdg module when it's less painful to have as a dependency
+    XDG_DATA_HOME = Path(
+        os.environ.get('XDG_DATA_HOME', os.path.expanduser('~/.local/share')))
+
+    return Path(os.path.join(
+        XDG_DATA_HOME,
+        'git-cache/v1',
+        _repo_hashname(repo)))
 
 
 def is_ancestor(repo: Repo, descendant: RefOrRev, ancestor: RefOrRev) -> bool:
@@ -54,6 +80,36 @@ def verify_ancestry(
         raise Exception('%s is not an ancestor of %s' % (ancestor, descendant))
 
 
+def _read_fetch_log(repo: Repo) -> Iterator[_LogEntry]:
+    filename = _log_filename(repo)
+    if not os.path.exists(filename):
+        return
+    with open(filename, 'r') as f:
+        for line in f:
+            _, _, rev, ref = line.strip().split(maxsplit=3)
+            yield _LogEntry(ref, rev)
+
+
+def _last(it: Iterator[T]) -> Optional[T]:
+    return functools.reduce(lambda a, b: b, it, None)
+
+
+def _previous_fetched_rev(repo: Repo, ref: Ref) -> Optional[Rev]:
+    return _last(entry.rev for entry in _read_fetch_log(
+        repo) if entry.ref == ref)
+
+
+def _log_fetch(repo: Repo, ref: Ref, rev: Rev) -> None:
+    prev_rev = _previous_fetched_rev(repo, ref)
+    if prev_rev is not None:
+        verify_ancestry(repo, rev, prev_rev)
+    filename = _log_filename(repo)
+    os.makedirs(os.path.dirname(filename), exist_ok=True)
+    with open(filename, 'a') as f:
+        f.write('%s fetch %s %s\n' %
+                (time.strftime('%Y-%m%d-%H:%M:%S%z'), rev, ref))
+
+
 @backoff.on_exception(
     backoff.expo,
     subprocess.CalledProcessError,
@@ -78,6 +134,7 @@ def fetch(repo: Repo, ref: Ref) -> Tuple[Path, Rev]:
     with open(os.path.join(cachedir, 'refs', 'heads', ref)) as rev_file:
         rev = Rev(rev_file.read(999).strip())
     verify_ancestry(repo, ref, rev)
+    _log_fetch(repo, ref, rev)
 
     return cachedir, rev
 
@@ -99,13 +156,29 @@ def ensure_rev_available(repo: Repo, ref: Ref, rev: Rev) -> Path:
 
 
 def _main() -> None:
-    if len(sys.argv) == 3:
-        print('{1} {0}'.format(*fetch(Repo(sys.argv[1]), Ref(sys.argv[2]))))
-    elif len(sys.argv) == 4:
-        print(ensure_rev_available(
-            Repo(sys.argv[1]), Ref(sys.argv[2]), Rev(sys.argv[3])))
+    parser = argparse.ArgumentParser(
+        description='Cache remote git repositories locally.',
+        epilog='example usage: git-cache https://github.com/NixOS/nixpkgs.git master')
+    parser.add_argument(
+        'repo',
+        metavar='Repo',
+        type=Repo,
+        help='Git repository URL')
+    parser.add_argument(
+        'ref',
+        metavar='Ref',
+        type=Ref,
+        help='Ref (branch or tag) in the git repo')
+    parser.add_argument(
+        'rev',
+        metavar='Rev',
+        type=Rev,
+        nargs='?',
+        help='Ensure that this revision is present.  ' +
+        'If this revision is already present locally, no network operations are performed.')
+    args = parser.parse_args()
+
+    if args.rev is None:
+        print('{1} {0}'.format(*fetch(args.repo, args.ref)))
     else:
-        usage = '''usage: git-cache repo ref [rev]
-example: git-cache https://github.com/NixOS/nixpkgs.git master'''
-        print(usage, file=sys.stderr)
-        sys.exit(1)
+        print(ensure_rev_available(args.repo, args.ref, args.rev))