Implement attribute consistency check for LFS files

pre_commit_hooks/check_lfs_attributes.py

@@ -0,0 +1,69 @@
+from __future__ import annotations
+
+import argparse
+import subprocess
+from typing import Sequence
+
+from pre_commit_hooks.util import zsplit
+
+
+def select_lfs_attr_files(filenames: set[str]) -> set[str]:  # pragma: no cover (lfs)
+    """Select files tracked by git-lfs from the set."""
+    if not filenames:
+        return filenames
+
+    check_attr = subprocess.run(
+        ('git', 'check-attr', 'filter', '-z', '--stdin'),
+        stdout=subprocess.PIPE,
+        stderr=subprocess.DEVNULL,
+        encoding='utf-8',
+        check=True,
+        input='\0'.join(filenames),
+    )
+    stdout = zsplit(check_attr.stdout)
+    # stdout values are triplets:
+    #   ['b.txt', 'filter', 'unspecified', 'a.bin', 'filter', 'lfs']
+    return {stdout[i] for i in range(0, len(stdout), 3) if stdout[i + 2] == 'lfs'}
+
+
+def select_lfs_tree_files(filenames: set[str]) -> set[str]:  # pragma: no cover
+    """Select LSF files found in the tree."""
+    if not filenames:
+        return filenames
+
+    output = subprocess.check_output(('git', 'lfs', 'ls-files', '-n'), text=True)
+    lfs_files = set(output.split())
+
+    return lfs_files & set(filenames)
+
+
+def check_lfs_attributes(filenames: Sequence[str]) -> int:
+    unique_filenames = set(filenames)
+
+    lfs_attr_files = select_lfs_attr_files(unique_filenames)
+    lfs_tree_files = select_lfs_tree_files(unique_filenames)
+
+    retv = 0
+    for filename in lfs_attr_files - lfs_tree_files:
+        print(f'{filename} is tracked by LFS but added as a regular object')
+        retv = 1
+    for filename in lfs_tree_files - lfs_attr_files:
+        print(f'{filename} is added as LFS object but not tracked')
+        retv = 1
+
+    return retv
+
+
+def main(argv: Sequence[str] | None = None) -> int:
+    parser = argparse.ArgumentParser()
+    parser.add_argument(
+        'filenames', nargs='*',
+        help='Filenames pre-commit believes are changed.',
+    )
+    args = parser.parse_args(argv)
+
+    return check_lfs_attributes(args.filenames)
+
+
+if __name__ == '__main__':
+    raise SystemExit(main())