Generate digest of a manifest

get_digest.py

#!/usr/bin/python

"""
This script takes at least one argument: path to a file with manifest.
It then computes and prints digest of the manifest(s).

The main reason this is not done properly, by decoding json, removing `signatures` and
encoding again, is that docker messes up encoding of strings and makes it impossible
to do right. For more info see:

    https://github.com/docker/distribution/issues/1065
"""

import os
import re
import sys
import hashlib


def prepare_file(fd):
    """
    prepare manifest for generating digest

    fd: file object returned by open
    """
    # 0 start -> signatures
    # 1 signatures
    # 2 after signatures
    states = 0
    sig_re = r'(\s+)\"signatures\":\s*\['
    stop = None

    ls = []

    for line in fd.readlines():
        if line == stop:
            states = 2
        if states == 1:
            continue
        m = re.match(sig_re, line)
        if m is not None:
            states = 1
            ls = ls[:-1]
            stop = m.groups()[0] + "]\n"
            continue
        else:
            ls.append(line)
    return "".join(ls)


def compute_digest(manifest):
    """
    manifest: str
    """
    return hashlib.sha256(manifest).hexdigest()


def main():
    if len(sys.argv[1:]) <= 0:
        print "Provide please at least one path to a file with manifest."
        sys.exit(1)
    result = []
    for fp in sys.argv[1:]:
        p = os.path.abspath(os.path.expanduser(fp))
        with open(p, "r") as fd:
            content = prepare_file(fd)
            digest = compute_digest(content)
            result.append((fp, digest))
    if len(result) == 1:
        print result[0][1]
    else:
        for r in result:
            print "{} {}".format(*r)
    sys.exit(0)


if __name__ == "__main__":
    main()