treehash512.py

#!/usr/bin/env python3
# Copyright (c) 2017 The Bitcoin Core developers
# Distributed under the MIT software license, see the accompanying
# file COPYING or http://www.opensource.org/licenses/mit-license.php.
'''
This script will show the SHA512 tree has for a certain commit, or HEAD
by default.

Usage:

    treehash512.py [<commithash>]
'''
import os
from sys import stdin,stdout,stderr
import sys
import argparse
import hashlib
import subprocess

# External tools (can be overridden using environment)
GIT = os.getenv('GIT','git')

def tree_sha512sum(commit='HEAD'):
    # request metadata for entire tree, recursively
    files = []
    blob_by_name = {}
    for line in subprocess.check_output([GIT, 'ls-tree', '--full-tree', '-r', commit]).splitlines():
        name_sep = line.index(b'\t')
        metadata = line[:name_sep].split() # perms, 'blob', blobid
        assert(metadata[1] == b'blob')
        name = line[name_sep+1:]
        files.append(name)
        blob_by_name[name] = metadata[2]

    files.sort()
    # open connection to git-cat-file in batch mode to request data for all blobs
    # this is much faster than launching it per file
    p = subprocess.Popen([GIT, 'cat-file', '--batch'], stdout=subprocess.PIPE, stdin=subprocess.PIPE)
    overall = hashlib.sha512()
    for f in files:
        blob = blob_by_name[f]
        # request blob
        p.stdin.write(blob + b'\n')
        p.stdin.flush()
        # read header: blob, "blob", size
        reply = p.stdout.readline().split()
        assert(reply[0] == blob and reply[1] == b'blob')
        size = int(reply[2])
        # hash the blob data
        intern = hashlib.sha512()
        ptr = 0
        while ptr < size:
            bs = min(65536, size - ptr)
            piece = p.stdout.read(bs)
            if len(piece) == bs:
                intern.update(piece)
            else:
                raise IOError('Premature EOF reading git cat-file output')
            ptr += bs
        dig = intern.hexdigest()
        assert(p.stdout.read(1) == b'\n') # ignore LF that follows blob data
        # update overall hash with file hash
        overall.update(dig.encode("utf-8"))
        overall.update("  ".encode("utf-8"))
        overall.update(f)
        overall.update("\n".encode("utf-8"))
    p.stdin.close()
    if p.wait():
        raise IOError('Non-zero return value executing git cat-file')
    return overall.hexdigest()

def main():
    if len(sys.argv)>1:
        commit = sys.argv[1]
    else:
        commit = 'HEAD'
    print(tree_sha512sum(commit))

if __name__ == '__main__':
    main()