tests/md5sum.py

similar: compare between actual file contents for exact identity

Before this patch, similarity detection logic (for addremove and
automv) depends entirely on SHA-1 digesting. But this causes incorrect
rename detection, if:

  - removing file A and adding file B occur at same committing, and
  - SHA-1 hash values of file A and B are same

This may prevent security experts from managing sample files for
SHAttered issue in Mercurial repository, for example.

  https://security.googleblog.com/2017/02/announcing-first-sha1-collision.html
  https://shattered.it/

Hash collision itself isn't so serious for core repository
functionality of Mercurial, described by mpm as below, though.

  https://www.mercurial-scm.org/wiki/mpm/SHA1

This patch compares between actual file contents after hash comparison
for exact identity.

Even after this patch, SHA-1 is still used, because it is reasonable
enough to quickly detect existence of "(almost) same" file.

  - replacing SHA-1 causes decreasing performance, and
  - replacement of it has ambiguity, yet

Getting content of removed file (= rfctx.data()) at each exact
comparison should be cheap enough, even though getting content of
added one costs much.

  ======= ============== =====================
  file    fctx           data() reads from
  ======= ============== =====================
  removed filectx        in-memory revlog data
  added   workingfilectx storage
  ======= ============== =====================

#!/usr/bin/env python
#
# Based on python's Tools/scripts/md5sum.py
#
# This software may be used and distributed according to the terms
# of the PYTHON SOFTWARE FOUNDATION LICENSE VERSION 2, which is
# GPL-compatible.

from __future__ import absolute_import

import os
import sys

try:
    import hashlib
    md5 = hashlib.md5
except ImportError:
    import md5
    md5 = md5.md5

try:
    import msvcrt
    msvcrt.setmode(sys.stdout.fileno(), os.O_BINARY)
    msvcrt.setmode(sys.stderr.fileno(), os.O_BINARY)
except ImportError:
    pass

for filename in sys.argv[1:]:
    try:
        fp = open(filename, 'rb')
    except IOError as msg:
        sys.stderr.write('%s: Can\'t open: %s\n' % (filename, msg))
        sys.exit(1)

    m = md5()
    try:
        for data in iter(lambda: fp.read(8192), ''):
            m.update(data)
    except IOError as msg:
        sys.stderr.write('%s: I/O error: %s\n' % (filename, msg))
        sys.exit(1)
    sys.stdout.write('%s  %s\n' % (m.hexdigest(), filename))

sys.exit(0)