Change the current process to belong to the specified user (and the groups
    to which it belongs)

def chugid(runas, group=None):
    '''
    Change the current process to belong to the specified user (and the groups
    to which it belongs)
    '''
    uinfo = pwd.getpwnam(runas)
    supgroups = []
    supgroups_seen = set()

    if group:
        try:
            target_pw_gid = grp.getgrnam(group).gr_gid
        except KeyError as err:
            raise CommandExecutionError(
                'Failed to fetch the GID for {0}. Error: {1}'.format(
                    group, err
                )
            )
    else:
        target_pw_gid = uinfo.pw_gid

    # The line below used to exclude the current user's primary gid.
    # However, when root belongs to more than one group
    # this causes root's primary group of '0' to be dropped from
    # his grouplist.  On FreeBSD, at least, this makes some
    # command executions fail with 'access denied'.
    #
    # The Python documentation says that os.setgroups sets only
    # the supplemental groups for a running process.  On FreeBSD
    # this does not appear to be strictly true.
    group_list = get_group_dict(runas, include_default=True)
    if sys.platform == 'darwin':
        group_list = dict((k, v) for k, v in six.iteritems(group_list)
                          if not k.startswith('_'))
    for group_name in group_list:
        gid = group_list[group_name]
        if (gid not in supgroups_seen
           and not supgroups_seen.add(gid)):
            supgroups.append(gid)

    if os.getgid() != target_pw_gid:
        try:
            os.setgid(target_pw_gid)
        except OSError as err:
            raise CommandExecutionError(
                'Failed to change from gid {0} to {1}. Error: {2}'.format(
                    os.getgid(), target_pw_gid, err
                )
            )

    # Set supplemental groups
    if sorted(os.getgroups()) != sorted(supgroups):
        try:
            os.setgroups(supgroups)
        except OSError as err:
            raise CommandExecutionError(
                'Failed to set supplemental groups to {0}. Error: {1}'.format(
                    supgroups, err
                )
            )

    if os.getuid() != uinfo.pw_uid:
        try:
            os.setuid(uinfo.pw_uid)
        except OSError as err:
            raise CommandExecutionError(
                'Failed to change from uid {0} to {1}. Error: {2}'.format(
                    os.getuid(), uinfo.pw_uid, err
                )
            )