Comment 3 for bug 909569

 In swift/common/utils.py the function drop_privileges can be found.

(line 464):
def drop_privileges(user):
    """
    Sets the userid/groupid of the current process, get session leader, etc.

    :param user: User name to change privileges to
    """
    user = pwd.getpwnam(user)
    os.setgid(user[3])
    os.setuid(user[2])
    try:
        os.setsid()
    except OSError:
        pass
    os.chdir('/') # in case you need to rmdir on where you started the daemon
    os.umask(0) # ensure files are created with the correct privileges

<--- On linux without dropping the starting user's groups it is possible for the drop_privileges method to not properly drop all intended groups(and keep group id of the user).
The drop_privileges method appears to be used by swift/common/daemon.py (on a quick inspection). I will have more time to look at this tomorrow and will do so then.