[server] avoid a possible race condition on _CnxSetPool.close()
The pool could become empty between time to check and time to use.
from __future__ import print_function
from logilab.common.shellutils import generate_password
from cubicweb.server.utils import crypt_password
for user in rql('CWUser U WHERE U cw_source S, S name "system", U upassword P, U login L').entities():
salt = user.upassword.getvalue()
if crypt_password('', salt) == salt:
passwd = generate_password()
print('setting random password for user %s' % user.login)
user.set_attributes(upassword=passwd)
commit()