[ldns-users] contrib/python: memory leak in ldns_pkt.new_query

[Johannes Naab]

Hi,

The python wrapper for ldns appears to leak memory when packets are
created by ldns_pkt.new_query.

The following is a short example (tested in ldns-1.6.16, Python 2.7.5
Linux 3.10, amd64):
#!/usr/bin/env python2.7
import ldns

dname = ldns.ldns_dname("test.nic.cz")

def leak():
    pkt = ldns.ldns_pkt.new_query(dname, ldns.LDNS_RR_TYPE_A,
ldns.LDNS_RR_CLASS_IN, 0)

while True:
    leak()


Root of the problem seems to be, that the pointer to the ldns_pkt struct
is not considered owned by the swig wrapper, and thus the pkt struct is
not freed upon garbage collection in python.

import ldns
dname = ldns.ldns_dname("test.nic.cz")
pkt = ldns.ldns_pkt.new_query(dname, ldns.LDNS_RR_TYPE_A,
                              ldns.LDNS_RR_CLASS_IN, 0)
print pkt.thisown

=> False.

If new_query is replaced by new_query_frm_str the packet is "owned" by
the swig wrapper, and the packet does not leak.


pkt = ldns.ldns_pkt.new_query_frm_str(str(dname), ldns.LDNS_RR_TYPE_A,
                                      ldns.LDNS_RR_CLASS_IN, 0)
print pkt.thisown

=> True

I tried to convince swig that the struct should be owned by the wrapper
by inserting a "%newobject ldns_pkt_query_new;" below line 44 in
contrib/python/ldns_packet.i. This made swig own the pointer, but
freeing the packet on garbage collection made python crash (memory
corruption, double free). My knowledge and understanding of swig is limited.


Johannes