Another way to reproduce this crash

1) prepare a dir pool  named default
# virsh pool-list
  Name                 State      Autostart
-------------------------------------------
  default              active     yes
# virsh pool-dumpxml default
<pool type='dir'>
   <name>default</name>
   <uuid>fd9db86b-ab31-44fe-02b2-9ccdf1a2c52a</uuid>
   <capacity unit='bytes'>42141450240</capacity>
   <allocation unit='bytes'>7179096064</allocation>
   <available unit='bytes'>34962354176</available>
   <source>
   </source>
   <target>
     <path>/var/lib/libvirt/images</path>
     <permissions>
       <mode>0755</mode>
       <owner>-1</owner>
       <group>-1</group>
     </permissions>
   </target>
</pool>

2) Create a volume with a gluster+tcp or gluster+rmda as backing file
# qemu-img info gluster+rdma://10.66.6.111/gluster-vol1/rhel65.img
image: gluster+rdma://10.66.6.111/gluster-vol1/rhel65.img
file format: raw
virtual size: 8.0G (8589934592 bytes)
disk size: 8.0G

# qemu-img create -f qcow2 /var/lib/libvirt/images/test.img -b
gluster+rdma://10.66.6.111/gluster-vol1/rhel65.img -o backing_fmt=raw
Formatting '/var/lib/libvirt/images/test.img', fmt=qcow2 size=8589934592
backing_file='gluster+rdma://10.66.6.111/gluster-vol1/rhel65.img'
backing_fmt='raw' encryption=off cluster_size=65536 lazy_refcounts=off

3) Refresh the default pool will lead libvirtd crash

4) same story with "gluster+tcp"

Remember to delete the volume you created in step 2) or 4), otherwise
you can't start libvirtd daemon anymore.

Fixed upstream. The problem will happen with any file that has a backing file containing a transport protocol as a part of the URI schema.

commit 98784369fd52ed6aa9bab2a9a9d213c52019e6ee
Author: Peter Krempa <pkrempa>
Date:   Wed Oct 29 10:55:23 2014 +0100

    storage: Fix crash when parsing backing store URI with schema
    
    The code that parses the schema from the URI touches the "hosts[0]"
    member of the storage file source structure in case the URI contains a
    schema. The hosts array was not yet allocated at the point in the code
    where the transport protocol was parsed and set. This lead to a crash of
    libvirtd.
    
    Fix the code by allocating the "hosts" array upfront and add a test case
    to verify this scenario. (Unfortunately this requires shuffling the test
    case numbers too).

Transport='tcp' is the default thus it's omitted from the XML. If you use RDMA for the backing file it should be recorded in the XML.

Hi,

What I want to ask is: 
from libvirt side, Is "gluster+tcp://ip/gluster-volume/file" and "gluster://ip/gluster-volume/file" different if they show in backing chains.

When rh6.s1's backing file is "gluster+tcp://ip/gluster-volume/file", libvirt will discard the backing file; when rh6.s1's backing file is "gluster://ip/gluster-volume/file", libvirt will recognise it.

Reproduce this with libvirt-1.2.8-5.el7.x86_64, verify with libvirt-1.2.8-6.el7.x86_64

Step as comment 0

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://rhn.redhat.com/errata/RHSA-2015-0323.html