I hit a bug with libvirt+ qemu+kvm when doing follow steps:

1.initial status :vm = base+z(z is a snapshot)

2. call snapshotCreateXML( ): vm =  base+z +z'

3. call blockRebase( ): vm = base+z''(z rebase to z')

4 .(after rebasing work done) vm shutdown and then start

an error accord like this:

I debugged with gdb and found that the error message was cauesed by 

virSecurityDACSetSecurityDiskLabel misuse the disk chain(that is base+z +z', 

after rebase), and z' was deleted yet.

why virSecurityDACSetSecurityDiskLabel  used old disk chain after rebasing ? 

I finally find that In qemuProcessHandleBlockJob when rebase complete, disk chain in vm->def is updated but vm->newDef is not(when vm->newDef is not null). In shutdown( actrully in qemuProcessStop ), if vm->newDef is not null , vm->def will be replaced  by vm->newDef, which contains a wrong disk chain!!! the next start will failed with above message.