drive_mirror crashes on full disk copy of a resized disk with a backing file

This bug was discovered using libvirt on ubuntu with a build of qemu 1.3 but it is trivailly reproducible with the curent git version.

Repro steps:

qemu-img create -f qcow2 base 32M
qemu-img create -f qcow2 -o backing_file=base disk
qemu-img resize /home/vishvananda/disk 64M
qemu-system-x86_64 -drive file=disk,id=vda -vnc :1 -monitor stdio
QEMU 1.3.0 monitor - type 'help' for more information
(qemu) drive_mirror -f vda test
Formatting 'test', fmt=qcow2 size=67108864 encryption=off cluster_size=65536 lazy_refcounts=off 
qemu-system-x86_64: /build/buildd/qemu-1.3.0+dfsg/block/mirror.c:129: mirror_run: Assertion `n > 0' failed.
Aborted

Note that the command works just fine if the front image is not resized:

qemu-img create -f qcow2 base 32M
qemu-img create -f qcow2 -o backing_file=base disk
qemu-system-x86_64 -drive file=disk,id=vda -vnc :1 -monitor stdio

or if the backing file is resized as well:

qemu-img create -f qcow2 base 32M
qemu-img create -f qcow2 -o backing_file=base disk
qemu-img resize /home/vishvananda/disk 64M
qemu-img resize /home/vishvananda/base 64M
qemu-system-x86_64 -drive file=disk,id=vda -vnc :1 -monitor stdio

or if we don't use -f when creating the mirror:

QEMU 1.3.0 monitor - type 'help' for more information
(qemu) drive_mirror vda test
Formatting 'test', fmt=qcow2 size=33554432 backing_file='base' backing_fmt='qcow2' encryption=off cluster_size=65536 lazy_refcounts=off 

although in this final case the mirror is created the same size as the backing file which seems wrong:

qemu-img info test
image: test
file format: qcow2
virtual size: 32M (33554432 bytes)
disk size: 196K
cluster_size: 65536
backing file: base
backing file format: qcow2

separated the final size issue into a separate bug here:

https://bugs.launchpad.net/qemu/+bug/1103903

Haven't found a good workaround for this. Best I've come up with is to use the workaround described in the other bug and then coalesce the files afterwards via qemu-img convert



Reformatted patch and submitted upstream:

http://lists.gnu.org/archive/html/qemu-devel/2013-01/msg04585.html

Patch has been included here:
http://git.qemu.org/?p=qemu.git;a=commitdiff;h=63ba17d39f1a8d262b31e
... so I think it's OK to close this bug now.