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[libvirt] [PATCH v3] snapshot: fix rollback failure in transaction mode

when qemu supports the 'transaction' monitor command,
and libvirt's --reuse-ext flag was not specified, libvirt created
a stub file with zero size in first place. After the failure of
QEMU transaction command performing qcow2 snapshots on more than
one drives, the stub file is left behind with non-empty
by the QEMU transaction command.
In order to unlink the file, the patch removes the file size checking.

Steps to reproduce the issue:
 1, Create a qemu instance with two drive images of qcow2 type (root user)
    /usr/libexec/qemu-kvm -m 1024 -smp 1 -name "rhel6u1" \
      -drive file=/var/lib/libvirt/images/firstqcow2,if=none,id=drive-virtio-disk0,format=qcow2,cache=none
      -device virtio-blk-pci,scsi=off,bus=pci.0,addr=0x5,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1 \
      -drive file=/var/lib/libvirt/images/secondqcow2,if=none,id=drive-virtio-disk1,format=qcow2,cache=none \
      -device virtio-blk-pci,scsi=off,bus=pci.0,addr=0x7,drive=drive-virtio-disk1,id=virtio-disk1 -qmp stdio

 2, Initialize qemu qmp

 3, Remove the second drive image file
    rm -f /var/lib/libvirt/images/secondqcow2

 4, Run 'transaction' command with snapshot qemu commands in.

 5, Got the error as follows:
    {"id": "libvirt-6",
      "error": {"class": "OpenFileFailed", "desc": "Could not open '/var/lib/libvirt/images/secondqcow2-snapshot.img'",
                "data": {"filename": "/var/lib/libvirt/images/secondqcow2-snapshot.img"}

 6, List first newly-created snapshot file:
    -rw-r--r--. 1 root root     262144 Sep 13 11:43 firstqcow2-snapshot.img
 src/qemu/qemu_driver.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/qemu/qemu_driver.c b/src/qemu/qemu_driver.c
index a410521..d11bd19 100644
--- a/src/qemu/qemu_driver.c
+++ b/src/qemu/qemu_driver.c
@@ -10833,7 +10833,7 @@ qemuDomainSnapshotUndoSingleDiskActive(struct qemud_driver *driver,
     if (virDomainLockDiskDetach(driver->lockManager, vm, disk) < 0)
         VIR_WARN("Unable to release lock on %s", disk->src);
     if (need_unlink && stat(disk->src, &st) == 0 &&
-        st.st_size == 0 && S_ISREG(st.st_mode) && unlink(disk->src) < 0)
+        S_ISREG(st.st_mode) && unlink(disk->src) < 0)
         VIR_WARN("Unable to remove just-created %s", disk->src);
     /* Update vm in place to match changes.  */

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