attempt to undo qom-set only if created (#26)

if, for some reason, setting up the cbw device fails, attempting to
revert the blockdev replace operation the qemu process might cause the
qemu process to stall out. Attempt to undo the blockdev replace
operation only if it has been executed prior.

Author: abbbi

libqmpbackup/qmpcommon.py

@@ -214,6 +214,8 @@ async def blockdev_replace(self, devices, action):
                 },
             )
 
+        return True
+
     async def add_cbw_device(self, argv, devices, uuid):
         """Add copy-before-write device operation"""
         self.log.info("Adding cbw devices to virtual machine")

qmpbackup

@@ -460,11 +460,12 @@ async def main():
             if qga:
                 fs.quiesce(qga)
 
+        cbw_active = False
         try:
             await qemu_client.prepare_target_devices(argv, blockdev, targetfiles)
             await qemu_client.prepare_fleece_devices(blockdev, fleecefiles)
             await qemu_client.add_cbw_device(argv, blockdev, uuid)
-            await qemu_client.blockdev_replace(blockdev, action="enable")
+            cbw_active = await qemu_client.blockdev_replace(blockdev, action="enable")
             await qemu_client.add_snapshot_access_devices(blockdev)
             await qemu_client.backup(argv, blockdev, qga, uuid)
         except (QMPError, RuntimeError) as errmsg:
@@ -481,7 +482,8 @@ async def main():
             except QMPError as errmsg:
                 log.warning("Unable to cleanup snapshot device: %s", errmsg)
             try:
-                await qemu_client.blockdev_replace(blockdev, action="disable")
+                if cbw_active is True:
+                    await qemu_client.blockdev_replace(blockdev, action="disable")
             except QMPError as errmsg:
                 log.warning("Unable to execute blockdev replace: %s", errmsg)
             try: