Comment 14 for bug 937352

I've just uploaded what we believe/hope will be a fix to this issue.
After much discussion, we decided to add a 'udevadm settle' prior to invoking 'growpart'.

The suspicion is that in the initial flurry of device activation, one of the udev spawned events still had an open filehandle on the block device (/dev/vda in this case) when sfdisk made the BLKRRPART call.

Unfortunately we did not definitively prove this, or even catch a 'ps' output where there was a failure. The one thing I fear here is that udevadm settle is quite likely to add enough delay to hide the issue if it is not the real solution.