[libvirt] Querying block device job status and semantics of virDomainBlockJobInfo()

In OpenStack Nova, we're trying to analyze a potential race condition[0]. The operation flow is something like: perform a live shallow blockRebase(), check for progress with blockJobInfo(), followed by a blockJobAbort() (QMP 'block-job-cancel') to get a live point-in-time snapshot, then convert the resulting shallow copy with a backing file into a flattenned image with (`qemu-img convert`), and Nova uploads that into the image repository (Glance). Nova uses some wrappers around the above APIs, and calls them this way, in its _live_snapshot() method[1]: [...] dev.rebase(disk_delta, copy=True, reuse_ext=True, shallow=True) while dev.wait_for_job(): time.sleep(0.5) dev.abort_job() [...] Looking at a failed blockRebase() operation from libvirt debug log, the root cause turns out to be libvirt issuing QMP command 'block-job-cancel' (blockJobAbort()) when the block device job status is "ready": false, which results in a corrupted destination file. (In a successful case, QMP command 'block-job-cancel' should be issued when the job status is: "ready": true). My libvirtd log analysis in the Nova bug is here[2] from a failed case, which has QMP traffic back-n-forth. - - - So, I'm trying to understand how libvirt reports the "cur" and "end" values. I've read the virDomainBlockJobInfo() struct, it wasn't crystal clear. It states: /* * The following fields provide an indication of block job progress. @cur * indicates the current position and will be between 0 and @end. @end is * the final cursor position for this operation and represents completion. * To approximate progress, divide @cur by @end. */ Now, if a job hasn't started, what would libvirt report? Talking to Michal Privoznik on IRC: [mprivozn] I wonder if libvirt should report something else than start=0 end=0 in order to tell the caller that job hasn't been started yet. I suspect that libvirt does not report correctly whether job has started already, which in turns forces Nova to use some workarounds. [kashyap] So, if the job hasn't started yet, what should libvirt report? [mprivozn] That's the question. We can't change the [virDomainBlockJobInfo] struct (otherwise we won't be ABI compatible), so we can't really add a bolean there 'bool job_started'. Or we can introduce new "job type" which wouldn't really be a job type, but we will fill status.job with it to say explicitly job hasn't started yet So, any other thoughts here, on how to proceed here? - - - I realize that a copy job has two phases, as clearly stated in the API documenation of virDomainBlockRebase()[3]: "[...] The job transitions to the second phase when the job info states cur == end, and remains alive to mirror all further changes to both source and destination. The user must call virDomainBlockJobAbort() to end the mirroring while choosing whether to revert to source or pivot to the destination. [...]" [0] https://bugs.launchpad.net/nova/+bug/1530275 [1] https://github.com/openstack/nova/blob/master/nova/virt/libvirt/driver.py... [2] https://bugs.launchpad.net/nova/+bug/1530275/comments/16 -- Live snapshot is corrupted (possibly race condition?) [3] https://libvirt.org/html/libvirt-libvirt-domain.html#virDomainBlockRebase -- /kashyap