Comment 8 for bug 1627134

Looking at the log...

Here we can the libvirt blockRebase() (QEMU 'block-stream') job has started:

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Sep 20 15:10:39 localhost libvirtd: 85852: info : qemuMonitorSend:1007 : QEMU_MONITOR_SEND_MSG: mon=0x7f64f8010bc0 msg={"execute":"block-stream","arguments":{"device":"drive-virtio-disk1","base":"/opt/stack/data/nova/mnt/530180c6e38955fc032f76c2c0b8b443/volume-0a1fd4cf-643b-4077-943d-69a6647ccf9b.8ffec8b3-fba4-4b41-969f-640dc7ea792a","backing-file":"volume-0a1fd4cf-643b-4077-943d-69a6647ccf9b.8ffec8b3-fba4-4b41-969f-640dc7ea792a"},"id":"libvirt-109"}#015#012 fd=-1
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libvirt queries QEMU for block job status:

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Sep 20 15:10:39 localhost libvirtd: 85849: info : qemuMonitorIOWrite:531 : QEMU_MONITOR_IO_WRITE: mon=0x7f64f8010bc0 buf={"execute":"query-block-jobs","id":"libvirt-110"}#015#012 len=51 ret=51 errno=11
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QEMU responds, saying that it is busy (and you can see, that len != offset; so it is still copying):

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Sep 20 15:10:39 localhost libvirtd: 85849: info : qemuMonitorJSONIOProcessLine:209 : QEMU_MONITOR_RECV_REPLY: mon=0x7f64f8010bc0 reply={"return": [{"io-status": "ok", "device": "drive-virtio-disk1", "busy": false, "len": 1073741824, "offset": 613941248, "paused": false, "speed": 0, "ready": false, "type": "stream"}], "id": "libvirt-110"}
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Shortly after that, we can see that the "len == offset", meaning copy job (type "stream") is finished, and QEMU has emitted the event BLOCK_JOB_COMPLETED:

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Sep 20 15:10:39 localhost libvirtd: 85849: info : qemuMonitorEmitEvent:1272 : mon=0x7f64f8010bc0 event=BLOCK_JOB_COMPLETED data={"device":"drive-virtio-disk1","len":1073741824,"offset":1073741824,"speed":0,"type":"stream"}
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Now libvirt queries QEMU for job status (line-148) once again:

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Sep 20 15:10:39 localhost libvirtd: 85854: info : qemuMonitorSend:1007 : QEMU_MONITOR_SEND_MSG: mon=0x7f64f8010bc0 msg={"execute":"query-block-jobs","id":"libvirt-111"}#015#012 fd=-1
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Response (notice the ID 'libvirt-111' matches the above query), is an empty dict:

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Sep 20 15:10:39 localhost libvirtd: 85849: info : qemuMonitorIOProcess:426 : QEMU_MONITOR_IO_PROCESS: mon=0x7f64f8010bc0 buf={"return": [], "id": "libvirt-111"}#015#012 len=37
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The above response from QEMU means: no jobs are active; so libvirt should _not_ be reporting a job status, with current libvirt versions.

Upstream libvirt says that maybe future libvirt could track that a job has finished and provide state even when QEMU didn't report any. (I'll file an upstream libvirt bug to track this).