Chain: [A] <- [B]

QEMU launched with:

$ ~/build/build-qemu-block-stream-crash-test/x86_64-softmmu/qemu-system-x86_64 -M q35 -display none -nodefconfig -nodefaults -m 512 -blockdev node-name=node-A,driver=qcow2,file.driver=file,file.filename=./a.qcow2 -device virtio-blk,drive=node-A,id=virtio0 -netdev user,id=net0,hostfwd=tcp::5555-:22 -device e1000,netdev=net0 -monitor stdio -qmp unix:./qmp-sock,server,nowait

QMP shell:

    $ ./qmp-shell -v -p /tmp/qmp-sock
    (QEMU)


Short (only `qmp-shell`):
=======================================================================
(QEMU) blockdev-snapshot-sync node-name=node-A snapshot-file=b.qcow2 snapshot-node-name=node-B format=qcow2
(QEMU) block-commit device=node-B base=a.qcow2 top=b.qcow2 job-id=job0                                                                                                                        
(QEMU) query-block-jobs
(QEMU) block-job-complete device=job0
(QEMU) query-block-jobs
=======================================================================

Long (`qmp-shell` output along with raw JSON):
-----------------------------------------------------------------------
(QEMU) query-version
{
    "execute": "query-version", 
    "arguments": {}
}
{
    "return": {
        "qemu": {
            "micro": 50, 
            "major": 2, 
            "minor": 9
        }, 
        "package": " (v2.9.0-557-g0e17187-dirty)"
    }
}
(QEMU) 
-----------------------------------------------------------------------
(QEMU) blockdev-snapshot-sync node-name=node-A snapshot-file=b.qcow2 snapshot-node-name=node-B format=qcow2
{
    "execute": "blockdev-snapshot-sync", 
    "arguments": {
        "node-name": "node-A", 
        "snapshot-file": "b.qcow2", 
        "format": "qcow2", 
        "snapshot-node-name": "node-B"
    }
}
{
    "return": {}
}
-----------------------------------------------------------------------
(QEMU) block-commit device=node-B base=a.qcow2 top=b.qcow2 job-id=job0
{
    "execute": "block-commit", 
    "arguments": {
        "device": "node-B", 
        "job-id": "job0", 
        "top": "b.qcow2", 
        "base": "a.qcow2"
    }
}
{
    "return": {}
}
(QEMU) 
{u'timestamp': {u'seconds': 1495924024, u'microseconds': 225595}, u'data': {u'device': u'job0', u'type': u'commit', u'speed': 0, u'len': 983040, u'offset': 983040}, u'event': u'BLOCK_JOB_COMPLETED'}
{u'timestamp': {u'seconds': 1495924062, u'microseconds': 901494}, u'data': {u'device': u'job0', u'type': u'commit', u'speed': 0, u'len': 0, u'offset': 0}, u'event': u'BLOCK_JOB_READY'}
(QEMU) 
-----------------------------------------------------------------------
(QEMU) query-block-jobs
{
    "execute": "query-block-jobs", 
    "arguments": {}
}
{
    "return": [
        {
            "busy": false, 
            "type": "commit", 
            "len": 0, 
            "paused": false, 
            "ready": true, 
            "io-status": "ok", 
            "offset": 0, 
            "device": "job0", 
            "speed": 0
        }
    ]
}
(QEMU) 
-----------------------------------------------------------------------
(QEMU) block-job-complete device=job0
{
    "execute": "block-job-complete", 
    "arguments": {
        "device": "job0"
    }
}
{
    "return": {}
}
-----------------------------------------------------------------------
(QEMU) query-block-jobs
{
    "execute": "query-block-jobs", 
    "arguments": {}
}
{
    "return": []
}
(QEMU) 
-----------------------------------------------------------------------