Created attachment 1246616 [details]
engine log

Description of problem:
In bz: https://bugzilla.redhat.com/show_bug.cgi?id=1414867 the ability to invoke the sparsify action is disabled in UI if the disk is a direct lun disk.
On REST API when trying to invoke the sparsify action on the same disk, the action fails as expected but the error is too general:
<detail>java.lang.NullPointerException</detail>



Version-Release number of selected component (if applicable):
rhevm-4.1.0.3-0.1.el7.noarch

How reproducible:
always

Steps to Reproduce:
1. Create a vm.
2. Create a disk for the vm which is a direct lun disk.
3. Invoke sparsify action from rest api:

https://{engine_url}/ovirt-engine/api/disks/{disk_id}/sparsify

body:
<action>
    <async>false</async>
    <grace_period>
        <expiry>10</expiry>
    </grace_period>
</action>

Actual results:

<action>
<async>false</async>
<fault>
<detail>java.lang.NullPointerException</detail>
<reason>Operation Failed</reason>
</fault>
<grace_period>
<expiry>10</expiry>
</grace_period>
<status>failed</status>
</action>

attached is engine log with fool trace of the NullPointerException.

Expected results:
An informative message explaining that the action is disabled for this disk type.

Additional info: