[Qemu-devel] Plan for error handling in QMP

[anthony@codemonkey.ws]

Hi,

We had a violent^Wheated discussion on IRC about how to move forward
with Luiz's proposed error series.  I think we reached consensus.  This
note attempts to outline that.

Principles
----------
1. Errors should be free formed strings with a class code

2. There should be a small number of class codes (10-15) added
   strictly when there are specific users of a code.

3. The class code should be expressed as an enum data type in the normal
   QMP schema.  Other than this, errors should have no structure in the
   schema.[*]

4. We should drop all dictionary arguments in the current error
   mechanisms beyond 'class' and 'desc'.

5. The following errors are used by libvirt:
   - CommandNotFound: QMP parsing
   - DeviceNotActive/KVMMissingCap: ballooning
   - DeviceNotFound: drive_del
   - MigrationExpected: cont

6. We need to make sure that these errors are preserved while other
   errors should be consolidated.
   - We need to state very clear for 1.2 which errors are going away.

7. We need to make sure that anything we expose in 1.2 stays that way.
   If we're dropping 'InvalidParameterType' as a class code, it should be
   dropped in 1.2.  This could be achieved by making all existing codes
   except for those in (5) report 'UnknownError' or something.[*]

[*] I took a little bit of license with these.  Hopefully it's not
controversial.

Regards,

Anthony Liguori