[Qemu-devel] Re: [PATCH 00/10]: QError v4

[Paolo Bonzini]

> 2. It falls short of the requirement that clients can reasonably
>     classify errors they don't know.

True.  (Though adding a classification mechanism can be done later once 
we have an idea of what errors are there at all).

> 3. It falls short of the requirement that clients can easily present a
>     human-readable error description to their human users, regardless of
>     whether they know the error or not.

That's true.  I would definitely have the interpolated desc field go on 
the wire too, as part of the JSON form of QError.

> If I understand Dan correctly, machine-readable error code +
> human-readable description is just fine, as long as the error code is
> reasonably specific and the description is precise and complete.  Have
> we heard anything else from client developers?

Then (except for the asynchronicity) the current qemu monitor protocol 
is also just fine, including the fact that we send migrate and get 
m\rmi\rmig\rmigr etc. back on the socket.

"error while creating snapshot on '%s'" may be easy to parse, but 
looking at a dictionary field in { "filename" : "blah.img" } is easier, 
and at the same time solves issues with escaping weird characters in the 
file name.

> * The crucial question for the client isn't "what exactly went wrong".
>    It is "how should I handle this error".  Answering that question
>    should be easy (say, check the error code).  Figuring out what went
>    wrong should still be possible for a human operator of the client.

The same error can be handled in a different way depending on the 
situation.  A missing image is in general a fatal error, but maybe 
another client could create images on the fly.

> Based on what I've learned about client requirements so far, I figure
> that solution is "easily classified error code + human-readable
> description".

How would you classify the error code?  By subsystem or by (for example) 
severity or anything else?  Does QEMU have subsystems that are 
well-identified enough to be carved in stone in QError?  (All genuine 
questions.  I have no idea).

Paolo