Re: [Qemu-devel] Adding errno to QMP errors

[Anthony Liguori]

On 06/13/2012 12:49 PM, Luiz Capitulino wrote:
> On Thu, 31 May 2012 16:54:47 +0200
> Paolo Bonzini<address@hidden>  wrote:
>
> > Wait, I think you're conflating two things.
> >
> > One is "do not shoehorn errors into errno values".  So for QOM invalid values we
> > have PropertyValueBad, not a generic InvalidArgument value.  We convert 
> > everything
> > to Error rather than returning negative errno values and then returning generic
> > error codes, because those would be ugly and non-descriptive.  I agree with 
> > that.
> >
> > The other is "when errors come straight from the OS, _do_ use errno values".
> > This is for OpenFileFailed, for the new socket errors and so on.  This is what
> > I am proposing.
>
> [...]
>
> >      $ echo | sed p>  /dev/full
> >      sed: couldn't flush stdout: No space left on device
> >           ^^^^^^^^^^^^^^                                 error type
> >                          ^^^^^^  ^^^^^^^^^^^^^^^^^^^^^^^ arguments
> >
> > That would become, in JSON:
> >
> >      { 'error': 'FlushFailed',
> >        'file': 'stdout',
> >        'os_error': 'enospc' }
>
> This is not a new discussion and what we're doing today is to return errno
> as a QError class name. So, for the example above we'd return something like:
>
>   { 'error': 'NoSpace' }


No, you're confusing things I think.  { 'error': 'NoSpace' } is bad.  errno is 
not an intrinsically bad thing but errno critically relies on the *caller* to 
understand the context that the error has occurred in.  Just returning { 
'error': 'NoSpace' } is not good enough in QMP because the caller doesn't know 
the context.  What was the command doing such that that error was returning?

In many cases, errno has different meanings depending on the context.  EINVAL is 
a good example of this.

The devil is in the details here.  Having an error like:

{ 'error': 'OpenFileFailed', 'file': 'filename', 'mode': 'r/w', 'os_error': 
'enospc' }

is actually pretty reasonable for something like a memory dump command where the 
user specifies a file.

OTOH, for something complex like live snapshotting which many involve opening 
multiple files, it may not be good enough.

So context is really everything here.

Regards,

Anthony Liguori

> It's possible to add new optional values if you need more information, but
> I know that that's not what you're asking.
>
> I mostly agree that your version would be better, the only problem I see
> is that this is probably going to mess a bit more our API as we have been
> doing like my example above for some time.
>
> Anthony, the current design was mostly influenced by you and you had
> objections on doing what Paolo and Kevin are suggesting. What do you think?