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

[Luiz Capitulino]

On Fri, 15 Jun 2012 11:52:57 -0500
Anthony Liguori <address@hidden> wrote:

> 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?

The screendump command (not merged yet) can return it during open() or write().

> 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.

And WriteFailed, ReadFailed, etc? This is what Paolo is suggesting I think. If
you agree on doing this, then I'll switch the screendump conversion to do this
and we'll have to extend existing errors.

And, I'm afraid that some not so new qapi conversions did the NoSpace example
above...

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

Kevin, I think you had a different proposal for the live snapshot command?