Re: [Qemu-devel] [PATCH 02/14] qerror: add new errors

[Luiz Capitulino]

On Thu, 31 May 2012 17:49:42 +0200
Paolo Bonzini <address@hidden> wrote:

> Il 31/05/2012 17:44, Luiz Capitulino ha scritto:
> >> 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.
> >>
> >> These two rules together match what most other programs do.
> >>
> >>     $ 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' }
> > 
> > Actually, I did propose something similar in the past but Anthony objected.
> 
> Looks like in the meanwhile we moved closer to this mechanism
> (OpenFileFailed, Sock*, etc.), except we have no clear way to identify
> _what_ error actually happened rather than _where_.

In fact, it's the other way around. OpenFileFailed, for example, is an old
error. We thought it would be ok to have it that way (also because of shallow
QMP conversion, as it would take ages to convert all possible errors). But today
it's clear it's bad and we're trying to move away from it.

The socket ones repeat this, but it's probably because people usually go
for the generic error first because having detailed errors with qerror is
cumbersome. I have some ideas to improve it that could _mitigate_ that problem,
like having a schema file qapi-errors.json:

 { 'error': 'InvalidParameter', 'data': { 'name': 'str' } }

vs. having to edit qerror.[ch]. This is not difficult to do, and I have a rfc
for it already.

But I really don't have the ultimate answer to the question of how to make
error reporting and infrastructure near perfect.