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

[Daniel P. Berrange]

On Fri, Jun 15, 2012 at 11:52:57AM -0500, Anthony Liguori wrote:
> On 06/13/2012 12:49 PM, Luiz Capitulino wrote:
> 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.

I can't help thinking that we're still over-engineering the error
reporting for QMP, and that really all we need is a reasonably
coarse error code/class, and an informal string.

eg,

   { 'error': "SystemError", msg = "failed to open file '/foo/bar' for writing: 
no space on device" }

   { 'error': "DNSError", msg = "unable to resolve hostname 'foo': cannot reach 
nameserver"}

   etc

In libvirt we started with a ridiculously complicated virErrorPtr
struct, which no one ever remembered to fill our details in, or
filledout details inconsistently. These days we only ever bother
with a coarse error class, and a string, and in the case of a
system error, we also include the raw errno value.

Pretty much all common APIs / languages focus primarily on just
an error code/class and a informal string too, with the odd
exception eg Python's OSException provides you the errno value
too

Are any users of QMP actually asking for this kind of advanced
error reporting ?  From libvirt's POV we're perfectly content
with just an error class & string.

Regards,
Daniel
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