A home for underappreciated errors

[Andrew Wilkins]

On Tue, Feb 10, 2015 at 5:57 PM, roger peppe <roger.peppe at canonical.com>
wrote:

> On 10 February 2015 at 08:55, Andrew Wilkins
> <andrew.wilkins at canonical.com> wrote:
> > Hi all,
> >
> > Ian raised a good point in http://reviews.vapour.ws/r/885/ (caution
> advised,
> > may cause eyebleed) about a change I made to errors:
> > https://github.com/juju/errors/pull/17
> >
> > The NotAssigned error, which previously was only raised by
> > state.Unit.AssignedMachineId, is now also raised by
> > state.Volume.StorageInstance. I removed the error from the state package
> so
> > we didn't have apiserver/common poking its head in there, and moved it
> over
> > to the juju/errors package.
> >
> > The issue Ian raised is that juju/errors should contain relatively
> generic
> > error types, and we should keep domain-specific things elsewhere.
> Examples
> > are NotAssigned, and NotProvisioned. We're thinking of moving these
> > domain-specific error types into a new package, juju/juju/errors. What do
> > you think about this?
>
> I agree with Ian here. Personally, I think that specific error
> types/values work well when defined in or close to the module that returns
> them,
> which is aligned to my general feeling that the set of possible returned
> errors is part of the contract of the method it was returned from.
>
> For example, the mgo package returns mgo.ErrNotFound when
> an item isn't found, and that's fine - we translate from that
> error to a more local not-found error when necessary.
>
> For errors that pass over an API, they can be considered a part of the API
> and defined along with the other API types.
>
> So, rather than a single package for juju-related errors,
> I would suggest that domain-specific errors are defined
> in packages close to the domain in question rather than
> in a single package used by everything. To my mind this
> is a more modular and scalable approach.
>

That all sounds good, but it doesn't work very well with the existing
error code thingy in apiserver/common, unless we're okay with
apiserver/common poking into state. I'm not a fan of that.

Perhaps we just need a state-internal package which registers predicates
in apiserver/common to do that magic. That wouldn't be so bad IMO.

It's also the standard Go idiom.
>
>   cheers,
>     rog.
>
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