A home for underappreciated errors

[William Reade]

I think that NotAssigned is *too* generic. UnitNotAssignedToMachineErr
is one thing, and VolumeNotAssignedToStorageInstance is another; if we
use the same error type to represent those distinct situations, we
really ought to include a bunch more mechanism for finding out
*precisely* what should have been assigned to what (because
refactoring happens, and methods start returning the same generic
error for new reasons, potentially breaking client code that makes
assumptions about what that generic error type implies. See
worker/uniter/filter/filter.go and search for ErrTerminateAgent for a
particularly hair-raising example...)

I see what you're saying re apiserver/common, but I wouldn't really
characterise it as "poking into" state: apiserver is generally
implemented in terms of state, so that's a perfectly reasonable
dependency IMO. For exactly the reasons stated re generic error types
in code, the generic error codes in the api are the scary bits I now
wish we'd done differently...

Cheers
William

On Tue, Feb 10, 2015 at 10:55 AM, 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?
>
> Cheers,
> Andrew
>
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