Format errors as JSON-API

[nono]

@aenario @jinroh I'm working on JSON-API formatting. This PR is not yet ready to be merged, but you may want to look at it to see where I'm going

It's now ready!

[jinroh]

This is not really an InvalidParameter or StatusUnprocessableEntity 422 (the MD5 given has the good "format") but a StatusPreconditionFailed 412 (the MD5 calculated does not match the given one)

[jinroh]

Same remark for ErrContentLengthMismatch which can fall into the same category.

[nono]

You are right, I'm going to fix that!

[jinroh]

@nono no api2go finally, or it's coming in ?

[nono]

@jinroh errors in api2go was identified as a limitation. It's tied to using the routing stuff from api2go. I don't want to use the routing stuff of api2go, only the serialization/deserialization of objects. So, I have to code it myself ;-)

[aenario]

I am OK to merge this PR as is, but in the future think we should have a similar structure for error handling in both CRUD and VFS api.

In CRUD, I opted to use the gin error handling mechanism, with the combo HTTPStatus(), c.AbortWithError and then an error handling middleware which renders the couchdb.Error to JSON if it is one. This is not so clean :

• HTTPStatus is only useful because, we need the status code to call AbortWithError. If there was another (*Context) function equivalent to c.Error(err) && c.Abort(), it could all be handled in the error handling middleware (like you did in jsonapi.AbortWithError())
• gin wraps the errors we pass into gin.Error and maintains a slice of them, which is pretty useless, but it can be configured to log some errors, which might prove useful.

In VFS, you went with having a jsonapi.AbortWithError() which immediately do the render, without passing the error to gin, which is way cleaner and doesnt involve

---

In the future we could

A. switch to echo v3, where we simply return error and then have an error handling function wich does all the triaging/formating.
B. If we stay with gin, we might do something like :

• a common interface, type StackError interface Error(), Title(), Reason(), JSON(), JSONAPI() StatusCode() implemented by both VFS errors, couchdb errors, other errors we will need in the future and a oneline constructor.
• a function Abort(context, StackError), which call c.Error(), c.JSON(), c.Abort() and renders the errors with proper formating depending on c.NegotiateFormat.

my 2c

[jinroh]

Rebased and merged 226e75a