Add input path in unmarshaling errors

[bowd]

Related to #1113

Problem summary

When relying on custom scalar, especially in mutations, you offload an aspect of validation to the unmarshaling step. This means that you ensure these custom types have the right structure in one place and then you can rely on them in the next layers.

The problem with the current implementation is that errors that happen as a result of scalar unmarshaling only have path information up to the mutation that they occur in. And it's impossible to reconstruct what field in the input is responsible for the error if you have multiple instances of the scalar in a nested input structure. And makes it impossible for the frontend to link the error to a form field, for example.

Solution

I've added a new context type FieldInputContext which acts similarly to FieldContext but is responsible for keeping track of the nesting inside of the arguments received by queries or mutations. This context is then used during unmarshaling to wrap the error returned into a gqlerror.Error if it's not already or just inject the path if it is. This lets developers return gqlerror.Error from their unmarshaling logic in order to inject additional data in extensions

See the updated docs for additional details.

Codegen touchpoints:

• args.gotpl - created nested input context for each argument
• input.gotpl - created nested input context for each field in an input
• type.gotpl - treat all unmarshaling scenarios

I have:

• Added tests covering the bug / feature (see testing)
• Updated any relevant documentation (see docs)

[bowd]

I was thinking now that this is, in a way, a breaking change because some implementations might be expecting the old pat somewhere downstream. So this feature could be enabled through configuration so you can switch between the partial field path or the full path for the errors.

[bowd]

An extension to this that I would like to make and I didn't handle in this PR would be to not fail on the first unmarshal error in the arg tree, but collect all errors in the tree using a multierror package or something similar to handle merging of errors. This way we could reduce round-trips by returning all errors that we can find in the first call.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[coveralls]

Coverage increased (+0.2%) to 66.419% when pulling bde4291 on bowd:add-input-path-for-unmarshaling into 6be2e9d on 99designs:master.

[vektah]

Thanks, this has annoyed me for a long time. 🎉

I've added a small commit to drop the custom context names to be consistent with the rest of the context usage (eg field context).