Warn when input attributes is missing in /v1/data body

[anderseknert]

Every now and then we see users new to OPA missing the fact that /v1/data requires the JSON request body to wrap the input document, ala: {"input": {"x": "y"}}. Unless e.g. default assignment is used, OPA currently returns just an empty response for a request missing the "input" wrapper, i.e. {"x": "y"} ... it's easy to see why this would be, given how people likely have worked on their policy in the Rego Playground or using opa eval before running the server.

It would be an improvement if we informed the user of what's missing! Suggested response might look something like:

HTTP/1.1 200 OK
Content-Type: application/json

{
  "result": <JSON>,
  "warning": {
    "code": "api_usage_warning",
    "message": "The 'input' key was missing from the request. See <DOC_URL> for more details."
  }
}

Note that while it's tempting to return a 400 response, this would both break a lot of existing integrations, and would need to differentiate between input missing entirely (i.e. no JSON body) and JSON body but with missing input attribute. For that reason we'll keep the 200 response but with an added warning.

[aflmp]

@anderseknert Can I work on this please? like others, I have also been bitten by this issue in the past.

[anderseknert]

Sure! Let me know if you need any help along the way, either here or in the #development channel on the OPA slack.

[aflmp]

from a quick parse, looks like we want to address the issue here ?

[anderseknert]

Yeah, at least you have the input variable there and can check if it's nil or not.. but since it shouldn't be treated as an error but rather just as a warning to add to the response, you'll probably want to do that check where the response is built, and append the warning to it... somewhere here.