feat(dbless): improve validation errors from /config

[flrgh]

summary

This is a quality-of-life feature for the /config endpoint in dbless mode.

the problem

When the configuration passed into POST /config has validation errors, the raw structures from kong.db.schema are returned. Here's an example where a single service is missing the value for the host property and contains an invalid value for the port property:

{
  "message": "declarative config is invalid: {services={{host=\"required field missing\",port=\"value should be between 0 and 65535\"}}}",
  "code": 14,
  "fields": {
    "services": [
      {
        "host": "required field missing",
        "port": "value should be between 0 and 65535"
      }
    ]
  },
  "name": "invalid declarative configuration"
}

The errors are not correlated to the input entity in any meaningful way, so this gets very confusing once you have more than a couple entities--especially if they are nested underneath other entities (i.e. service.routes).

the solution

This PR adds an optional flatten_errors query param to the endpoint. When truth-y (e.g. POST /config?flatten_errors=1), errors are reshaped and attached to the response in a flattened_errors array at the top level (old fields are retained for backwards compatibility). Here's what the previous response looks like with flatten_errors turned on:

{
  "message": "declarative config is invalid: {services={{host=\"required field missing\",port=\"value should be between 0 and 65535\"}}}",
  "code": 14,
  "fields": {
    "services": [
      {
        "host": "required field missing",
        "port": "value should be between 0 and 65535"
      }
    ]
  },
  "name": "invalid declarative configuration",
  "flattened_errors": [
    {
      "entity_type": "service",
      "entity_name": "jeff",
      "entity_tags": [
        "jeff-is-cool"
      ],
      "entity": {
        "name": "jeff",
        "protocol": "http",
        "port": -1,
        "tags": [
          "jeff-is-cool"
        ]
      },
      "errors": [
        {
          "type": "field",
          "field": "host",
          "message": "required field missing"
        },
        {
          "type": "field",
          "field": "port",
          "message": "value should be between 0 and 65535"
        }
      ]
    }
  ]
}

PR status

This is ready for merge after an approving review or two.

todo

• more input/response test cases
  • many-to-one
  • one-to-one
  • plugin entity test cases
• changelog
• fix tests broken by the changes made to kong.db.declarative
• make the flattened array optional via query param (?)

[samugi]

looks great to me so far. I verified that the flattened output reflects what was discussed in the design. I understand it is still WIP so I am leaving the review as comment for now until it's ready

[flrgh]

The initial goal was to use the processed input when generating the final error array.
This means updating kong.db.schema.others.declarative_config:flatten() to attach/return the processed config table when returning an error. I couldn't come up with a means of doing so without making lots of breaking and/or intrusive changes, so I compromised a bit.

[flrgh]

Rebased onto master just now. I'm cherry-picking to EE to hopefully make sure there are no surprises over on that side.

[tyler-ball]

A few questions but overall I think its looking great. I would like to get 1 more review's eyes on this if possible, but the test cases look good