Valid JSON Schema not working for null-able response

[andyburke]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the bug has not already been reported

Fastify version

3.22.1

Plugin version

No response

Node.js version

14.x

Operating system

Linux

Operating system version (i.e. 20.04, 11.3, 10)

20.04

Description

When trying to define a GET request that can return either an object or a null, the valid JSON Schema of type: [ 'object', 'null' ] is not supported by Fastify.

FastifyError [FST_ERR_SCH_SERIALIZATION_BUILD]: Failed building the serialization schema for GET: /test, due to error object,null unsupported

schema: {
	summary: 'Test',
	response: {
		200: {
			type: [ 'object', 'null' ],
			properties: {
				foo: {
					type: 'string'
				}
			}
		}
	}
}

I understand you are supposed to pass nullable but we do a lot of automatic schema generation, and needing to do this only in the case where the top-level response is in this situation is frustrating. It seems like this is at the intersection of Fastify, Open API 3 and JSON Schema, so I don't envy trying to navigate this, but as it stands this is a pretty confusing error to get.

Steps to Reproduce

Create an endpoint whose response can be either a valid value or null.

Expected Behavior

Passing valid JSON Schema for a response that's a possible null object should work as it does at other levels schema definition.

[VigneshMurugan]

Let me work on this

[matthyk]

If you need a quick fix

schema: {
	summary: 'Test',
	response: {
		200: {
			type: 'object',
                         nullable: true,
			properties: {
				foo: {
					type: 'string'
				}
			}
		}
	}
}

does the trick.

[andyburke]

Yeah, we are currently doing something like:

200: { ...as_json_schema( model ), nullable: true }

Although there is a workaround, this still feels out of place because it's only necessary at this top level (at least as I currently understand the issue).

[mcollina]

@Eomm why is it a good first issue? I'd not have a clue on how to fix this in Fastify.

[matthyk]

As I understand it, the problem lies in fast-json-stringify in this switch case

fast-json-stringify/index.js

Line 102 in 8cde564

switch (schema.type) {

as this does not support an array of types. So the problem is not with the null but with the array. Accordingly, { type: [ "string", "number" ] } does not work either.
So the question is whether we want to support arrays to specify different types?

[mcollina]

I think it would be a great idea!

[andyburke]

@matthyk FWIW, this "type as an array" comes from the JSON Schema way of specifying these. Not sure what impacts that would have on fast-json-stringify, as I'm not very familiar with it.