compileAsync causes infinite chain of requests when metaschema is specified

[yesennes]

What version of Ajv are you using? 8.8
Does the issue happen if you use the latest version? yes

Ajv options object

{
  loadSchema: async (url) => {
    if (url.startsWith("http://")) {
      url = "https://" + url.substring(7)
    }
    const ans = await (await fetch(url)).json()
    console.log(ans)
    return ans;
  }
}

JSON Schema

{ "$ref": "http://json-schema.org/draft-04/schema#", "$schema": "http://json-schema.org/draft-04/schema#" }

Sample data

The schema never finishes compiling

Your code

const ajv = new Ajv(options)

async function compile() {
  await ajv.compileAsync(schema)
  console.log("Done compiling")
}

Here's a sandbox replicating the problem: https://runkit.com/yesennes/619429f6e904c40008bcc644

Validation result, data AFTER validation, error messages

The schema never finish compiling.

What results did you expect?
The schema to finish compiling.
Are you going to resolve the issue?
I can take a stab at it.

[MatthewPinheiro]

Hello! Not sure if you solved this, but I believe there are two problems here: one with your code, but the other really is with Ajv.

First, the issue with Ajv: I changed the $schema and $ref in your schema to draft-07, and it worked fine. I then tested this with draft-06, but this also started an infinite request loop, just like you're experiencing with draft-04. This implies that when Ajv reads a $schema value of exactly http://json-schema.org/draft-07/schema, it understands that this is its default, pre-cached meta-schema and so it does no additional requesting/processing. However, when it reads a $ref or a $schema linking to an older draft, such as http://json-schema.org/draft-04/schema, it actively makes a request to that URL. The schema is fetched, but before being cached, Ajv needs to process said schema to resolve any additional references. But since every meta-schema has its own $schema property pointing to itself, and meta-schemas aren't cached until all its references can be resolved, Ajv requests that schema, see its schema reference, requests that schema, sees its schema reference, and so on.

To work around that, you can simply preload meta-schemas in advance. If I was using draft-06, I could preload draft-06 before attempting compilation, like so:

const draft06 = require('ajv/dist/refs/json-schema-draft-06.json');
ajv.addMetaSchema(draft06);

However, this will not work with draft-04 because draft-04 is not included with Ajv, since it doesn't work with Ajv out of the box. That's where the issue with your code comes in, but there's an easy fix.

From this documentation:

You can use JSON Schema draft-04 schemas with Ajv from v8.5.0 and the additional package ajv-draft-04 (both ajv and ajv-draft-04 should be installed).

const Ajv = require("ajv-draft-04")
const ajv = new Ajv()

This will add a version of Ajv that works with draft-04, and has draft-04 preloaded as its meta-schema already.

[udondan]

Facing the same problem with draft-07, but the suggested solution does not (maybe no longer) work.

const draft07 = require('ajv/dist/refs/json-schema-draft-07.json');
ajv.addMetaSchema(draft07);

This solves the infinite loop problem, but the loadSchema function is still called for this schema, which should already be known.

ajv then fails with:

Error: schema with key or id "http://json-schema.org/draft-07/schema" already exists

The only way I was able to pass this step was to actually remove the schema from the loaded files:

delete schema.$schema