Federation: Add support for federationMetadata for executable schemas

[gdorsi]

Hello!

As for now the federationMetadata can be enabled only on string schemas.

Would be useful to support also "executable" schemas.

[mcollina]

I would like to do have that too! Would you like to send a PR?

[gdorsi]

Yes I'd like to, but it could take a while because ATM I don't have so much time to work on it.

If somebody else picks it up in the meantime, that's fine for me.

[xxSkyy]

I've made a dirty solution for this problem to provide _service { sdl } query for executable schema as gateway needs. Then applied middlewares works aswell as gateway reads schema and passing queries.

const { makeExecutableSchema } = require("@graphql-tools/schema")
const fs = require("fs")

const makeGatewayExecutableSchema = async ({
  schemaPath,
  schemaString,
  resolvers,
}) => {
  const serviceInfo = `
  type Service {
    sdl: String
  }

  type Query {
    _service: Service
  }
  type Mutation {
    _service: Service
  }
  type Subscription {
    _service: Service
  }
  `

  let schema

  if (schemaPath) {
    try {
      schema = await fs.readFileSync(schemaPath, "utf-8")
    } catch (e) {
      console.log(e)
      throw Error("Can't load schema file")
    }
  } else {
    schema = schemaString
  }

  // TODO: check if there are type query and swap to extend
  if (
    schema.indexOf("extend type Query") === -1 &&
    schema.indexOf("type Query") !== -1
  ) {
    throw Error(
      `You can't have 'type Query' in your schema, 
      only 'extend type Query' and so on with mutations and subscriptions`
    )
  }

  resolvers.Query._service = () => {
    return { sdl: `${schema}` }
  }

  return makeExecutableSchema({
    typeDefs: serviceInfo + schema,
    resolvers,
  })
}

Edit: Updated, it should work fine for many services, I've also made temporary module mercurius-exec-gateway-fix till it will get proper fix