Add 5.2.4 Operation Type Exists

[Shane32]

Closes #1097

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[benjie]

Great find!

All points were addressed

[benjie]

Looks good to me, though some slight editorial on the wording to match the style of the spec as a whole might be warranted.

[JoviDeCroock]

This is a great validation rule 🙌

[benjie]

The lack of this validation rule can be demonstrated in GraphQL.js with this:

import { GraphQLSchema, GraphQLInt, graphql, GraphQLObjectType } from 'graphql';

const Query = new GraphQLObjectType({
  name: 'Query',
  fields: {
    a: {
      type: GraphQLInt,
      resolve() {
        return 42;
      },
    },
  },
});

const schema = new GraphQLSchema({
  query: Query,
});

const result = await graphql({ schema, source: `mutation { __typename }` });
console.dir(result);

which produces:

{
  data: null,
  errors: [
    GraphQLError: Schema is not configured to execute mutation operation.
        at executeOperation [...]
      path: undefined,
      locations: [Array],
      extensions: [Object: null prototype] {}
    }
  ]
}

We should really mark this as an invalid request, and thus it should not have a data key in the response.

[yaacovCR]

This is a refresh of #955 which has a pending PR at the implementation graphql/graphql-js#3601

[benjie]

Good spot @yaacovCR; it seems both @Shane32 and I had commented on that previous PR!

I have merged this PR into that PR and applied my own editorial edits on top of both:

• Add validation rule that operation types exist #955