Unhandled rejection if an object has both a synchronous and asynchronous "Cannot return null for non-nullable field" error

[asztal]

Sorry about the title but it's a pretty specific scenario.

It looks like this error occurs because of how errors are handled for non-nullable fields. (I saw this with the Cannot return null for non-nullable field error, but it seems if the resolver throws an error it would be handled the same way.)

The problem appears to be here: https://github.com/graphql/graphql-js/blob/main/src/execution/execute.ts#L450
Let's say in the example below, the City object type has a resolver for country, which is asynchronous, and a resolver for population, which is synchronous, and they both return null (which they shouldn't, of course). The resolver for the country field is run, and returns a promise. Then the resolver for the population field is run, and because it's synchronous, the error is thrown straight away, and the promise returned by the country resolver is discarded. When that promise eventually rejects, the process crashes with an unhandledRejection error.

// File: testcase.mjs
import { graphql, GraphQLInt, GraphQLList, GraphQLNonNull, GraphQLObjectType, GraphQLSchema, GraphQLString } from "graphql";

process.on("unhandledRejection", reason => {
    console.error("Unhandled rejection!", reason);
});

const City = new GraphQLObjectType({
    name: "City",
    fields: {
        name: { type: new GraphQLNonNull(GraphQLString) },
        country: { type: new GraphQLNonNull(GraphQLString), resolve: () => Promise.resolve(null) },
        population: { type: new GraphQLNonNull(GraphQLInt), resolve: () => null },
    }
});

const Query = new GraphQLObjectType({
    name: "Query",
    fields: {
        cities: {
            type: new GraphQLNonNull(new GraphQLList(new GraphQLNonNull(City))),
            resolve() {
                return [{ name: "London", country: "GB", population: 8_982_000 }];
            }
        }
    }
});

const schema = new GraphQLSchema({
    types: [City],
    query: Query,
});

try {
    graphql(schema, `
        query {
            cities { name country population }
        }
    `).then(result => {
        if (result.errors)
            console.error("Error returned in result.errors[]:", result.errors);
        else
            console.log("Result:", result.data);
    }, error => {
        console.error("graphql() rejected:", error);
    });
} catch(err) {
    console.error("Error caught by try/catch:", err);
}

The output of this is:

Error returned in result.errors[]: [
  Error: Cannot return null for non-nullable field City.population.
      at completeValue (/home/lee/restapi/node_modules/graphql/execution/execute.js:566:13)
      at resolveField (/home/lee/restapi/node_modules/graphql/execution/execute.js:478:19)
      at executeFields (/home/lee/restapi/node_modules/graphql/execution/execute.js:292:18)
      at collectAndExecuteSubfields (/home/lee/restapi/node_modules/graphql/execution/execute.js:755:10)
      at completeObjectValue (/home/lee/restapi/node_modules/graphql/execution/execute.js:745:10)
      at completeValue (/home/lee/restapi/node_modules/graphql/execution/execute.js:597:12)
      at completeValue (/home/lee/restapi/node_modules/graphql/execution/execute.js:563:21)
      at /home/lee/restapi/node_modules/graphql/execution/execute.js:627:25
      at Array.map (<anonymous>)
      at safeArrayFrom (/home/lee/restapi/node_modules/graphql/jsutils/safeArrayFrom.js:36:23) {
    locations: [ [Object] ],
    path: [ 'cities', 0, 'population' ]
  }
]
Unhandled rejection! Error: Cannot return null for non-nullable field City.country.
    at completeValue (/home/lee/restapi/node_modules/graphql/execution/execute.js:566:13)
    at /home/lee/restapi/node_modules/graphql/execution/execute.js:475:16 {
  locations: [ { line: 3, column: 27 } ],
  path: [ 'cities', 0, 'country' ]
}