`defaultTypeResolver` can result in unhandled promise rejection

[benjie]

I was just looking over the source code around polymorphism and I think I found an issue here:

graphql-js/src/execution/execute.ts

Lines 1000 to 1018 in 7dd7812

	const isTypeOfResult = type.isTypeOf(value, contextValue, info);
	if (isPromise(isTypeOfResult)) {
	promisedIsTypeOfResults[i] = isTypeOfResult;
	} else if (isTypeOfResult) {
	return type.name;
	}
	}
	}
	if (promisedIsTypeOfResults.length) {
	return Promise.all(promisedIsTypeOfResults).then((isTypeOfResults) => {
	for (let i = 0; i < isTypeOfResults.length; i++) {
	if (isTypeOfResults[i]) {
	return possibleTypes[i].name;
	}
	}
	});
	}

Specifically: if we're looking over types A, B and C, and A yields a promise, and B returns a match, we will return B.name and the promise we added to promisedIsTypeOfResults for A will never be handled. If that promise were to reject then Node would raise an unhandled promise rejection error.

I think we can solve this by adding a line above return type.name:

       } else if (isTypeOfResult) {
+        // Ignore errors from promises already created
+        if (promisedIsTypeOfResults.length) {
+          void Promise.allSettled(promisedIsTypeOfResults);
+        }
         return type.name;
       }