Make error handling consistent in createSourceEventStream

src/subscription/subscribe.js

@@ -35,8 +35,9 @@ import { getOperationRootType } from '../utilities/getOperationRootType';
  * Implements the "Subscribe" algorithm described in the GraphQL specification.
  *
  * Returns a Promise which resolves to either an AsyncIterator (if successful)
- * or an ExecutionResult (client error). The promise will be rejected if a
- * server error occurs.
+ * or an ExecutionResult (error). The promise will be rejected if the schema or
+ * other arguments to this function are invalid, or if the resolved event stream
+ * is not an async iterable.
  *
  * If the client-provided arguments to this function do not result in a
  * compliant subscription, a GraphQL Response (ExecutionResult) with
@@ -171,19 +172,28 @@ function subscribeImpl(
             reportGraphQLError,
           )
         : ((resultOrStream: any): ExecutionResult),
-    reportGraphQLError,
   );
 }
 
 /**
  * Implements the "CreateSourceEventStream" algorithm described in the
  * GraphQL specification, resolving the subscription source event stream.
  *
- * Returns a Promise<AsyncIterable>.
+ * Returns a Promise which resolves to either an AsyncIterable (if successful)
+ * or an ExecutionResult (error). The promise will be rejected if the schema or
+ * other arguments to this function are invalid, or if the resolved event stream
+ * is not an async iterable.
  *
- * If the client-provided invalid arguments, the source stream could not be
- * created, or the resolver did not return an AsyncIterable, this function will
- * will throw an error, which should be caught and handled by the caller.
+ * If the client-provided arguments to this function do not result in a
+ * compliant subscription, a GraphQL Response (ExecutionResult) with
+ * descriptive errors and no data will be returned.
+ *
+ * If the the source stream could not be created due to faulty subscription
+ * resolver logic or underlying systems, the promise will resolve to a single
+ * ExecutionResult containing `errors` and no `data`.
+ *
+ * If the operation succeeded, the promise resolves to the AsyncIterable for the
+ * event stream returned by the resolver.
  *
  * A Source Event Stream represents a sequence of events, each of which triggers
  * a GraphQL execution for that event.
@@ -270,7 +280,11 @@ export function createSourceEventStream(
     return Promise.resolve(result).then(eventStream => {
       // If eventStream is an Error, rethrow a located error.
       if (eventStream instanceof Error) {
-        throw locatedError(eventStream, fieldNodes, responsePathAsArray(path));
+        return {
+          errors: [
+            locatedError(eventStream, fieldNodes, responsePathAsArray(path)),
+          ],
+        };
       }
 
       // Assert field returned an event stream, otherwise yield an error.
@@ -284,6 +298,8 @@ export function createSourceEventStream(
       );
     });
   } catch (error) {
-    return Promise.reject(error);
+    return error instanceof GraphQLError
+      ? Promise.resolve({ errors: [error] })
+      : Promise.reject(error);
   }
 }