Simplify subscriptions. (#860)

There was a secondary function in subscribe.js which doesn't correspond to a spec function or provide much value. It was an artifact of an earlier implementation. Collapsing it into the defined createSourceEventStream makes the alignment to spec more clear. It also moves this function below `subscribe` in the file, making its purpose more clear.

Author: leebyron

src/subscription/subscribe.js

@@ -25,51 +25,8 @@ import { GraphQLSchema } from '../type/schema';
 import invariant from '../jsutils/invariant';
 import mapAsyncIterator from './mapAsyncIterator';
 
-import type {
-  ExecutionContext,
-  ExecutionResult,
-} from '../execution/execute';
-import type {
-  DocumentNode,
-  OperationDefinitionNode,
-} from '../language/ast';
-
-/**
- * Implements the "CreateSourceEventStream" algorithm described in the 
- * GraphQL specification, resolving the subscription source event stream.
- *
- * Returns an AsyncIterable
- *
- * A Source Stream represents the sequence of events, each of which is 
- * expected to be used to trigger a GraphQL execution for that event.
- */
-export function createSourceEventStream(
-  schema: GraphQLSchema,
-  document: DocumentNode,
-  rootValue?: mixed,
-  contextValue?: mixed,
-  variableValues?: ?{[key: string]: mixed},
-  operationName?: ?string,
-): AsyncIterable<mixed> {
-  // If a valid context cannot be created due to incorrect arguments,
-  // this will throw an error.
-  const exeContext = buildExecutionContext(
-    schema,
-    document,
-    rootValue,
-    contextValue,
-    variableValues,
-    operationName
-  );
-
-  // Call the `subscribe()` resolver or the default resolver to produce an
-  // AsyncIterable yielding raw payloads.
-  return resolveSubscription(
-    exeContext,
-    exeContext.operation,
-    rootValue
-  );
-}
+import type { ExecutionResult } from '../execution/execute';
+import type { DocumentNode } from '../language/ast';
 
 /**
  * Implements the "Subscribe" algorithm described in the GraphQL specification.
@@ -97,6 +54,10 @@ export function subscribe(
 
   // For each payload yielded from a subscription, map it over the normal
   // GraphQL `execute` function, with `payload` as the rootValue.
+  // This implements the "MapSourceToResponseEvent" algorithm described in
+  // the GraphQL specification. The `execute` function provides the
+  // "ExecuteSubscriptionEvent" algorithm, as it is nearly identical to the
+  // "ExecuteQuery" algorithm, for which `execute` is also used.
   return mapAsyncIterator(
     subscription,
     payload => execute(
@@ -110,12 +71,40 @@ export function subscribe(
   );
 }
 
-function resolveSubscription(
-  exeContext: ExecutionContext,
-  operation: OperationDefinitionNode,
-  rootValue: mixed
+/**
+ * Implements the "CreateSourceEventStream" algorithm described in the
+ * GraphQL specification, resolving the subscription source event stream.
+ *
+ * Returns an AsyncIterable, may through a GraphQLError.
+ *
+ * A Source Stream represents the sequence of events, each of which is
+ * expected to be used to trigger a GraphQL execution for that event.
+ *
+ * This may be useful when hosting the stateful subscription service in a
+ * different process or machine than the stateless GraphQL execution engine,
+ * or otherwise separating these two steps. For more on this, see the
+ * "Supporting Subscriptions at Scale" information in the GraphQL specification.
+ */
+export function createSourceEventStream(
+  schema: GraphQLSchema,
+  document: DocumentNode,
+  rootValue?: mixed,
+  contextValue?: mixed,
+  variableValues?: ?{[key: string]: mixed},
+  operationName?: ?string,
 ): AsyncIterable<mixed> {
-  const type = getOperationRootType(exeContext.schema, exeContext.operation);
+  // If a valid context cannot be created due to incorrect arguments,
+  // this will throw an error.
+  const exeContext = buildExecutionContext(
+    schema,
+    document,
+    rootValue,
+    contextValue,
+    variableValues,
+    operationName
+  );
+
+  const type = getOperationRootType(schema, exeContext.operation);
   const fields = collectFields(
     exeContext,
     type,
@@ -131,12 +120,14 @@ function resolveSubscription(
   const responseName = responseNames[0];
   const fieldNodes = fields[responseName];
   const fieldNode = fieldNodes[0];
-  const fieldDef = getFieldDef(exeContext.schema, type, fieldNode.name.value);
+  const fieldDef = getFieldDef(schema, type, fieldNode.name.value);
   invariant(
     fieldDef,
     'This subscription is not defined by the schema.'
   );
 
+  // Call the `subscribe()` resolver or the default resolver to produce an
+  // AsyncIterable yielding raw payloads.
   const resolveFn = fieldDef.subscribe || defaultFieldResolver;
 
   const info = buildResolveInfo(
@@ -148,7 +139,7 @@ function resolveSubscription(
   );
 
   // resolveFieldValueOrError implements the "ResolveFieldEventStream"
-  // algorithm from GraphQL specification. It differs from 
+  // algorithm from GraphQL specification. It differs from
   // "ResolveFieldValue" due to providing a different `resolveFn`.
   const subscription = resolveFieldValueOrError(
     exeContext,