refactor: subscribe: introduce buildPerEventExecutionContext (#3639)

= introduces `buildPerEventExecutionContext` that creates an `ExecutionContext` for each subscribe event from the original `ExecutionContext` used to create the event stream
= `subscribe` now directly builds the `ExecutionContext` instead of relying on `createSourceEventStream`
= introduces `createSourceEventStreamImpl` and `executeImpl` functions that operate on the built `ExecutionContext` rather the `ExecutionArgs`
= `subscribe` calls the `createSourceEventStreamImpl` function on the original context and eventuallys calls `executeImpl` on the per event context created by `buildEventExecutionContext`.

Motivation:
1. remove unnecessary `buildExecutionContext` call, reducing duplicate work
2. paves the way for easily enhancing the `buildPerEventExecutionContext` to add a new `perEventContextFactory` argument to augment the context argument passed to resolvers as need per event.

Author: yaacovCR

src/execution/__tests__/subscribe-test.ts

@@ -14,7 +14,7 @@ import { GraphQLList, GraphQLObjectType } from '../../type/definition';
 import { GraphQLBoolean, GraphQLInt, GraphQLString } from '../../type/scalars';
 import { GraphQLSchema } from '../../type/schema';
 
-import type { ExecutionResult } from '../execute';
+import type { ExecutionArgs, ExecutionResult } from '../execute';
 import { createSourceEventStream, subscribe } from '../execute';
 
 import { SimplePubSub } from './simplePubSub';
@@ -189,9 +189,15 @@ function subscribeWithBadFn(
   });
   const document = parse('subscription { foo }');
 
+  return subscribeWithBadArgs({ schema, document });
+}
+
+function subscribeWithBadArgs(
+  args: ExecutionArgs,
+): PromiseOrValue<ExecutionResult | AsyncIterable<unknown>> {
   return expectEqualPromisesOrValues(
-    subscribe({ schema, document }),
-    createSourceEventStream({ schema, document }),
+    subscribe(args),
+    createSourceEventStream(args),
   );
 }
 
@@ -394,7 +400,7 @@ describe('Subscription Initialization Phase', () => {
     const schema = new GraphQLSchema({ query: DummyQueryType });
     const document = parse('subscription { unknownField }');
 
-    const result = subscribe({ schema, document });
+    const result = subscribeWithBadArgs({ schema, document });
     expectJSON(result).toDeepEqual({
       errors: [
         {
@@ -418,7 +424,7 @@ describe('Subscription Initialization Phase', () => {
     });
     const document = parse('subscription { unknownField }');
 
-    const result = subscribe({ schema, document });
+    const result = subscribeWithBadArgs({ schema, document });
     expectJSON(result).toDeepEqual({
       errors: [
         {
@@ -441,7 +447,7 @@ describe('Subscription Initialization Phase', () => {
     });
 
     // @ts-expect-error
-    expect(() => subscribe({ schema, document: {} })).to.throw();
+    expect(() => subscribeWithBadArgs({ schema, document: {} })).to.throw();
   });
 
   it('throws an error if subscribe does not return an iterator', async () => {
@@ -526,7 +532,7 @@ describe('Subscription Initialization Phase', () => {
 
     // If we receive variables that cannot be coerced correctly, subscribe() will
     // resolve to an ExecutionResult that contains an informative error description.
-    const result = subscribe({ schema, document, variableValues });
+    const result = subscribeWithBadArgs({ schema, document, variableValues });
     expectJSON(result).toDeepEqual({
       errors: [
         {

src/execution/execute.ts

@@ -174,6 +174,12 @@ export function execute(args: ExecutionArgs): PromiseOrValue<ExecutionResult> {
     return { errors: exeContext };
   }
 
+  return executeImpl(exeContext);
+}
+
+function executeImpl(
+  exeContext: ExecutionContext,
+): PromiseOrValue<ExecutionResult> {
   // Return a Promise that will eventually resolve to the data described by
   // The "Response" section of the GraphQL specification.
   //
@@ -319,6 +325,17 @@ export function buildExecutionContext(
   };
 }
 
+function buildPerEventExecutionContext(
+  exeContext: ExecutionContext,
+  payload: unknown,
+): ExecutionContext {
+  return {
+    ...exeContext,
+    rootValue: payload,
+    errors: [],
+  };
+}
+
 /**
  * Implements the "Executing operations" section of the spec.
  */
@@ -1017,20 +1034,29 @@ export function subscribe(
 ): PromiseOrValue<
   AsyncGenerator<ExecutionResult, void, void> | ExecutionResult
 > {
-  const resultOrStream = createSourceEventStream(args);
+  // If a valid execution context cannot be created due to incorrect arguments,
+  // a "Response" with only errors is returned.
+  const exeContext = buildExecutionContext(args);
+
+  // Return early errors if execution context failed.
+  if (!('schema' in exeContext)) {
+    return { errors: exeContext };
+  }
+
+  const resultOrStream = createSourceEventStreamImpl(exeContext);
 
   if (isPromise(resultOrStream)) {
     return resultOrStream.then((resolvedResultOrStream) =>
-      mapSourceToResponse(resolvedResultOrStream, args),
+      mapSourceToResponse(exeContext, resolvedResultOrStream),
     );
   }
 
-  return mapSourceToResponse(resultOrStream, args);
+  return mapSourceToResponse(exeContext, resultOrStream);
 }
 
 function mapSourceToResponse(
+  exeContext: ExecutionContext,
   resultOrStream: ExecutionResult | AsyncIterable<unknown>,
-  args: ExecutionArgs,
 ): PromiseOrValue<
   AsyncGenerator<ExecutionResult, void, void> | ExecutionResult
 > {
@@ -1045,10 +1071,7 @@ function mapSourceToResponse(
   // "ExecuteSubscriptionEvent" algorithm, as it is nearly identical to the
   // "ExecuteQuery" algorithm, for which `execute` is also used.
   return mapAsyncIterator(resultOrStream, (payload: unknown) =>
-    execute({
-      ...args,
-      rootValue: payload,
-    }),
+    executeImpl(buildPerEventExecutionContext(exeContext, payload)),
   );
 }
 
@@ -1092,6 +1115,12 @@ export function createSourceEventStream(
     return { errors: exeContext };
   }
 
+  return createSourceEventStreamImpl(exeContext);
+}
+
+function createSourceEventStreamImpl(
+  exeContext: ExecutionContext,
+): PromiseOrValue<AsyncIterable<unknown> | ExecutionResult> {
   try {
     const eventStream = executeSubscription(exeContext);
     if (isPromise(eventStream)) {