refactor: DRY with context building and *Impl functions

The `execute`/`executeImpl` and `createSourceEventStream`/`createSourceEventStreamImpl` functions follow the same basic pattern of building the contet and using it to run a function. This PR extracts that pattern into a separate function.

For good measure, the same pattern in applied to the soon-to-be-deprecated `subscribe` function.

Hheavier refactoring is on the way from @IvanGoncharov (see #3639 (review)), but in the meantime, this consolidates the common pattern without any breaking changes.

Author: yaacovCR

src/execution/execute.ts

@@ -165,6 +165,13 @@ export interface ExecutionArgs {
  * a GraphQLError will be thrown immediately explaining the invalid input.
  */
 export function execute(args: ExecutionArgs): PromiseOrValue<ExecutionResult> {
+  return prepareContextAndRunFn(args, executeImpl);
+}
+
+function prepareContextAndRunFn<T>(
+  args: ExecutionArgs,
+  fn: (exeContext: ExecutionContext) => T,
+): ExecutionResult | T {
   // If a valid execution context cannot be created due to incorrect arguments,
   // a "Response" with only errors is returned.
   const exeContext = buildExecutionContext(args);
@@ -174,7 +181,7 @@ export function execute(args: ExecutionArgs): PromiseOrValue<ExecutionResult> {
     return { errors: exeContext };
   }
 
-  return executeImpl(exeContext);
+  return fn(exeContext);
 }
 
 function executeImpl(
@@ -1034,15 +1041,14 @@ export function subscribe(
 ): PromiseOrValue<
   AsyncGenerator<ExecutionResult, void, void> | ExecutionResult
 > {
-  // 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 };
-  }
+  return prepareContextAndRunFn(args, subscribeImpl);
+}
 
+export function subscribeImpl(
+  exeContext: ExecutionContext,
+): PromiseOrValue<
+  AsyncGenerator<ExecutionResult, void, void> | ExecutionResult
+> {
   const resultOrStream = createSourceEventStreamImpl(exeContext);
 
   if (isPromise(resultOrStream)) {
@@ -1106,16 +1112,7 @@ function mapSourceToResponse(
 export function createSourceEventStream(
   args: ExecutionArgs,
 ): PromiseOrValue<AsyncIterable<unknown> | ExecutionResult> {
-  // 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 };
-  }
-
-  return createSourceEventStreamImpl(exeContext);
+  return prepareContextAndRunFn(args, createSourceEventStreamImpl);
 }
 
 function createSourceEventStreamImpl(