deduplicate (mask) leaf values that have already been sent

causes some additional minor changes to `hasNext` property on some payloads due to modification of returned promises resulting in an extra tick

src/execution/__tests__/defer-test.ts

@@ -591,7 +591,7 @@ describe('Execute: defer directive', () => {
     ]);
   });
 
-  it('Can deduplicate initial fields with deferred fragments at multiple levels', async () => {
+  it('Can deduplicate fields with deferred fragments at multiple levels', async () => {
     const document = parse(`
       query {
         hero {
@@ -650,19 +650,14 @@ describe('Execute: defer directive', () => {
           },
           {
             data: {
-              deeperObject: {
-                bar: 'bar',
-                baz: 'baz',
-              },
+              deeperObject: {},
             },
             path: ['hero', 'nestedObject'],
           },
           {
             data: {
               nestedObject: {
-                deeperObject: {
-                  bar: 'bar',
-                },
+                deeperObject: {},
               },
             },
             path: ['hero'],
@@ -732,7 +727,7 @@ describe('Execute: defer directive', () => {
     ]);
   });
 
-  it('can deduplicate initial fields with deferred fragments in different branches at multiple non-overlapping levels', async () => {
+  it('can deduplicate fields with deferred fragments in different branches at multiple non-overlapping levels', async () => {
     const document = parse(`
       query {
         a {
@@ -789,9 +784,7 @@ describe('Execute: defer directive', () => {
             data: {
               a: {
                 b: {
-                  e: {
-                    f: 'f',
-                  },
+                  e: {},
                 },
               },
               g: {

src/execution/__tests__/stream-test.ts

@@ -1170,9 +1170,6 @@ describe('Execute: stream directive', () => {
             ],
           },
         ],
-        hasNext: true,
-      },
-      {
         hasNext: false,
       },
     ]);
@@ -1355,9 +1352,6 @@ describe('Execute: stream directive', () => {
             ],
           },
         ],
-        hasNext: true,
-      },
-      {
         hasNext: false,
       },
     ]);

src/execution/execute.ts

@@ -124,6 +124,7 @@ export interface ExecutionContext {
   errors: Array<GraphQLError>;
   subsequentPayloads: Set<AsyncPayloadRecord>;
   branches: WeakMap<GroupedFieldSet, Set<string>>;
+  leaves: Set<string>;
 }
 
 /**
@@ -503,6 +504,7 @@ export function buildExecutionContext(
     subscribeFieldResolver: subscribeFieldResolver ?? defaultFieldResolver,
     subsequentPayloads: new Set(),
     branches: new WeakMap(),
+    leaves: new Set(),
     errors: [],
   };
 }
@@ -516,6 +518,7 @@ function buildPerEventExecutionContext(
     rootValue: payload,
     subsequentPayloads: new Set(),
     branches: new WeakMap(),
+    leaves: new Set(),
     errors: [],
   };
 }
@@ -703,6 +706,8 @@ function executeFields(
   const results = Object.create(null);
   let containsPromise = false;
 
+  const shouldMask = Object.create(null);
+
   try {
     for (const [responseName, fieldSet] of groupedFieldSet) {
       const fieldPath = addPath(path, responseName, parentType.name);
@@ -716,6 +721,20 @@ function executeFields(
       const returnType = fieldDef.type;
 
       if (shouldExecuteFieldSet(returnType, fieldSet, deferDepth)) {
+        if (
+          asyncPayloadRecord !== undefined &&
+          isLeafType(getNamedType(returnType))
+        ) {
+          shouldMask[responseName] = () => {
+            const key = pathToArray(fieldPath).join('.');
+            if (exeContext.leaves.has(key)) {
+              return true;
+            }
+            exeContext.leaves.add(key);
+            return false;
+          };
+        }
+
         const result = executeField(
           exeContext,
           parentType,
@@ -748,13 +767,27 @@ function executeFields(
 
   // If there are no promises, we can just return the object
   if (!containsPromise) {
-    return results;
+    return asyncPayloadRecord === undefined
+      ? results
+      : new Proxy(results, {
+          ownKeys: (target) =>
+            Reflect.ownKeys(target).filter((key) => !shouldMask[key]?.()),
+        });
   }
 
   // Otherwise, results is a map from field name to the result of resolving that
   // field, which is possibly a promise. Return a promise that will return this
   // same map, but with any promises replaced with the values they resolved to.
-  return promiseForObject(results);
+  const promisedResult = promiseForObject(results);
+  return asyncPayloadRecord === undefined
+    ? promisedResult
+    : promisedResult.then(
+        (resolved) =>
+          new Proxy(resolved, {
+            ownKeys: (target) =>
+              Reflect.ownKeys(target).filter((key) => !shouldMask[key]?.()),
+          }),
+      );
 }
 
 /**