allow nested defers at the same level

if we allow nesting of defers at the same level then we have to handle the case where a defer is completely empty -- not just empty because its fields are also contained within a parent.

this means that collectFields must directly return the new defer usages so that even empty defer usages can be used to create fragments so that we can track their children

src/execution/__tests__/defer-test.ts

@@ -355,28 +355,32 @@ describe('Execute: defer directive', () => {
         data: {
           hero: {},
         },
-        pending: [
-          { id: '0', path: ['hero'], label: 'DeferTop' },
-          { id: '1', path: ['hero'], label: 'DeferNested' },
-        ],
+        pending: [{ id: '0', path: ['hero'], label: 'DeferTop' }],
         hasNext: true,
       },
       {
+        pending: [{ id: '1', path: ['hero'], label: 'DeferNested' }],
         incremental: [
           {
             data: {
               id: '1',
             },
             id: '0',
           },
+        ],
+        completed: [{ id: '0' }],
+        hasNext: true,
+      },
+      {
+        incremental: [
           {
             data: {
               friends: [{ name: 'Han' }, { name: 'Leia' }, { name: 'C-3PO' }],
             },
             id: '1',
           },
         ],
-        completed: [{ id: '0' }, { id: '1' }],
+        completed: [{ id: '1' }],
         hasNext: false,
       },
     ]);
@@ -472,6 +476,35 @@ describe('Execute: defer directive', () => {
     });
   });
 
+  it('Emits children of empty defer fragments', async () => {
+    const document = parse(`
+      query HeroNameQuery {
+        hero {
+          ... @defer {
+            ... @defer {
+              name
+            }
+          }
+        }
+      }
+    `);
+    const result = await complete(document);
+    expectJSON(result).toDeepEqual([
+      {
+        data: {
+          hero: {},
+        },
+        pending: [{ id: '0', path: ['hero'] }],
+        hasNext: true,
+      },
+      {
+        incremental: [{ data: { name: 'Luke' }, id: '0' }],
+        completed: [{ id: '0' }],
+        hasNext: false,
+      },
+    ]);
+  });
+
   it('Can separately emit defer fragments with different labels with varying fields', async () => {
     const document = parse(`
       query HeroNameQuery {

src/execution/buildFieldPlan.ts

@@ -8,7 +8,6 @@ export type DeferUsageSet = ReadonlySet<DeferUsage>;
 export interface FieldGroup {
   fields: ReadonlyArray<FieldDetails>;
   deferUsages?: DeferUsageSet | undefined;
-  knownDeferUsages?: DeferUsageSet | undefined;
 }
 
 export type GroupedFieldSet = Map<string, FieldGroup>;
@@ -21,21 +20,15 @@ export interface NewGroupedFieldSetDetails {
 export function buildFieldPlan(
   fields: Map<string, ReadonlyArray<FieldDetails>>,
   parentDeferUsages: DeferUsageSet = new Set<DeferUsage>(),
-  knownDeferUsages: DeferUsageSet = new Set<DeferUsage>(),
 ): {
   groupedFieldSet: GroupedFieldSet;
   newGroupedFieldSetDetailsMap: Map<DeferUsageSet, NewGroupedFieldSetDetails>;
-  newDeferUsages: ReadonlyArray<DeferUsage>;
 } {
-  const newDeferUsages: Set<DeferUsage> = new Set<DeferUsage>();
-  const newKnownDeferUsages = new Set<DeferUsage>(knownDeferUsages);
-
   const groupedFieldSet = new Map<
     string,
     {
       fields: Array<FieldDetails>;
       deferUsages: DeferUsageSet;
-      knownDeferUsages: DeferUsageSet;
     }
   >();
 
@@ -47,7 +40,6 @@ export function buildFieldPlan(
         {
           fields: Array<FieldDetails>;
           deferUsages: DeferUsageSet;
-          knownDeferUsages: DeferUsageSet;
         }
       >;
       shouldInitiateDefer: boolean;
@@ -72,10 +64,6 @@ export function buildFieldPlan(
         continue;
       }
       deferUsageSet.add(deferUsage);
-      if (!knownDeferUsages.has(deferUsage)) {
-        newDeferUsages.add(deferUsage);
-        newKnownDeferUsages.add(deferUsage);
-      }
     }
     if (inOriginalResult) {
       deferUsageSet.clear();
@@ -99,7 +87,6 @@ export function buildFieldPlan(
         fieldGroup = {
           fields: [],
           deferUsages: deferUsageSet,
-          knownDeferUsages: newKnownDeferUsages,
         };
         groupedFieldSet.set(responseKey, fieldGroup);
       }
@@ -140,7 +127,6 @@ export function buildFieldPlan(
       fieldGroup = {
         fields: [],
         deferUsages: deferUsageSet,
-        knownDeferUsages: newKnownDeferUsages,
       };
       newGroupedFieldSet.set(responseKey, fieldGroup);
     }
@@ -150,7 +136,6 @@ export function buildFieldPlan(
   return {
     groupedFieldSet,
     newGroupedFieldSetDetailsMap,
-    newDeferUsages: Array.from(newDeferUsages),
   };
 }
 

src/execution/collectFields.ts

@@ -60,8 +60,12 @@ export function collectFields(
   variableValues: { [variable: string]: unknown },
   runtimeType: GraphQLObjectType,
   operation: OperationDefinitionNode,
-): Map<string, ReadonlyArray<FieldDetails>> {
+): {
+  fields: Map<string, ReadonlyArray<FieldDetails>>;
+  newDeferUsages: ReadonlyArray<DeferUsage>;
+} {
   const groupedFieldSet = new AccumulatorMap<string, FieldDetails>();
+  const newDeferUsages: Array<DeferUsage> = [];
   const context: CollectFieldsContext = {
     schema,
     fragments,
@@ -71,8 +75,13 @@ export function collectFields(
     visitedFragmentNames: new Set(),
   };
 
-  collectFieldsImpl(context, operation.selectionSet, groupedFieldSet);
-  return groupedFieldSet;
+  collectFieldsImpl(
+    context,
+    operation.selectionSet,
+    groupedFieldSet,
+    newDeferUsages,
+  );
+  return { fields: groupedFieldSet, newDeferUsages };
 }
 
 /**
@@ -93,7 +102,10 @@ export function collectSubfields(
   operation: OperationDefinitionNode,
   returnType: GraphQLObjectType,
   fieldDetails: ReadonlyArray<FieldDetails>,
-): Map<string, ReadonlyArray<FieldDetails>> {
+): {
+  fields: Map<string, ReadonlyArray<FieldDetails>>;
+  newDeferUsages: ReadonlyArray<DeferUsage>;
+} {
   const context: CollectFieldsContext = {
     schema,
     fragments,
@@ -103,6 +115,7 @@ export function collectSubfields(
     visitedFragmentNames: new Set(),
   };
   const subGroupedFieldSet = new AccumulatorMap<string, FieldDetails>();
+  const newDeferUsages: Array<DeferUsage> = [];
 
   for (const fieldDetail of fieldDetails) {
     const node = fieldDetail.node;
@@ -111,19 +124,23 @@ export function collectSubfields(
         context,
         node.selectionSet,
         subGroupedFieldSet,
+        newDeferUsages,
         fieldDetail.deferUsage,
       );
     }
   }
 
-  return subGroupedFieldSet;
+  return {
+    fields: subGroupedFieldSet,
+    newDeferUsages,
+  };
 }
 
 function collectFieldsImpl(
   context: CollectFieldsContext,
   selectionSet: SelectionSetNode,
   groupedFieldSet: AccumulatorMap<string, FieldDetails>,
-  parentDeferUsage?: DeferUsage,
+  newDeferUsages: Array<DeferUsage>,
   deferUsage?: DeferUsage,
 ): void {
   const {
@@ -143,7 +160,7 @@ function collectFieldsImpl(
         }
         groupedFieldSet.add(getFieldEntryKey(selection), {
           node: selection,
-          deferUsage: deferUsage ?? parentDeferUsage,
+          deferUsage,
         });
         break;
       }
@@ -159,16 +176,27 @@ function collectFieldsImpl(
           operation,
           variableValues,
           selection,
-          parentDeferUsage,
+          deferUsage,
         );
 
-        collectFieldsImpl(
-          context,
-          selection.selectionSet,
-          groupedFieldSet,
-          parentDeferUsage,
-          newDeferUsage ?? deferUsage,
-        );
+        if (!newDeferUsage) {
+          collectFieldsImpl(
+            context,
+            selection.selectionSet,
+            groupedFieldSet,
+            newDeferUsages,
+            deferUsage,
+          );
+        } else {
+          newDeferUsages.push(newDeferUsage);
+          collectFieldsImpl(
+            context,
+            selection.selectionSet,
+            groupedFieldSet,
+            newDeferUsages,
+            newDeferUsage,
+          );
+        }
 
         break;
       }
@@ -179,7 +207,7 @@ function collectFieldsImpl(
           operation,
           variableValues,
           selection,
-          parentDeferUsage,
+          deferUsage,
         );
 
         if (
@@ -199,15 +227,23 @@ function collectFieldsImpl(
         }
         if (!newDeferUsage) {
           visitedFragmentNames.add(fragName);
+          collectFieldsImpl(
+            context,
+            fragment.selectionSet,
+            groupedFieldSet,
+            newDeferUsages,
+            deferUsage,
+          );
+        } else {
+          newDeferUsages.push(newDeferUsage);
+          collectFieldsImpl(
+            context,
+            fragment.selectionSet,
+            groupedFieldSet,
+            newDeferUsages,
+            newDeferUsage,
+          );
         }
-
-        collectFieldsImpl(
-          context,
-          fragment.selectionSet,
-          groupedFieldSet,
-          parentDeferUsage,
-          newDeferUsage ?? deferUsage,
-        );
         break;
       }
     }

src/execution/execute.ts

@@ -91,19 +91,18 @@ const buildSubFieldPlan = memoize3(
     returnType: GraphQLObjectType,
     fieldGroup: FieldGroup,
   ) => {
-    const subFields = collectSubfields(
+    const { fields: subFields, newDeferUsages } = collectSubfields(
       exeContext.schema,
       exeContext.fragments,
       exeContext.variableValues,
       exeContext.operation,
       returnType,
       fieldGroup.fields,
     );
-    return buildFieldPlan(
-      subFields,
-      fieldGroup.deferUsages,
-      fieldGroup.knownDeferUsages,
-    );
+    return {
+      ...buildFieldPlan(subFields, fieldGroup.deferUsages),
+      newDeferUsages,
+    };
   },
 );
 
@@ -408,14 +407,14 @@ function executeOperation(
     );
   }
 
-  const fields = collectFields(
+  const { fields, newDeferUsages } = collectFields(
     schema,
     fragments,
     variableValues,
     rootType,
     operation,
   );
-  const { groupedFieldSet, newGroupedFieldSetDetailsMap, newDeferUsages } =
+  const { groupedFieldSet, newGroupedFieldSetDetailsMap } =
     buildFieldPlan(fields);
 
   const newDeferMap = addNewDeferredFragments(
@@ -1807,7 +1806,7 @@ function executeSubscription(
     );
   }
 
-  const fields = collectFields(
+  const { fields } = collectFields(
     schema,
     fragments,
     variableValues,

src/validation/rules/SingleFieldSubscriptionsRule.ts

@@ -49,7 +49,7 @@ export function SingleFieldSubscriptionsRule(
               fragments[definition.name.value] = definition;
             }
           }
-          const fields = collectFields(
+          const { fields } = collectFields(
             schema,
             fragments,
             variableValues,