feat(core): Support Apollo Federation subscriptions multipart payloads (#3499)

Author: JoviDeCroock

.changeset/small-cheetahs-jam.md

@@ -0,0 +1,5 @@
+---
+'@urql/core': minor
+---
+
+Support [Apollo Federation's format](https://www.apollographql.com/docs/router/executing-operations/subscription-multipart-protocol/) for subscription results in `multipart/mixed` responses (result properties essentially are namespaced on a `payload` key)

packages/core/src/types.ts

@@ -223,6 +223,7 @@ export interface ExecutionResult {
    * @see {@link https://github.com/graphql/graphql-wg/blob/main/rfcs/DeferStream.md#payload-format} for the DeferStream spec
    */
   hasNext?: boolean;
+  payload?: Omit<ExecutionResult, 'payload'>;
 }
 
 /** A source of {@link OperationResult | OperationResults}, convertable to a promise, subscribable, or Wonka Source.

packages/core/src/utils/result.test.ts

@@ -2,6 +2,8 @@ import { describe, it, expect } from 'vitest';
 import { OperationResult } from '../types';
 import { queryOperation, subscriptionOperation } from '../test-utils';
 import { makeResult, mergeResultPatch } from './result';
+import { GraphQLError } from '@0no-co/graphql.web';
+import { CombinedError } from './error';
 
 describe('makeResult', () => {
   it('adds extensions and errors correctly', () => {
@@ -180,6 +182,62 @@ describe('mergeResultPatch (defer/stream pre June-2023)', () => {
     expect(merged.hasNext).toBe(true);
   });
 
+  it('should work with the payload property', () => {
+    const prevResult: OperationResult = {
+      operation: subscriptionOperation,
+      data: {
+        __typename: 'Subscription',
+        event: 1,
+      },
+      stale: false,
+      hasNext: true,
+    };
+
+    const merged = mergeResultPatch(prevResult, {
+      payload: {
+        data: {
+          __typename: 'Subscription',
+          event: 2,
+        },
+      },
+    });
+
+    expect(merged.data).not.toBe(prevResult.data);
+    expect(merged.data.event).toBe(2);
+    expect(merged.hasNext).toBe(true);
+  });
+
+  it('should work with the payload property and errors', () => {
+    const prevResult: OperationResult = {
+      operation: subscriptionOperation,
+      data: {
+        __typename: 'Subscription',
+        event: 1,
+      },
+      stale: false,
+      hasNext: true,
+    };
+
+    const merged = mergeResultPatch(prevResult, {
+      payload: {
+        data: {
+          __typename: 'Subscription',
+          event: 2,
+        },
+      },
+      errors: [new GraphQLError('Something went horribly wrong')],
+    });
+
+    expect(merged.data).not.toBe(prevResult.data);
+    expect(merged.data.event).toBe(2);
+    expect(merged.error).toEqual(
+      new CombinedError({
+        graphQLErrors: [new GraphQLError('Something went horribly wrong')],
+      })
+    );
+    expect(merged.hasNext).toBe(true);
+  });
+
   it('should ignore invalid patches', () => {
     const prevResult: OperationResult = {
       operation: queryOperation,

packages/core/src/utils/result.ts

@@ -92,8 +92,12 @@ export const mergeResultPatch = (
   pending?: ExecutionResult['pending']
 ): OperationResult => {
   let errors = prevResult.error ? prevResult.error.graphQLErrors : [];
-  let hasExtensions = !!prevResult.extensions || !!nextResult.extensions;
-  const extensions = { ...prevResult.extensions, ...nextResult.extensions };
+  let hasExtensions =
+    !!prevResult.extensions || !!(nextResult.payload || nextResult).extensions;
+  const extensions = {
+    ...prevResult.extensions,
+    ...(nextResult.payload || nextResult).extensions,
+  };
 
   let incremental = nextResult.incremental;
 
@@ -146,8 +150,11 @@ export const mergeResultPatch = (
       }
     }
   } else {
-    withData.data = nextResult.data || prevResult.data;
-    errors = (nextResult.errors as any[]) || errors;
+    withData.data = (nextResult.payload || nextResult).data || prevResult.data;
+    errors =
+      (nextResult.errors as any[]) ||
+      (nextResult.payload && nextResult.payload.errors) ||
+      errors;
   }
 
   return {