fix(core): Add missing error patching and support incremental results in subscriptionExchange

.changeset/fuzzy-baboons-add.md

@@ -0,0 +1,5 @@
+---
+'@urql/core': minor
+---
+
+Update `subscriptionExchange` to support incremental results out of the box. If a subscription proactively completes, results are also now updated with `hasNext: false`.

.changeset/poor-pumpkins-greet.md

@@ -0,0 +1,5 @@
+---
+'@urql/core': patch
+---
+
+Fix incremental results not merging `errors` from subsequent non-incremental results.

packages/core/src/exchanges/subscription.ts

@@ -15,6 +15,7 @@ import {
   makeResult,
   makeErrorResult,
   makeOperation,
+  mergeResultPatch,
 } from '../utils';
 
 import {
@@ -158,14 +159,23 @@ export const subscriptionExchange = ({
     return make<OperationResult>(({ next, complete }) => {
       let isComplete = false;
       let sub: Subscription | void;
+      let result: OperationResult | void;
 
       Promise.resolve().then(() => {
         if (isComplete) return;
 
         sub = observableish.subscribe({
-          next: result => next(makeResult(operation, result)),
-          error: err => next(makeErrorResult(operation, err)),
-          complete: () => {
+          next(nextResult) {
+            next(
+              (result = result
+                ? mergeResultPatch(result, nextResult)
+                : makeResult(operation, nextResult))
+            );
+          },
+          error(error) {
+            next(makeErrorResult(operation, error));
+          },
+          complete() {
             if (!isComplete) {
               isComplete = true;
               if (operation.kind === 'subscription') {
@@ -174,6 +184,8 @@ export const subscriptionExchange = ({
                 );
               }
 
+              if (result && result.hasNext)
+                next(mergeResultPatch(result, { hasNext: false }));
               complete();
             }
           },

packages/core/src/utils/result.ts

@@ -71,9 +71,9 @@ export const mergeResultPatch = (
   response?: any
 ): OperationResult => {
   let data: ExecutionResult['data'];
+  let errors = prevResult.error ? prevResult.error.graphQLErrors : [];
   let hasExtensions = !!prevResult.extensions || !!nextResult.extensions;
   const extensions = { ...prevResult.extensions, ...nextResult.extensions };
-  const errors = prevResult.error ? prevResult.error.graphQLErrors : [];
 
   let incremental = nextResult.incremental;
 
@@ -115,6 +115,7 @@ export const mergeResultPatch = (
     }
   } else {
     data = nextResult.data || prevResult.data;
+    errors = (nextResult.errors as any[]) || errors;
   }
 
   return {