[6.x] Fix regression in CallClient, which caused request errors like timeouts to result in fatal errors. (#22558)

src/ui/public/courier/fetch/__tests__/call_client.js

@@ -119,6 +119,35 @@ describe('callClient', () => {
         expect(results).to.eql([1]);
       });
     });
+
+    it(`resolves the promise despite the request failing`, () => {
+      addSearchStrategy({
+        id: 'fail',
+        isViable: indexPattern => {
+          return indexPattern.type === 'fail';
+        },
+        search: () => {
+          return {
+            searching: Promise.reject(new Error('Search failed')),
+            failedSearchRequests: [],
+            abort: () => {},
+          };
+        },
+      });
+
+      const searchRequestFail = createSearchRequest('fail', {
+        source: {
+          getField: () => ({ type: 'fail' }),
+        },
+      });
+
+      searchRequests = [ searchRequestFail ];
+      const callingClient = callClient(searchRequests);
+
+      return callingClient.then(results => {
+        expect(results).to.eql(undefined);
+      });
+    });
   });
 
   describe('implementation', () => {

src/ui/public/courier/fetch/call_client.js

@@ -183,17 +183,18 @@ export function CallClientProvider(Private, Promise, es) {
       }
     });
 
-    // If there are any errors, notify the searchRequests of them.
-    defer.promise.catch((err) => {
+    // Return the promise which acts as our vehicle for providing search responses to the consumer.
+    // However, if there are any errors, notify the searchRequests of them *instead* of bubbling
+    // them up to the consumer.
+    return defer.promise.catch((err) => {
+      // By returning the return value of this catch() without rethrowing the error, we delegate
+      // error-handling to the searchRequest instead of the consumer.
       searchRequests.forEach((searchRequest, index) => {
         if (searchRequestsAndStatuses[index] !== ABORTED) {
           searchRequest.handleFailure(err);
         }
       });
     });
-
-    // Return the promise which acts as our vehicle for providing search responses to the consumer.
-    return defer.promise;
   }
 
   return callClient;