From 0d662d089d5483f44ebcf36a87904c920e4e9eee Mon Sep 17 00:00:00 2001
From: Kazuhiro Sera <ksera@slack-corp.com>
Date: Wed, 1 Apr 2020 19:12:52 +0900
Subject: [PATCH 1/2] Add failing tests

---
 src/App.spec.ts | 83 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 83 insertions(+)

diff --git a/src/App.spec.ts b/src/App.spec.ts
index aee109e35..8c5d5487f 100644
--- a/src/App.spec.ts
+++ b/src/App.spec.ts
@@ -1257,6 +1257,89 @@ describe('App', () => {
           assert.equal(fakeErrorHandler.callCount, dummyReceiverEvents.length);
         });
       });
+
+      describe('processBeforeResponse', () => {
+
+        it('should call ack() after event listener completion', async () => {
+          // Arrange
+          const App = await importApp(overrides); // tslint:disable-line:variable-name
+          const fakeLogger = createFakeLogger();
+          const app = new App({
+            logger: fakeLogger,
+            receiver: fakeReceiver,
+            authorize: sinon.fake.resolves(dummyAuthorizationResult),
+            processBeforeResponse: true,
+          });
+
+          let acknowledged = false;
+
+          app.event('app_home_opened', async ({}) => {
+            await new Promise(resolve => setTimeout(resolve, 200));
+            assert.isFalse(acknowledged);
+          });
+
+          const event = {
+            body: {
+              type: 'event_callback',
+              token: 'XXYYZZ',
+              team_id: 'TXXXXXXXX',
+              api_app_id: 'AXXXXXXXXX',
+              event: {
+                type: 'app_home_opened',
+                event_ts: '1234567890.123456',
+                user: 'UXXXXXXX1',
+                text: 'hello friends!',
+                tab: 'home',
+                view: {},
+              },
+            },
+            respond: noop,
+            ack: async (_res: any) => {
+              acknowledged = true;
+            },
+          };
+
+          // Act
+          await fakeReceiver.sendEvent(event);
+
+          // Assert
+          assert.isTrue(acknowledged);
+        });
+
+        it('should call ack() after command listener completion', async () => {
+          // Arrange
+          const App = await importApp(overrides); // tslint:disable-line:variable-name
+          const fakeLogger = createFakeLogger();
+          const app = new App({
+            logger: fakeLogger,
+            receiver: fakeReceiver,
+            authorize: sinon.fake.resolves(dummyAuthorizationResult),
+            processBeforeResponse: true,
+          });
+
+          let acknowledged = false;
+
+          app.command('/hello', async ({ ack }) => {
+            assert.isFalse(acknowledged);
+            await ack();
+          });
+
+          const event = {
+            ...baseEvent,
+            body: {
+              command: '/hello',
+            },
+            ack: async (_res: any) => {
+              acknowledged = true;
+            },
+          };
+
+          // Act
+          await fakeReceiver.sendEvent(event);
+          // Assert
+          assert.isTrue(acknowledged);
+        });
+      });
     });
   });
 });

From c2bb0221d7f456b64a0afff9c873b71a212a0158 Mon Sep 17 00:00:00 2001
From: Kazuhiro Sera <ksera@slack-corp.com>
Date: Wed, 1 Apr 2020 19:23:53 +0900
Subject: [PATCH 2/2] Re-fix #395 by adjusting the processBeforeResponse option

---
 src/App.ts             | 14 ++++++++++++--
 src/ExpressReceiver.ts | 22 +++++++++++++++++-----
 2 files changed, 29 insertions(+), 7 deletions(-)

diff --git a/src/App.ts b/src/App.ts
index 58b379ae0..fe8052abf 100644
--- a/src/App.ts
+++ b/src/App.ts
@@ -154,6 +154,9 @@ export default class App {
   /** Receiver - ingests events from the Slack platform */
   private receiver: Receiver;
 
+  /** Always call ack() after event listener completion if true */
+  private processBeforeResponse: boolean = false;
+
   /** Logger */
   private logger: Logger;
 
@@ -256,6 +259,7 @@ export default class App {
         });
       }
     }
+    this.processBeforeResponse = processBeforeResponse;
 
     // Conditionally use a global middleware that ignores events (including messages) that are sent from this app
     if (ignoreSelf) {
@@ -580,8 +584,11 @@ export default class App {
     if (type !== IncomingEventType.Event) {
       listenerArgs.ack = ack;
     } else {
-      // Events API requests are acknowledged right away, since there's no data expected
-      await ack();
+      // If processBeforeResponse is true, ack() call should be after the listener completion
+      if (!this.processBeforeResponse) {
+        // Events API requests are acknowledged right away, since there's no data expected
+        await ack();
+      }
     }
 
     // Get the client arg
@@ -635,6 +642,9 @@ export default class App {
           } else if (rejectedListenerResults.length > 1) {
             throw new MultipleListenerError(rejectedListenerResults.map(rlr => rlr.reason));
           }
+          if (this.processBeforeResponse && type === IncomingEventType.Event) {
+            await ack();
+          }
         },
       );
     } catch (error) {
diff --git a/src/ExpressReceiver.ts b/src/ExpressReceiver.ts
index f125bfafa..ea55d1560 100644
--- a/src/ExpressReceiver.ts
+++ b/src/ExpressReceiver.ts
@@ -96,12 +96,24 @@ export default class ExpressReceiver implements Receiver {
 
     try {
       await this.bolt?.processEvent(event);
-      if (storedResponse !== undefined) {
-        if (typeof storedResponse === 'string') {
-          res.send(storedResponse);
-        } else {
-          res.json(storedResponse);
+
+      if (this.processBeforeResponse) {
+        let spentMillis = 0;
+        const millisForTimeout = 10;
+        // Wait here until the isAcknowledged is marked as true or 3 seconds have passed
+        while (!isAcknowledged && spentMillis < 3000) {
+          await new Promise(resolve => setTimeout(resolve, millisForTimeout));
+          spentMillis += millisForTimeout;
         }
+        if (isAcknowledged) {
+          this.logger.debug(`The listener execution completed in ${spentMillis} millis`);
+          if (typeof storedResponse === 'string') {
+            res.send(storedResponse);
+          }  else {
+            res.json(storedResponse);
+          }
+          this.logger.debug('Acknowledged after the listener completion');
+        } // Otherwise, this Bolt app never responds to this request and above setTimeout outputs an error message
       }
     } catch (err) {
       res.send(500);