fix(fanout): match SlackApiError by name as well as instanceof (aws-samples#79 review aws-samples#7)

When a bundler ever duplicates the slack-notify module (rare with
NodejsFunction tree-shaking but possible if dual-bundled), two
distinct SlackApiError classes coexist and ``instanceof`` against
one fails for instances of the other. The dispatcher would see a
foreign-class SlackApiError, fall through to the rethrow branch,
and the router would treat it as an infra rejection — flipping a
channel-terminal swallow into infinite Lambda retries.

Add an ``err.name === 'SlackApiError'`` fallback so the swallow
branch fires either way. Mirrors the duck-typed
``GitHubCommentError`` check used elsewhere in the same handler.

Test: synthesise a plain Error with name === 'SlackApiError'
(NOT an instance of the mock's SlackApiError class) and assert
batchItemFailures stays empty — proving the swallow path catches
both shapes.

Author: bgagent

cdk/src/handlers/fanout-task-events.ts

@@ -347,14 +347,24 @@ async function dispatchToSlack(event: FanOutEvent): Promise<void> {
   try {
     await dispatchSlackEvent(effectiveEvent, ddb);
   } catch (err) {
-    if (err instanceof SlackApiError) {
+    // Match SlackApiError by class OR by ``name`` so a bundler that
+    // duplicates the slack-notify module (rare with NodejsFunction
+    // tree-shaking but possible if the module ever gets dual-bundled)
+    // can't make ``instanceof`` silently fail and turn a
+    // channel-terminal swallow into an infinite Lambda retry loop.
+    // Mirrors how ``GitHubCommentError`` is duck-typed by name in
+    // dispatchToGitHubComment (PR #79 review #7).
+    const isSlackApiErr =
+      err instanceof SlackApiError
+      || (err instanceof Error && err.name === 'SlackApiError');
+    if (isSlackApiErr) {
       logger.warn('[fanout/slack] Slack API error — swallowing per channel policy', {
         event: 'fanout.slack.api_error',
         task_id: event.task_id,
         event_id: event.event_id,
         event_type: event.event_type,
         effective_event_type: effectiveType,
-        error: err.message,
+        error: (err as Error).message,
       });
       return;
     }

cdk/test/handlers/fanout-task-events.test.ts

@@ -1185,6 +1185,28 @@ describe('fanout-task-events: Slack dispatcher (issue #64 migration)', () => {
     };
     await expect(handler(event)).resolves.toEqual({ batchItemFailures: [] });
   });
+
+  test('SlackApiError matched by name even when instanceof fails (PR #79 review #7)', async () => {
+    // Defense-in-depth: if a bundler ever duplicates the slack-notify
+    // module, two distinct SlackApiError classes coexist and
+    // ``instanceof`` against one fails for instances of the other.
+    // The dispatcher must fall back to ``err.name === 'SlackApiError'``
+    // so a duplicated-class scenario doesn't flip the channel-terminal
+    // swallow into an infinite retry loop. Synthesise that exact
+    // shape: a plain Error with name === 'SlackApiError', NOT an
+    // instance of the mock's SlackApiError class.
+    const fakeForeignSlackApiError = new Error(
+      'slack chat.postMessage failed: not_authed',
+    );
+    fakeForeignSlackApiError.name = 'SlackApiError';
+    mockDispatchSlackEvent.mockReset().mockRejectedValueOnce(fakeForeignSlackApiError);
+
+    const event: DynamoDBStreamEvent = {
+      Records: [mkEvent('task_completed', 't-slack-foreign-class')],
+    };
+    // Must still be caught — record advances, no batchItemFailures.
+    await expect(handler(event)).resolves.toEqual({ batchItemFailures: [] });
+  });
 });
 
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