From 874a4c0d9f5dd187e74f25e193a2a9c0c3ed84b9 Mon Sep 17 00:00:00 2001
From: Shuhui Luo <107524008+shuhuiluo@users.noreply.github.com>
Date: Sun, 14 Dec 2025 23:34:35 -0500
Subject: [PATCH] fix: retry thread lookup to handle anchor race condition
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

When a comment arrives before the PR anchor mapping is committed to DB,
the thread lookup fails and the comment is posted standalone. This adds
exponential backoff retries (250ms → 500ms → 1000ms) to handle the race.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
---
 src/constants.ts                         |  8 ++++++
 src/services/message-delivery-service.ts | 36 ++++++++++++++++++++----
 2 files changed, 38 insertions(+), 6 deletions(-)

diff --git a/src/constants.ts b/src/constants.ts
index aa32a0b..165b144 100644
--- a/src/constants.ts
+++ b/src/constants.ts
@@ -102,3 +102,11 @@ export const OAUTH_STATE_CLEANUP_INTERVAL_MS = 60 * 60 * 1000;
  * After this period, mappings are cleaned up and threads can no longer be grouped.
  */
 export const MESSAGE_MAPPING_EXPIRY_DAYS = 30;
+
+/**
+ * Thread lookup retry settings for race condition handling.
+ * When a comment arrives before anchor mapping is committed, retry with exponential backoff.
+ * Schedule: 250ms → 500ms → 1000ms (total max: 1.75s)
+ */
+export const THREAD_LOOKUP_INITIAL_DELAY_MS = 250;
+export const THREAD_LOOKUP_MAX_RETRIES = 3;
diff --git a/src/services/message-delivery-service.ts b/src/services/message-delivery-service.ts
index 1f6c6a6..46af3d7 100644
--- a/src/services/message-delivery-service.ts
+++ b/src/services/message-delivery-service.ts
@@ -1,6 +1,10 @@
 import { and, eq, gt, lt } from "drizzle-orm";
 
-import { MESSAGE_MAPPING_EXPIRY_DAYS } from "../constants";
+import {
+  MESSAGE_MAPPING_EXPIRY_DAYS,
+  THREAD_LOOKUP_INITIAL_DELAY_MS,
+  THREAD_LOOKUP_MAX_RETRIES,
+} from "../constants";
 import { db } from "../db";
 import { messageMappings } from "../db/schema";
 import type { TownsBot } from "../types/bot";
@@ -63,20 +67,40 @@ export class MessageDeliveryService {
     } = params;
 
     try {
-      // Thread lookup: non-anchors reply to their parent anchor
-      const threadId =
+      // Thread lookup with retry: handles race condition where comment arrives
+      // before anchor mapping is committed (exponential backoff: 0 → 250 → 500 → 1000ms)
+      let threadId: string | undefined;
+      if (
         entityContext &&
         !entityContext.isAnchor &&
         entityContext.parentType &&
         entityContext.parentNumber != null
-          ? ((await this.getMessageId(
+      ) {
+        for (let attempt = 0; attempt <= THREAD_LOOKUP_MAX_RETRIES; attempt++) {
+          const delay =
+            attempt > 0
+              ? THREAD_LOOKUP_INITIAL_DELAY_MS * 2 ** (attempt - 1)
+              : 0;
+          if (delay) await new Promise(r => setTimeout(r, delay));
+          threadId =
+            (await this.getMessageId(
               spaceId,
               channelId,
               repoFullName,
               entityContext.parentType,
               String(entityContext.parentNumber)
-            )) ?? undefined)
-          : undefined;
+            )) ?? undefined;
+          if (threadId) {
+            if (delay)
+              console.log(`Thread lookup succeeded after ${delay}ms retry`);
+            break;
+          }
+        }
+        if (!threadId)
+          console.log(
+            `Thread lookup failed after ${THREAD_LOOKUP_MAX_RETRIES} retries for ${entityContext.parentType}:${entityContext.parentNumber}`
+          );
+      }
 
       // Skip synthetic thread replies (closed/reopened) when anchor doesn't exist
       // These have githubEntityType === parentType (e.g., "pr" for PR close event)