From 6d0cd54ac17bf2f8720157861f2cce6b1eb9e1e7 Mon Sep 17 00:00:00 2001
From: Vignesh Natarajan <vigneshnatarajan92@gmail.com>
Date: Sat, 14 Feb 2026 17:55:13 -0800
Subject: [PATCH] Slack: bound thread starter cache growth

---
 .../media.thread-starter-cache.test.ts        | 87 +++++++++++++++++++
 src/slack/monitor/media.ts                    | 49 ++++++++++-
 2 files changed, 133 insertions(+), 3 deletions(-)
 create mode 100644 src/slack/monitor/media.thread-starter-cache.test.ts

diff --git a/src/slack/monitor/media.thread-starter-cache.test.ts b/src/slack/monitor/media.thread-starter-cache.test.ts
new file mode 100644
index 000000000..45278acfe
--- /dev/null
+++ b/src/slack/monitor/media.thread-starter-cache.test.ts
@@ -0,0 +1,87 @@
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { resetSlackThreadStarterCacheForTest, resolveSlackThreadStarter } from "./media.js";
+
+describe("resolveSlackThreadStarter cache", () => {
+  afterEach(() => {
+    resetSlackThreadStarterCacheForTest();
+    vi.useRealTimers();
+  });
+
+  it("returns cached thread starter without refetching within ttl", async () => {
+    const replies = vi.fn(async () => ({
+      messages: [{ text: "root message", user: "U1", ts: "1000.1" }],
+    }));
+    const client = {
+      conversations: { replies },
+    } as unknown as Parameters<typeof resolveSlackThreadStarter>[0]["client"];
+
+    const first = await resolveSlackThreadStarter({
+      channelId: "C1",
+      threadTs: "1000.1",
+      client,
+    });
+    const second = await resolveSlackThreadStarter({
+      channelId: "C1",
+      threadTs: "1000.1",
+      client,
+    });
+
+    expect(first).toEqual(second);
+    expect(replies).toHaveBeenCalledTimes(1);
+  });
+
+  it("expires stale cache entries and refetches after ttl", async () => {
+    vi.useFakeTimers();
+    vi.setSystemTime(new Date("2026-01-01T00:00:00.000Z"));
+
+    const replies = vi.fn(async () => ({
+      messages: [{ text: "root message", user: "U1", ts: "1000.1" }],
+    }));
+    const client = {
+      conversations: { replies },
+    } as unknown as Parameters<typeof resolveSlackThreadStarter>[0]["client"];
+
+    await resolveSlackThreadStarter({
+      channelId: "C1",
+      threadTs: "1000.1",
+      client,
+    });
+
+    vi.setSystemTime(new Date("2026-01-01T07:00:00.000Z"));
+    await resolveSlackThreadStarter({
+      channelId: "C1",
+      threadTs: "1000.1",
+      client,
+    });
+
+    expect(replies).toHaveBeenCalledTimes(2);
+  });
+
+  it("evicts oldest entries once cache exceeds bounded size", async () => {
+    const replies = vi.fn(async () => ({
+      messages: [{ text: "root message", user: "U1", ts: "1000.1" }],
+    }));
+    const client = {
+      conversations: { replies },
+    } as unknown as Parameters<typeof resolveSlackThreadStarter>[0]["client"];
+
+    // Cache cap is 2000; add enough distinct keys to force eviction of earliest keys.
+    for (let i = 0; i <= 2000; i += 1) {
+      await resolveSlackThreadStarter({
+        channelId: "C1",
+        threadTs: `1000.${i}`,
+        client,
+      });
+    }
+    const callsAfterFill = replies.mock.calls.length;
+
+    // Oldest key should be evicted and require fetch again.
+    await resolveSlackThreadStarter({
+      channelId: "C1",
+      threadTs: "1000.0",
+      client,
+    });
+
+    expect(replies.mock.calls.length).toBe(callsAfterFill + 1);
+  });
+});
diff --git a/src/slack/monitor/media.ts b/src/slack/monitor/media.ts
index 33390e45a..c943b9daa 100644
--- a/src/slack/monitor/media.ts
+++ b/src/slack/monitor/media.ts
@@ -233,17 +233,49 @@ export type SlackThreadStarter = {
   files?: SlackFile[];
 };
 
-const THREAD_STARTER_CACHE = new Map<string, SlackThreadStarter>();
+type SlackThreadStarterCacheEntry = {
+  value: SlackThreadStarter;
+  cachedAt: number;
+};
+
+const THREAD_STARTER_CACHE = new Map<string, SlackThreadStarterCacheEntry>();
+const THREAD_STARTER_CACHE_TTL_MS = 6 * 60 * 60_000;
+const THREAD_STARTER_CACHE_MAX = 2000;
+
+function evictThreadStarterCache(): void {
+  const now = Date.now();
+  for (const [cacheKey, entry] of THREAD_STARTER_CACHE.entries()) {
+    if (now - entry.cachedAt > THREAD_STARTER_CACHE_TTL_MS) {
+      THREAD_STARTER_CACHE.delete(cacheKey);
+    }
+  }
+  if (THREAD_STARTER_CACHE.size <= THREAD_STARTER_CACHE_MAX) {
+    return;
+  }
+  const excess = THREAD_STARTER_CACHE.size - THREAD_STARTER_CACHE_MAX;
+  let removed = 0;
+  for (const cacheKey of THREAD_STARTER_CACHE.keys()) {
+    THREAD_STARTER_CACHE.delete(cacheKey);
+    removed += 1;
+    if (removed >= excess) {
+      break;
+    }
+  }
+}
 
 export async function resolveSlackThreadStarter(params: {
   channelId: string;
   threadTs: string;
   client: SlackWebClient;
 }): Promise<SlackThreadStarter | null> {
+  evictThreadStarterCache();
   const cacheKey = `${params.channelId}:${params.threadTs}`;
   const cached = THREAD_STARTER_CACHE.get(cacheKey);
+  if (cached && Date.now() - cached.cachedAt <= THREAD_STARTER_CACHE_TTL_MS) {
+    return cached.value;
+  }
   if (cached) {
-    return cached;
+    THREAD_STARTER_CACHE.delete(cacheKey);
   }
   try {
     const response = (await params.client.conversations.replies({
@@ -263,13 +295,24 @@ export async function resolveSlackThreadStarter(params: {
       ts: message.ts,
       files: message.files,
     };
-    THREAD_STARTER_CACHE.set(cacheKey, starter);
+    if (THREAD_STARTER_CACHE.has(cacheKey)) {
+      THREAD_STARTER_CACHE.delete(cacheKey);
+    }
+    THREAD_STARTER_CACHE.set(cacheKey, {
+      value: starter,
+      cachedAt: Date.now(),
+    });
+    evictThreadStarterCache();
     return starter;
   } catch {
     return null;
   }
 }
 
+export function resetSlackThreadStarterCacheForTest(): void {
+  THREAD_STARTER_CACHE.clear();
+}
+
 export type SlackThreadMessage = {
   text: string;
   userId?: string;