feat(DIS-103): ChannelQueue serializes messages per channel (FIFO)

Add ChannelQueue in src/runtime/channel-queue.ts.
Router enqueues each message so same-channel requests run serially.
Different channels remain parallel.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/router.ts

@@ -2,6 +2,9 @@ import { Message, TextChannel } from "discord.js";
 import { DisclawDatabase } from "./db/database";
 import { DisclawConfig } from "./config/loader";
 import { runAgent } from "./agent/runner";
+import { ChannelQueue } from "./runtime/channel-queue";
+
+const channelQueue = new ChannelQueue();
 
 const DISCORD_MAX_LENGTH = 2000;
 
@@ -77,6 +80,7 @@ export async function routeMessage(
 
   const channel = message.channel as TextChannel;
 
+  await channelQueue.enqueue(channelId, async () => {
     // Show typing indicator while the agent works
     const typingInterval = setInterval(() => {
       channel.sendTyping().catch(() => {});
@@ -132,6 +136,7 @@ export async function routeMessage(
     } finally {
       clearInterval(typingInterval);
     }
+  });
 
   return true;
 }

src/runtime/channel-queue.ts

@@ -0,0 +1,37 @@
+type Task = () => Promise<void>;
+
+/**
+ * Per-channel FIFO task serializer.
+ *
+ * Messages arriving in the same Discord channel are enqueued and executed one
+ * at a time. A failing task logs the error and is swallowed so the next task
+ * is not blocked. Different channels run their queues in parallel.
+ */
+export class ChannelQueue {
+  private queues = new Map<string, Promise<void>>();
+
+  enqueue(channelId: string, task: Task): Promise<void> {
+    // Use the existing tail promise as the predecessor, or a pre-resolved one
+    // for a fresh channel.
+    const prev = this.queues.get(channelId) ?? Promise.resolve();
+
+    // Chain the new task behind the current tail. Errors are caught and
+    // swallowed so subsequent tasks always get a chance to run.
+    const next = prev.then(() => task()).catch((err) => {
+      console.error(`[ChannelQueue] Task error in channel ${channelId}:`, err);
+    });
+
+    // Register the new tail. Use a finalizer that removes the entry only when
+    // THIS chain link is still the stored tail — if another task was enqueued
+    // in the meantime the entry must stay.
+    this.queues.set(channelId, next);
+
+    next.finally(() => {
+      if (this.queues.get(channelId) === next) {
+        this.queues.delete(channelId);
+      }
+    });
+
+    return next;
+  }
+}

tests/integration/channel-queue-router.test.ts

@@ -0,0 +1,115 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
+
+/**
+ * Integration test: verifies that routeMessage serializes same-channel messages
+ * through ChannelQueue so they are processed in FIFO order.
+ *
+ * runAgent is mocked to record call order without spawning a real process.
+ * discord.js and the database are also mocked so no real I/O occurs.
+ */
+
+// ---- Mock runAgent BEFORE importing router ----
+const runOrder: string[] = [];
+
+vi.mock("../../src/agent/runner", () => ({
+  runAgent: vi.fn(async ({ userMessage }: { userMessage: string }) => {
+    // Simulate brief async work so the promise chain is exercised
+    await new Promise<void>((resolve) => setTimeout(resolve, 5));
+    runOrder.push(userMessage);
+    return `echo: ${userMessage}`;
+  }),
+}));
+
+// ---- Import after mock registration ----
+import { routeMessage } from "../../src/router";
+
+// ---- Helpers ----
+
+/** Builds a minimal fake discord.js Message object. */
+function makeMessage(channelId: string, content: string) {
+  const sent: string[] = [];
+  const channel = {
+    name: "test-channel",
+    sendTyping: vi.fn().mockResolvedValue(undefined),
+    send: vi.fn(async (text: string) => {
+      sent.push(text);
+      return { id: `msg-${Math.random()}` };
+    }),
+  };
+
+  return {
+    channelId,
+    content,
+    id: `mid-${content}`,
+    author: { username: "tester" },
+    channel,
+    _sent: sent,
+  } as unknown as import("discord.js").Message & { _sent: string[] };
+}
+
+/** Minimal fake DisclawDatabase. */
+function makeDb(channelId: string) {
+  return {
+    getWorkspaceByChannelId: vi.fn((_id: string) => ({
+      id: 1,
+      workspace_path: "/fake/workspace",
+    })),
+    getConversationHistory: vi.fn(() => []),
+    addConversation: vi.fn(),
+  } as unknown as import("../../src/db/database").DisclawDatabase;
+}
+
+/** Minimal fake DisclawConfig. */
+const fakeConfig = {} as import("../../src/config/loader").DisclawConfig;
+
+// ---- Tests ----
+
+describe("channel-queue / router integration", () => {
+  beforeEach(() => {
+    runOrder.length = 0;
+  });
+
+  afterEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  it("processes 3 messages in the same channel in FIFO order", async () => {
+    const channelId = "ch-999";
+    const db = makeDb(channelId);
+
+    // Fire all three without awaiting — they should serialise internally
+    const p1 = routeMessage(makeMessage(channelId, "first"), db, fakeConfig, null);
+    const p2 = routeMessage(makeMessage(channelId, "second"), db, fakeConfig, null);
+    const p3 = routeMessage(makeMessage(channelId, "third"), db, fakeConfig, null);
+
+    await Promise.all([p1, p2, p3]);
+
+    expect(runOrder).toEqual(["first", "second", "third"]);
+  });
+
+  it("returns true for every handled message", async () => {
+    const channelId = "ch-handled";
+    const db = makeDb(channelId);
+
+    const results = await Promise.all([
+      routeMessage(makeMessage(channelId, "a"), db, fakeConfig, null),
+      routeMessage(makeMessage(channelId, "b"), db, fakeConfig, null),
+    ]);
+
+    expect(results).toEqual([true, true]);
+  });
+
+  it("returns false when channel is the management channel", async () => {
+    const channelId = "ch-mgmt";
+    const db = makeDb(channelId);
+
+    const result = await routeMessage(
+      makeMessage(channelId, "hi"),
+      db,
+      fakeConfig,
+      channelId  // same id => management channel
+    );
+
+    expect(result).toBe(false);
+  });
+});

tests/unit/channel-queue.test.ts

@@ -0,0 +1,100 @@
+import { describe, it, expect, vi } from "vitest";
+import { ChannelQueue } from "../../src/runtime/channel-queue";
+
+/**
+ * Returns a task that resolves after `ms` milliseconds and pushes its label
+ * into the shared `order` array.
+ */
+function makeTask(order: string[], label: string, ms = 0): () => Promise<void> {
+  return () =>
+    new Promise<void>((resolve) => {
+      setTimeout(() => {
+        order.push(label);
+        resolve();
+      }, ms);
+    });
+}
+
+describe("ChannelQueue", () => {
+  it("(a) executes tasks in FIFO order within the same channel", async () => {
+    const queue = new ChannelQueue();
+    const order: string[] = [];
+
+    const p1 = queue.enqueue("ch1", makeTask(order, "first"));
+    const p2 = queue.enqueue("ch1", makeTask(order, "second"));
+    const p3 = queue.enqueue("ch1", makeTask(order, "third"));
+
+    await Promise.all([p1, p2, p3]);
+
+    expect(order).toEqual(["first", "second", "third"]);
+  });
+
+  it("(b) a failing task does not block the next task", async () => {
+    const queue = new ChannelQueue();
+    const order: string[] = [];
+
+    const failing: () => Promise<void> = () =>
+      Promise.reject(new Error("intentional failure"));
+
+    // Suppress the console.error output from ChannelQueue internals
+    const spy = vi.spyOn(console, "error").mockImplementation(() => {});
+
+    const p1 = queue.enqueue("ch1", failing);
+    const p2 = queue.enqueue("ch1", makeTask(order, "after-error"));
+
+    await Promise.all([p1, p2]);
+
+    spy.mockRestore();
+
+    expect(order).toEqual(["after-error"]);
+  });
+
+  it("(c) map entry is removed after all tasks complete", async () => {
+    const queue = new ChannelQueue();
+
+    const p1 = queue.enqueue("ch1", () => Promise.resolve());
+    const p2 = queue.enqueue("ch1", () => Promise.resolve());
+
+    await Promise.all([p1, p2]);
+
+    // Allow the finally-cleanup microtask to run
+    await Promise.resolve();
+
+    expect((queue as unknown as { queues: Map<string, unknown> }).queues.size).toBe(0);
+  });
+
+  it("(d) different channels run in parallel, not serialized", async () => {
+    const queue = new ChannelQueue();
+    const completionOrder: string[] = [];
+
+    // ch-slow gets a 50 ms task; ch-fast gets a 10 ms task.
+    // If channels were serialized (ch-slow first), ch-fast would finish after
+    // ch-slow. With true parallelism ch-fast finishes first.
+    const pSlow = queue.enqueue(
+      "ch-slow",
+      () =>
+        new Promise<void>((resolve) => {
+          setTimeout(() => {
+            completionOrder.push("slow");
+            resolve();
+          }, 50);
+        })
+    );
+
+    const pFast = queue.enqueue(
+      "ch-fast",
+      () =>
+        new Promise<void>((resolve) => {
+          setTimeout(() => {
+            completionOrder.push("fast");
+            resolve();
+          }, 10);
+        })
+    );
+
+    await Promise.all([pSlow, pFast]);
+
+    // fast must finish before slow
+    expect(completionOrder).toEqual(["fast", "slow"]);
+  });
+});