perf: stop v2 host-service from scaling linearly with worktree count

packages/host-service/package.json

@@ -50,6 +50,7 @@
 		"test:integration": "bun test --pass-with-no-tests test/integration",
 		"test:integration:daemon": "node --experimental-strip-types --test src/terminal/DaemonClient/DaemonClient.node-test.ts src/daemon/DaemonSupervisor.node-test.ts",
 		"test:e2e": "bun run scripts/test-e2e.ts",
+		"bench": "bun test test/integration/pull-requests-scaling.bench.ts",
 		"typecheck": "tsc --noEmit --emitDeclarationOnly false"
 	},
 	"dependencies": {

packages/host-service/src/app.ts

@@ -7,7 +7,7 @@ import { Hono } from "hono";
 import { cors } from "hono/cors";
 import { createApiClient } from "./api";
 import { createDb, type HostDb } from "./db";
-import { EventBus, registerEventBusRoute } from "./events";
+import { EventBus, GitWatcher, registerEventBusRoute } from "./events";
 import type { ApiAuthProvider } from "./providers/auth";
 import type { HostAuthProvider } from "./providers/host-auth";
 import type { ModelProviderRuntimeResolver } from "./providers/model-providers";
@@ -76,13 +76,19 @@ export function createApp(options: CreateAppOptions): CreateAppResult {
 			return new Octokit({ auth: token });
 		});
 
+	const filesystem = new WorkspaceFilesystemManager({ db });
+	// GitWatcher is the single source of truth for `.git/` and worktree fs
+	// activity per workspace. Both EventBus (broadcasts to clients) and the
+	// pull-requests runtime (event-driven branch sync) subscribe to it.
+	const gitWatcher = new GitWatcher(db, filesystem);
+	gitWatcher.start();
 	const pullRequestRuntime = new PullRequestRuntimeManager({
 		db,
 		git,
 		github,
+		gitWatcher,
 	});
 	pullRequestRuntime.start();
-	const filesystem = new WorkspaceFilesystemManager({ db });
 	const chatRuntime =
 		options.chatRuntime ??
 		new ChatRuntimeManager({
@@ -111,7 +117,7 @@ export function createApp(options: CreateAppOptions): CreateAppResult {
 		}),
 	);
 
-	const eventBus = new EventBus({ db, filesystem });
+	const eventBus = new EventBus({ db, filesystem, gitWatcher });
 	eventBus.start();
 
 	// Backfill `kind='main'` v2 workspaces for projects already set up before
@@ -180,6 +186,11 @@ export function createApp(options: CreateAppOptions): CreateAppResult {
 		} catch (err) {
 			console.warn("[host-service] eventBus.close failed:", err);
 		}
+		try {
+			gitWatcher.close();
+		} catch (err) {
+			console.warn("[host-service] gitWatcher.close failed:", err);
+		}
 		if (ownsDb) {
 			try {
 				(db as unknown as { $client?: { close: () => void } }).$client?.close();

packages/host-service/src/events/event-bus.test.ts

@@ -4,13 +4,17 @@ import type { HostDb } from "../db";
 import { portManager } from "../ports/port-manager";
 import type { WorkspaceFilesystemManager } from "../runtime/filesystem";
 import { EventBus } from "./event-bus";
+import type { GitWatcher } from "./git-watcher";
 
 function createEventBus(): EventBus {
 	return new EventBus({
 		db: {} as unknown as HostDb,
 		filesystem: {
 			resolveWorkspaceRoot: () => "/tmp/missing-workspace",
 		} as unknown as WorkspaceFilesystemManager,
+		gitWatcher: {
+			onChanged: () => () => {},
+		} as unknown as GitWatcher,
 	});
 }
 

packages/host-service/src/events/event-bus.ts

@@ -6,7 +6,7 @@ import type { HostDb } from "../db/index.ts";
 import { portManager } from "../ports/port-manager.ts";
 import { getLabelsForWorkspace } from "../ports/static-ports.ts";
 import type { WorkspaceFilesystemManager } from "../runtime/filesystem/index.ts";
-import { GitWatcher } from "./git-watcher.ts";
+import type { GitWatcher } from "./git-watcher.ts";
 import type { ClientMessage, ServerMessage } from "./types.ts";
 
 type WsSocket = {
@@ -52,6 +52,7 @@ function parseClientMessage(data: unknown): ClientMessage | null {
 export interface EventBusOptions {
 	db: HostDb;
 	filesystem: WorkspaceFilesystemManager;
+	gitWatcher: GitWatcher;
 }
 
 /**
@@ -71,13 +72,12 @@ export class EventBus {
 
 	constructor(options: EventBusOptions) {
 		this.filesystem = options.filesystem;
-		this.gitWatcher = new GitWatcher(options.db, options.filesystem);
+		this.gitWatcher = options.gitWatcher;
 	}
 
 	start(): void {
 		if (this.removeGitListener || this.removePortListeners) return;
 
-		this.gitWatcher.start();
 		this.removeGitListener = this.gitWatcher.onChanged((event) => {
 			this.broadcast({
 				type: "git:changed",
@@ -105,7 +105,6 @@ export class EventBus {
 		this.removeGitListener = null;
 		this.removePortListeners?.();
 		this.removePortListeners = null;
-		this.gitWatcher.close();
 		for (const [socket, state] of this.clients) {
 			this.cleanupClient(socket, state);
 		}

packages/host-service/src/events/index.ts

@@ -1,4 +1,5 @@
 export { EventBus, registerEventBusRoute } from "./event-bus.ts";
+export { type GitChangedEvent, GitWatcher } from "./git-watcher.ts";
 export {
 	type AgentLifecycleEventType,
 	mapEventType,

packages/host-service/src/runtime/pull-requests/pull-requests.test.ts

@@ -148,6 +148,7 @@ function createManager(state: FakeState) {
 		github: async () => {
 			throw new Error("github should not be used for direct PR linking");
 		},
+		gitWatcher: { onChanged: () => () => {} } as never,
 	});
 }