Fix auth session race

frontend/app/lib/claw-auth.ts

@@ -1,6 +1,6 @@
 import { spawn } from "node:child_process";
 import { randomBytes, scryptSync, timingSafeEqual } from "node:crypto";
-import { mkdir, readFile, writeFile } from "node:fs/promises";
+import { mkdir, readFile, rename, writeFile } from "node:fs/promises";
 import path from "node:path";
 import { cookies } from "next/headers";
 
@@ -14,6 +14,7 @@ const COOKIE_NAME = authCookieName();
 const SESSION_MAX_AGE_SECONDS = 60 * 60 * 24 * 30;
 const SESSION_REFRESH_INTERVAL_MS = 60 * 60 * 12 * 1000;
 const LINUX_ACCOUNT_WORKSPACE_NAME = "zk-agent";
+let sessionsWriteQueue: Promise<unknown> = Promise.resolve();
 
 type UserRecord = {
 	id: string;
@@ -131,9 +132,10 @@ export async function logoutAccount() {
 	const cookieStore = await cookies();
 	const token = cookieStore.get(COOKIE_NAME)?.value;
 	if (token) {
-		const sessionsFile = await readSessions();
+		await mutateSessions((sessionsFile) => {
 			delete sessionsFile.sessions[token];
-		await writeSessions(sessionsFile);
+			return sessionsFile;
+		});
 	}
 	cookieStore.delete(COOKIE_NAME);
 }
@@ -151,8 +153,12 @@ export async function getCurrentAccount(): Promise<AccountSession | null> {
 	const account = usersFile.users.find((user) => user.id === session.accountId);
 	if (!account) return null;
 	if (shouldRefreshSession(session.updatedAt ?? session.createdAt)) {
-		session.updatedAt = new Date().toISOString();
-		await writeSessions(sessionsFile);
+		await mutateSessions((latestSessionsFile) => {
+			const latestSession = latestSessionsFile.sessions[token];
+			if (!latestSession) return latestSessionsFile;
+			latestSession.updatedAt = new Date().toISOString();
+			return latestSessionsFile;
+		});
 		await setSessionCookie(token);
 	}
 	return { id: account.id, username: account.username };
@@ -199,13 +205,15 @@ export function accountSessionRoot(accountId: string, sessionId: string) {
 
 async function createSession(account: UserRecord) {
 	const token = randomBytes(32).toString("hex");
-	const sessionsFile = await readSessions();
+	const now = new Date().toISOString();
+	await mutateSessions((sessionsFile) => {
 		sessionsFile.sessions[token] = {
 			accountId: account.id,
-		createdAt: new Date().toISOString(),
-		updatedAt: new Date().toISOString(),
+			createdAt: now,
+			updatedAt: now,
 		};
-	await writeSessions(sessionsFile);
+		return sessionsFile;
+	});
 
 	await setSessionCookie(token);
 	return { id: account.id, username: account.username };
@@ -257,7 +265,22 @@ async function readSessions(): Promise<SessionsFile> {
 
 async function writeSessions(payload: SessionsFile) {
 	await mkdir(AUTH_STATE_ROOT, { recursive: true });
-	await writeFile(SESSIONS_PATH, JSON.stringify(payload, null, 2), "utf8");
+	const tempPath = `${SESSIONS_PATH}.${process.pid}.${Date.now()}.${randomBytes(4).toString("hex")}.tmp`;
+	await writeFile(tempPath, JSON.stringify(payload, null, 2), "utf8");
+	await rename(tempPath, SESSIONS_PATH);
+}
+
+async function mutateSessions(
+	mutator: (payload: SessionsFile) => SessionsFile | Promise<SessionsFile>,
+) {
+	const run = sessionsWriteQueue.then(async () => {
+		const current = await readSessions();
+		const next = await mutator(current);
+		await writeSessions(next);
+		return next;
+	});
+	sessionsWriteQueue = run.catch(() => undefined);
+	return run;
 }
 
 function normalizeUsername(username: string) {