fix(browse): lockfile prevents concurrent server start races

Adds exclusive lockfile (O_CREAT|O_EXCL) around ensureServer to prevent
TOCTOU race where two CLI invocations could both kill the old server and
start new ones, leaving an orphaned chromium process. Second caller now
waits for the first to finish starting.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

browse/src/cli.ts

@@ -206,6 +206,34 @@ async function startServer(): Promise<ServerState> {
   throw new Error(`Server failed to start within ${MAX_START_WAIT / 1000}s`);
 }
 
+/**
+ * Acquire an exclusive lockfile to prevent concurrent ensureServer() races (TOCTOU).
+ * Returns a cleanup function that releases the lock.
+ */
+function acquireServerLock(): (() => void) | null {
+  const lockPath = `${config.stateFile}.lock`;
+  try {
+    // O_CREAT | O_EXCL — fails if file already exists (atomic check-and-create)
+    const fd = fs.openSync(lockPath, fs.constants.O_CREAT | fs.constants.O_EXCL | fs.constants.O_WRONLY);
+    fs.writeSync(fd, `${process.pid}\n`);
+    fs.closeSync(fd);
+    return () => { try { fs.unlinkSync(lockPath); } catch {} };
+  } catch {
+    // Lock already held — check if the holder is still alive
+    try {
+      const holderPid = parseInt(fs.readFileSync(lockPath, 'utf8').trim(), 10);
+      if (holderPid && isProcessAlive(holderPid)) {
+        return null; // Another live process holds the lock
+      }
+      // Stale lock — remove and retry
+      fs.unlinkSync(lockPath);
+      return acquireServerLock();
+    } catch {
+      return null;
+    }
+  }
+}
+
 async function ensureServer(): Promise<ServerState> {
   const state = readState();
 
@@ -234,12 +262,36 @@ async function ensureServer(): Promise<ServerState> {
     }
   }
 
-  // Need to (re)start — kill the old server first to avoid orphaned chromium processes
-  if (state && state.pid) {
-    await killServer(state.pid);
+  // Acquire lock to prevent concurrent restart races (TOCTOU)
+  const releaseLock = acquireServerLock();
+  if (!releaseLock) {
+    // Another process is starting the server — wait for it
+    console.error('[browse] Another instance is starting the server, waiting...');
+    const start = Date.now();
+    while (Date.now() - start < MAX_START_WAIT) {
+      const freshState = readState();
+      if (freshState && isProcessAlive(freshState.pid)) return freshState;
+      await Bun.sleep(200);
+    }
+    throw new Error('Timed out waiting for another instance to start the server');
+  }
+
+  try {
+    // Re-read state under lock in case another process just started the server
+    const freshState = readState();
+    if (freshState && isProcessAlive(freshState.pid)) {
+      return freshState;
+    }
+
+    // Kill the old server to avoid orphaned chromium processes
+    if (state && state.pid) {
+      await killServer(state.pid);
+    }
+    console.error('[browse] Starting server...');
+    return await startServer();
+  } finally {
+    releaseLock();
   }
-  console.error('[browse] Starting server...');
-  return startServer();
 }
 
 // ─── Command Dispatch ──────────────────────────────────────────