fix(daemon): add backpressure control and command serialization to prevent IPC EAGAIN (#529)

- Add AGENT_BROWSER_DEFAULT_TIMEOUT env var to override Playwright's
  default 60s timeout (CDP/recording 10s timeouts unaffected)
- Add backpressure-aware safeWrite() that waits for drain when socket
  buffer is full, preventing data loss under load
- Serialize command execution per socket via queue to prevent concurrent
  writes that cause buffer contention

These daemon-side fixes complement #329 (CLI-side EAGAIN retry) by
addressing the root causes: Playwright operations that outlast the
CLI's IPC timeout, and concurrent socket.write() calls that overflow
the kernel buffer.

Tested with heavy React app (1000+ DOM nodes) — 10 consecutive
snapshot commands complete without os error 35/11.

Refs #322

Co-authored-by: shohu <shohu@users.noreply.github.com>

src/daemon.ts

@@ -21,6 +21,47 @@ import {
 // Manager type - either desktop browser or iOS
 type Manager = BrowserManager | IOSManager;
 
+/**
+ * Backpressure-aware socket write.
+ * If the kernel buffer is full (socket.write returns false),
+ * waits for the 'drain' event before resolving.
+ */
+function safeWrite(socket: net.Socket, payload: string): Promise<void> {
+  return new Promise((resolve, reject) => {
+    if (socket.destroyed) {
+      resolve();
+      return;
+    }
+    const canContinue = socket.write(payload);
+    if (canContinue) {
+      resolve();
+    } else if (socket.destroyed) {
+      resolve();
+    } else {
+      const cleanup = () => {
+        socket.removeListener('drain', onDrain);
+        socket.removeListener('error', onError);
+        socket.removeListener('close', onClose);
+      };
+      const onDrain = () => {
+        cleanup();
+        resolve();
+      };
+      const onError = (err: Error) => {
+        cleanup();
+        reject(err);
+      };
+      const onClose = () => {
+        cleanup();
+        resolve();
+      };
+      socket.once('drain', onDrain);
+      socket.once('error', onError);
+      socket.once('close', onClose);
+    }
+  });
+}
+
 // Platform detection
 const isWindows = process.platform === 'win32';
 
@@ -321,35 +362,25 @@ export async function startDaemon(options?: {
     let buffer = '';
     let httpChecked = false;
 
-    socket.on('data', async (data) => {
-      buffer += data.toString();
+    // Command serialization: queue incoming lines and process them one at a time.
+    // This prevents concurrent command execution which can cause socket.write
+    // buffer contention and EAGAIN errors on the Rust CLI side.
+    const commandQueue: string[] = [];
+    let processing = false;
 
-      // Security: Detect and reject HTTP requests to prevent cross-origin attacks.
-      // Browsers using fetch() must send HTTP headers (e.g., "POST / HTTP/1.1"),
-      // while legitimate clients send raw JSON starting with "{".
-      if (!httpChecked) {
-        httpChecked = true;
-        const trimmed = buffer.trimStart();
-        if (/^(GET|POST|PUT|DELETE|HEAD|OPTIONS|PATCH|CONNECT|TRACE)\s/i.test(trimmed)) {
-          socket.destroy();
-          return;
-        }
-      }
+    async function processQueue(): Promise<void> {
+      if (processing) return;
+      processing = true;
 
-      // Process complete lines
-      while (buffer.includes('\n')) {
-        const newlineIdx = buffer.indexOf('\n');
-        const line = buffer.substring(0, newlineIdx);
-        buffer = buffer.substring(newlineIdx + 1);
-
-        if (!line.trim()) continue;
+      while (commandQueue.length > 0) {
+        const line = commandQueue.shift()!;
 
         try {
           const parseResult = parseCommand(line);
 
           if (!parseResult.success) {
             const resp = errorResponse(parseResult.id ?? 'unknown', parseResult.error);
-            socket.write(serializeResponse(resp) + '\n');
+            await safeWrite(socket, serializeResponse(resp) + '\n');
             continue;
           }
 
@@ -363,10 +394,11 @@ export async function startDaemon(options?: {
                 success: true as const,
                 data: { devices },
               };
-              socket.write(serializeResponse(response) + '\n');
+              await safeWrite(socket, serializeResponse(response) + '\n');
             } catch (err) {
               const message = err instanceof Error ? err.message : String(err);
-              socket.write(
+              await safeWrite(
+                socket,
                 serializeResponse(errorResponse(parseResult.command.id, message)) + '\n'
               );
             }
@@ -493,7 +525,7 @@ export async function startDaemon(options?: {
               isIOS && manager instanceof IOSManager
                 ? await executeIOSCommand(parseResult.command, manager)
                 : await executeCommand(parseResult.command, manager as BrowserManager);
-            socket.write(serializeResponse(response) + '\n');
+            await safeWrite(socket, serializeResponse(response) + '\n');
 
             if (!shuttingDown) {
               shuttingDown = true;
@@ -503,6 +535,9 @@ export async function startDaemon(options?: {
                 process.exit(0);
               }, 100);
             }
+
+            commandQueue.length = 0;
+            processing = false;
             return;
           }
 
@@ -520,12 +555,46 @@ export async function startDaemon(options?: {
             }
           }
 
-          socket.write(serializeResponse(response) + '\n');
+          await safeWrite(socket, serializeResponse(response) + '\n');
         } catch (err) {
           const message = err instanceof Error ? err.message : String(err);
-          socket.write(serializeResponse(errorResponse('error', message)) + '\n');
+          await safeWrite(socket, serializeResponse(errorResponse('error', message)) + '\n').catch(
+            () => {}
+          ); // Socket may already be destroyed
         }
       }
+
+      processing = false;
+    }
+
+    socket.on('data', (data) => {
+      buffer += data.toString();
+
+      // Security: Detect and reject HTTP requests to prevent cross-origin attacks.
+      // Browsers using fetch() must send HTTP headers (e.g., "POST / HTTP/1.1"),
+      // while legitimate clients send raw JSON starting with "{".
+      if (!httpChecked) {
+        httpChecked = true;
+        const trimmed = buffer.trimStart();
+        if (/^(GET|POST|PUT|DELETE|HEAD|OPTIONS|PATCH|CONNECT|TRACE)\s/i.test(trimmed)) {
+          socket.destroy();
+          return;
+        }
+      }
+
+      // Extract complete lines and enqueue them for serial processing
+      while (buffer.includes('\n')) {
+        const newlineIdx = buffer.indexOf('\n');
+        const line = buffer.substring(0, newlineIdx);
+        buffer = buffer.substring(newlineIdx + 1);
+
+        if (!line.trim()) continue;
+        commandQueue.push(line);
+      }
+
+      processQueue().catch(() => {
+        // Socket write failures during queue processing are non-fatal
+      });
     });
 
     socket.on('error', () => {