Improve latency of both process.stdout and asciinema stdout writes.

web/src/server/pty/asciinema-writer.ts

@@ -159,17 +159,19 @@ export class AsciinemaWriter {
     const eventJson = JSON.stringify(eventArray);
     this.writeStream.write(eventJson + '\n');
 
-    // Force immediate disk write to trigger file watchers
+    // Force immediate disk write to trigger file watchers asynchronously
     if (this.fd !== null) {
-      try {
-        /*fs.fsync(this.fd, (err) => {
-          if (err) {
-            logger.error(`Failed to fsync asciinema file: ${err.message}`);
-          }
-        });*/
-        fs.fsyncSync(this.fd);
-      } catch (_e) {
-        // Ignore sync errors
+      // Use setImmediate to avoid blocking the event loop
+      if (this.fd !== null) {
+        try {
+          fs.fsync(this.fd, (err) => {
+            if (err) {
+              // Ignore sync errors
+            }
+          });
+        } catch (_e) {
+          // Ignore sync errors
+        }
       }
     }
   }

web/src/server/pty/pty-manager.ts

@@ -334,7 +334,7 @@ export class PtyManager extends EventEmitter {
     const { ptyProcess, asciinemaWriter } = session;
 
     // Handle PTY data output
-    ptyProcess?.onData((data: string) => {
+    ptyProcess?.onData(async (data: string) => {
       try {
         // Check for bell character (ASCII 7) - filter out OSC sequences
         if (data.includes('\x07')) {
@@ -366,13 +366,25 @@ export class PtyManager extends EventEmitter {
           }
         }
 
-        // Write to asciinema file (now async, non-blocking)
-        asciinemaWriter?.writeOutput(Buffer.from(data, 'utf8'));
+        // Execute both writes in parallel
+        const promises: Promise<void>[] = [];
 
-        // Forward to stdout if requested (for fwd.ts) with batching
+        // Write to asciinema file
+        promises.push(
+          Promise.resolve().then(() => {
+            asciinemaWriter?.writeOutput(Buffer.from(data, 'utf8'));
+          })
+        );
+
+        // Forward to stdout if requested
         if (forwardToStdout) {
           process.stdout.write(data);
         }
+
+        // Wait for both operations to complete
+        if (promises.length > 0) {
+          await Promise.all(promises);
+        }
       } catch (error) {
         logger.error(`Failed to write PTY data for session ${session.id}:`, error);
       }