fix: avoid blocking audio writes

TTS responses were written synchronously from the async presentation pipeline, blocking concurrent requests. Offload byte writes to a worker thread and add a regression test that keeps the event loop responsive.

surfsense_backend/app/agents/video_presentation/nodes.py

@@ -17,6 +17,7 @@ from app.config import config as app_config
 from app.services.kokoro_tts_service import get_kokoro_tts_service
 from app.services.llm_service import get_agent_llm
 from app.utils.content_utils import extract_text_content, strip_markdown_fences
+from app.utils.file_io import write_bytes
 
 from .configuration import Configuration
 from .prompts import (
@@ -137,8 +138,7 @@ async def create_slide_audio(state: State, config: RunnableConfig) -> dict[str,
                 kwargs["api_base"] = app_config.TTS_SERVICE_API_BASE
 
             response = await aspeech(**kwargs)
-            with open(chunk_path, "wb") as f:
-                f.write(response.content)
+            await write_bytes(chunk_path, response.content)
 
         return chunk_path
 

surfsense_backend/app/utils/file_io.py

@@ -0,0 +1,7 @@
+import asyncio
+from pathlib import Path
+
+
+async def write_bytes(path: str, content: bytes) -> None:
+    """Write bytes without blocking the event loop."""
+    await asyncio.to_thread(Path(path).write_bytes, content)

surfsense_backend/tests/unit/agents/video_presentation/test_file_io.py

@@ -0,0 +1,29 @@
+import asyncio
+import threading
+
+import pytest
+
+from app.utils.file_io import write_bytes
+
+pytestmark = pytest.mark.unit
+
+
+@pytest.mark.asyncio
+async def test_write_bytes_does_not_block_event_loop(monkeypatch, tmp_path):
+    write_started = threading.Event()
+    release_write = threading.Event()
+
+    def blocking_write(_path, _content):
+        write_started.set()
+        release_write.wait(timeout=1)
+        return 5
+
+    monkeypatch.setattr("pathlib.Path.write_bytes", blocking_write)
+
+    task = asyncio.create_task(write_bytes(str(tmp_path / "audio.mp3"), b"audio"))
+    while not write_started.is_set():
+        await asyncio.sleep(0)
+
+    assert not task.done()
+    release_write.set()
+    await task