AsyncStreamHandler breaks when no `await` is used in `emit()`

[vvolhejn]

Hi again! I tried generating a sine wave using AsyncStreamHandler but stumbled upon a strange issue - the server silently fails unless I add await asyncio.sleep(0.01) into the emit() method:

import asyncio

import numpy as np
from fastrtc import AsyncStreamHandler, Stream

SAMPLE_RATE = 24000
OUTPUT_FRAME_SIZE = 1920


class SineHandler(AsyncStreamHandler):
    def __init__(self) -> None:
        super().__init__(input_sample_rate=SAMPLE_RATE)
        self.cur_time_samples = 0

    async def receive(self, frame: tuple[int, np.ndarray]) -> None:
        pass

    async def emit(self) -> tuple[int, np.ndarray]:
        times = np.arange(
            self.cur_time_samples,
            self.cur_time_samples + OUTPUT_FRAME_SIZE,
        )
        x = np.sin(2 * np.pi * 440 / SAMPLE_RATE * times) * 0.3
        x = x.astype(np.float32)
        self.cur_time_samples += OUTPUT_FRAME_SIZE

        # Uncomment to fix:
        # await asyncio.sleep(0.01)

        return (SAMPLE_RATE, x)

    def copy(self):
        return SineHandler()

    async def start_up(self) -> None:
        pass


if __name__ == "__main__":
    stream = Stream(
        handler=SineHandler(),
        modality="audio",
        mode="send-receive",
    )

    stream.ui.launch(debug=True)

(Side note - when I add the sleep(), I still get audio artifacts, see #202 )

Screen.Recording.2025-03-21.at.12.48.15.mov

FastRTC version 0.0.17, Arc browser (Chromium-based), MacOS

[freddyaboulton]

I think the instant emit may be flooding the event loop and so nothing else can run. I'll see if I can do something about this. In the meantime, can you add a small amount of sleep in the start_up and remove the sleep from emit and see if that fixes it?

[vvolhejn]

I tried

    async def start_up(self) -> None:
        while True:
            await asyncio.sleep(0.01)

and

    async def start_up(self) -> None:
        await asyncio.sleep(0.01)

but unfortunately neither worked.

[freddyaboulton]

Hi @vvolhejn - I actually can't reproduce this locally. I copy/pasted your code as-is and I don't get the hanging-behavior. Are you using the latest pypi release? What about using 480 as the output_frame_size?

2025-03-24_09-51-26.mp4

[vvolhejn]

Hm, I am on the latest version, and tried 480 as well... Strange, but it's not blocking me.

[freddyaboulton]

Ok glad it's not blocking you at least!

[freddyaboulton]

If someone else has this issue then maybe we can find a pattern