Bug: SSE not showing up in browser if not explicitly ending AsyncGenerator content with newline

[macabrus]

Description

I am using Litestar on the backend, HTMX + Jinja2 for templating. I have following code:

@get(path="/progress", sync_to_thread=False)
def handle_updates() -> ServerSentEvent:
    """This route pushes finish/progress events to the client."""

    async def result_notifier() -> AsyncGenerator[str, None]:
        # keep state per client, re-render template fragment
        ctx = {
            "progress": None,
            "tasks": (tasks := TASKS),
        }
        for i in itertools.count(1):
            html = f'<h1>{i}</h1>\n'   # this \n missing was the problem!!!
            yield html
            await asyncio.sleep(1)

    return ServerSentEvent(result_notifier())

and following event receiver in HTMX:

<div id="queue-status" hx-ext="sse" sse-connect="/progress" >
    <div sse-swap="message"></div>
</div>

also tried curling:

curl http://localhost:8000/progress
data: <h1>1</h1>
data: <h1>2</h1>
...

curl http://localhost:8000/progress
data: <h1>1</h1>

data: <h1>2</h1>

data: <h1>3</h1>

data: <h1>4</h1>
...

My issue is that i didn't explicitly end events with '\n' so browser was forever buffering them and not showing in event stream console (neither Safari nor Chrome). As I've learned, SSE should explicitly end in '\n\n' to prevent buffering behavior! Is this expected behaviour? It wasn't obvious for me at all and I suggest this to be in the docs.

URL to code causing the issue

No response

MCVE

No response

Steps to reproduce

No response

Screenshots

No response

Logs

server logs do not contain errors, client side behavior is odd.

Litestar Version

latest

Platform

• [ ] Linux
• [X] Mac
• [ ] Windows
• [ ] Other (Please specify in the description above)

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[JacobCoffee]

I think we could document this better in https://docs.litestar.dev/latest/usage/responses.html#server-sent-event-responses and also include a link to the living standard for it (https://html.spec.whatwg.org/multipage/server-sent-events.html)

[provinzkraut]

I think we should consider this a bug. It's very unintuitive and can be easily fixed by appending the newline if it's not already there :shrug:

[provinzkraut]

Now that I think about this, it might be better if simply were to add a parameter that could configure this. Something like disable_client_buffering?

[provinzkraut]

@euri10 Iiuc your PR #2888 would fix this one as well, right?

[euri10]

@euri10 Iiuc your PR #2888 would fix this one as well, right?

yes i think so :)