Sending a SSE:Event with leading Whitespace

[michaelfeil]

Using https://docs.rs/axum/latest/axum/response/sse/struct.Event.html, I am trying to send a SSE, where the data: field of SSe is a bit special, it starts with a single leading whitespace.

Sadly, I cannot modify this requirement, its in the openai streaming protocol https://github.com/openai/openai-python/blob/b82a3f7e4c462a8a10fa445193301a3cefef9a4a/openai/api_requestor.py#L102
Clients would expect as stop token for the SSE, python 3.9 would check for b"data: [DONE]" -> aka payload of event is b" [DONE]" -> [32, 91, 68, 79, 78, 69, 93] in utf-8 bytes as e.g. stop sequence.

let stop_seq = vec![32, 91, 68, 79, 78, 69, 93];
let stop_seq = std::str::from_utf8(&stop_seq).unwrap();
yield Ok(Event::default().data(stop_seq));

When trying this code above, I however yield the following sequence:
[100, 97, 116, 97, 58, 32, 32, 91, 68, 79, 78, 69, 93] for Event data vec![32, 91, 68, 79, 78, 69, 93];
and
[100, 97, 116, 97, 58, 91, 68, 79, 78, 69, 93] for Event data vec![91, 68, 79, 78, 69, 93];

How can I add a leading whitespace?
s

Same would go on how to add

stream_token: Box<dyn Serialize + Send> = ..
yield Ok(Event::default().json_data(stream_token).unwrap());

FYI:

• Context with some visualizations: Wrapping completions and chat/completions endpoint Preemo-Inc/text-generation-inference#2
• Mozilla SSE docu contains leading whitespace for SSE: https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events/Using_server-sent_events#examples
• This started all here Don't drop leading spaces in SSE responses #600 and here: https://github.com/tokio-rs/axum/blob/432289dd042d4f91af54dda9d9b2abaaae7d36be/axum/src/response/sse.rs#L540C1-L544C70
  0 whitespace -> 0 whitespace
  1 whitespace -> 2 whitespaces for data:

[davidpdrsn]

I was gonna suggest that you include an additional space yourself with .data(" foo") but it seems we have a bug where that causes an additional space to be added here https://github.com/tokio-rs/axum/blob/main/axum/src/response/sse.rs#L362-L364

I can fix that tomorrow. Would that work for you?

[michaelfeil]

Would work for me! Thanks a lot for checking this out!

I think the PR was a mistake, the implementation is not conform to the standard, a leading space after data: should be ignored, most other libraries in Java, Python implement data field with white-space included as b"data: " to avoid this confusion.
https://github.com/sysid/sse-starlette/blob/03e4fa1dc6dfbe63febb5fc8a1a48234e8a8f707/tests/test_sse.py#L14

https://html.spec.whatwg.org/multipage/server-sent-events.html#processField

The following stream fires two identical events:

data:test

data: test

This is because the space after the colon is ignored if present.

[davidpdrsn]

Yeah I see it in the spec as well https://html.spec.whatwg.org/multipage/server-sent-events.html#event-stream-interpretation

We also have this test specifically for the double space https://github.com/tokio-rs/axum/blob/main/axum/src/response/sse.rs#L543-L544

I suppose that means everything is working fine?

[michaelfeil]

No, just added some context. As you said earlier, it does not work, sending b"data: [DONE]" is impossible with the current implementation.

[davidpdrsn]

Shouldn't that be fixed in the client? Then all severs benefit.

[michaelfeil]

I am already doing so. The protocol is many ways reverse engineered, e.g. openai/openai-python#559