SseEventBuilder implementation is not standards compliant

[leon2LF]

When I was using SseEventBuilder in spring mvc 5.3.x, I noticed that no spaces were added when constructing "data:" in the data method of the SseEventBuilderImpl implementation class. As a result, when the output is in markdown format, when the content that starts with a space appears in the segmented interception, the space will be lost on the front end. When reviewing the protocol standards of SSE, it was found that when a space appears after ":" in the content, the space is removed. See server-sent-events 9.2.6, which references

If the line starts with a U+003A COLON character (:)
Ignore the line.
If the line contains a U+003A COLON character (:)
Collect the characters on the line before the first U+003A COLON character (:), and let be that string.field
Collect the characters on the line after the first U+003A COLON character (:), and let be that string. If starts with a U+0020 SPACE character, remove it from .valuevaluevalue

So I suggest modifying append("data:") in the SseEventBuilderImpl.data method of spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/SseEmitter.java to append("data: "); The other versions are modified in the same way

[quaff]

According to the reference, data:value is same to data: value, why do you think data:value is not standards compliant?

[bclozel]

Spring Framework is sending valid SSE chunks. In this case, I think the application is responsible for encoding/escaping the payloads to account for this.

[icguy]

the application is responsible for encoding/escaping the payloads to account for this

I believe this is incorrect. Spring is sending valid SSE chunks but they do not contain the supplied payload. I've implemented a minimal reproduction: https://github.com/icguy/spring33963

val payloads = listOf("These", " words", " should", " arrive", " with", " spaces", " between", " them")
val emitter = SseEmitter()
thread {
	for (p in payloads) {
		val event = SseEmitter.event().data(p).build()
		emitter.send(event)
	}
	emitter.complete()
}
return emitter

As you can see the payload for most events starts with a space character.

This is the raw response:

data:These

data: words

data: should

data: arrive

data: with

data: spaces

data: between

data: them

But according to the spec during parsing on the client side if value starts with a U+0020 SPACE character, remove it from value.

This is the behavior of the minimal client I wrote to demonstrate the issue (also in the repository) which simply concatenates the values received via an EventSource object:

So in conclusion: no, Spring doesn't send the payload in a standard compliant manner unless the intention was to implicitly drop a leading space from the payload.

[bclozel]

@icguy SseEmitter can accept many types as data. If you are providing such strings directly, I would argue that managing the space character here should be done by your implementation.

[icguy]

I don't think it's unreasonable for an API consumer to expect to receive on the client side exactly what they specified on the server side.

Is there a particular reason against the originally proposed solution of replacing data: with data: ?

[bclozel]

I'd say that both are completely standard, but if we make this behavior change we might break existing applications that already take this into account. Other contributors could say that data: is perfectly valid and is the minimum that the server side should do, and by forcing data: we take away an application choice.