chore(sse): add space between event name and value for compatibility

according to https://html.spec.whatwg.org/multipage/server-sent-events.html#event-stream-interpretation
using `field: value` style is OK.

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

other client side tools may detect the `data` field rely on the space,

like this one https://github.com/openai/openai-python/blob/b82a3f7e4c462a8a10fa445193301a3cefef9a4a/openai/api_requestor.py#L106

and this one: https://github.com/sashabaranov/go-openai/blob/71a24931dbc5b7029901ff963dc4d0d2509aa7ed/stream_reader.go#L14

axum/src/response/sse.rs

@@ -171,9 +171,9 @@ pub struct Event {
 }
 
 impl Event {
- /// Set the event's data data field(s) (`data:<content>`)
+ /// Set the event's data data field(s) (`data: <content>`)
  ///
- /// Newlines in `data` will automatically be broken across `data:` fields.
+ /// Newlines in `data` will automatically be broken across `data: ` fields.
  ///
  /// This corresponds to [`MessageEvent`'s data field].
  ///
@@ -202,7 +202,7 @@ impl Event {
  self
  }
 
- /// Set the event's data field to a value serialized as unformatted JSON (`data:<content>`).
+ /// Set the event's data field to a value serialized as unformatted JSON (`data: <content>`).
  ///
  /// This corresponds to [`MessageEvent`'s data field].
  ///
@@ -220,7 +220,7 @@ impl Event {
  panic!("Called `EventBuilder::json_data` multiple times");
  }
 
- self.buffer.extend_from_slice(b"data:");
+ self.buffer.extend_from_slice(b"data: ");
  serde_json::to_writer((&mut self.buffer).writer(), &data).map_err(axum_core::Error::new)?;
  self.buffer.put_u8(b'\n');
 
@@ -358,10 +358,7 @@ impl Event {
  );
  self.buffer.extend_from_slice(name.as_bytes());
  self.buffer.put_u8(b':');
- // Prevent values that start with spaces having that space stripped
- if value.starts_with(b" ") {
- self.buffer.put_u8(b' ');
- }
+ self.buffer.put_u8(b' ');
  self.buffer.extend_from_slice(value);
  self.buffer.put_u8(b'\n');
  }
@@ -538,7 +535,7 @@ mod tests {
  #[test]
  fn leading_space_is_not_stripped() {
  let no_leading_space = Event::default().data("\tfoobar");
- assert_eq!(&*no_leading_space.finalize(), b"data:\tfoobar\n\n");
+ assert_eq!(&*no_leading_space.finalize(), b"data: \tfoobar\n\n");
 
  let leading_space = Event::default().data(" foobar");
  assert_eq!(&*leading_space.finalize(), b"data: foobar\n\n");