Added comment message to all the legacy endpoints whic explains the d…

…eprecation and refers to documentation for limitation (#311)

Co-authored-by: Jakub Zajkowski <[email protected]>

LEGACY_SSE_EMULATION.md

@@ -8,7 +8,7 @@ SSE emulation is by default turned off, the instruction on how to enable it is i
 
 **BEFORE YOU ENABLE LEGACY SSE EMULATION** please consider the following:
 
-- The legacy SSE emulation is a temporary solution and can be removed in a future major release.
+- The legacy SSE emulation is a temporary solution and can be removed in a future major release. Consider it being _deprecated_.
 - The legacy SSE emulation is not a 1:1 mapping of the 2.x events to 1.x events. Some events will be omitted, some will be transformed, some will be passed as is. More details on the limitations of the emulation are explained below.
 - The legacy SSE emulation is an additional drain on resources. It will consume more resources than the "native" 2.x SSE API.
 

event_sidecar/src/event_stream_server/http_server.rs

@@ -90,6 +90,13 @@ async fn send_api_version_from_global_state(
         .send(ServerSentEvent::initial_event(protocol_version))
 }
 
+async fn send_legacy_comment(
+    subscriber: &NewSubscriberInfo,
+) -> Result<(), SendError<ServerSentEvent>> {
+    subscriber
+        .initial_events_sender
+        .send(ServerSentEvent::legacy_comment_event())
+}
 async fn send_sidecar_version(
     subscriber: &NewSubscriberInfo,
 ) -> Result<(), SendError<ServerSentEvent>> {
@@ -113,7 +120,8 @@ async fn handle_incoming_data(
             trace!("Event stream server received {:?}", data);
             let event = ServerSentEvent {
                 id: maybe_event_index,
-                data: data.clone(),
+                comment: None,
+                data: Some(data.clone()),
                 inbound_filter,
             };
             match data {
@@ -146,6 +154,9 @@ async fn register_new_subscriber(
     buffer: &WheelBuf<Vec<(ProtocolVersion, ServerSentEvent)>, (ProtocolVersion, ServerSentEvent)>,
     latest_protocol_version: Option<ProtocolVersion>,
 ) {
+    if subscriber.enable_legacy_filters {
+        let _ = send_legacy_comment(&subscriber).await;
+    }
     let _ = send_sidecar_version(&subscriber).await;
     let mut observed_events = false;
     // If the client supplied a "start_from" index, provide the buffered events.