Fix Sse client api (#6685)

* Use reqwest eventsource for get_events api

* await for Event::Open before returning stream

* fmt

* Merge branch 'unstable' into sse-client-fix

* Ignore lint

common/eth2/src/lib.rs

@@ -27,6 +27,7 @@ use reqwest::{
     Body, IntoUrl, RequestBuilder, Response,
 };
 pub use reqwest::{StatusCode, Url};
+use reqwest_eventsource::{Event, EventSource};
 pub use sensitive_url::{SensitiveError, SensitiveUrl};
 use serde::{de::DeserializeOwned, Serialize};
 use ssz::Encode;
@@ -52,6 +53,8 @@ pub const SSZ_CONTENT_TYPE_HEADER: &str = "application/octet-stream";
 pub enum Error {
     /// The `reqwest` client raised an error.
     HttpClient(PrettyReqwestError),
+    /// The `reqwest_eventsource` client raised an error.
+    SseClient(reqwest_eventsource::Error),
     /// The server returned an error message where the body was able to be parsed.
     ServerMessage(ErrorMessage),
     /// The server returned an error message with an array of errors.
@@ -93,6 +96,13 @@ impl Error {
     pub fn status(&self) -> Option<StatusCode> {
         match self {
             Error::HttpClient(error) => error.inner().status(),
+            Error::SseClient(error) => {
+                if let reqwest_eventsource::Error::InvalidStatusCode(status, _) = error {
+                    Some(*status)
+                } else {
+                    None
+                }
+            }
             Error::ServerMessage(msg) => StatusCode::try_from(msg.code).ok(),
             Error::ServerIndexedMessage(msg) => StatusCode::try_from(msg.code).ok(),
             Error::StatusCode(status) => Some(*status),
@@ -2592,16 +2602,29 @@ impl BeaconNodeHttpClient {
             .join(",");
         path.query_pairs_mut().append_pair("topics", &topic_string);
 
-        Ok(self
-            .client
-            .get(path)
-            .send()
-            .await?
-            .bytes_stream()
-            .map(|next| match next {
-                Ok(bytes) => EventKind::from_sse_bytes(bytes.as_ref()),
-                Err(e) => Err(Error::HttpClient(e.into())),
-            }))
+        let mut es = EventSource::get(path);
+        // If we don't await `Event::Open` here, then the consumer
+        // will not get any Message events until they start awaiting the stream.
+        // This is a way to register the stream with the sse server before
+        // message events start getting emitted.
+        while let Some(event) = es.next().await {
+            match event {
+                Ok(Event::Open) => break,
+                Err(err) => return Err(Error::SseClient(err)),
+                // This should never happen as we are guaranteed to get the
+                // Open event before any message starts coming through.
+                Ok(Event::Message(_)) => continue,
+            }
+        }
+        Ok(Box::pin(es.filter_map(|event| async move {
+            match event {
+                Ok(Event::Open) => None,
+                Ok(Event::Message(message)) => {
+                    Some(EventKind::from_sse_bytes(&message.event, &message.data))
+                }
+                Err(err) => Some(Err(Error::SseClient(err))),
+            }
+        })))
     }
 
     /// `POST validator/duties/sync/{epoch}`