Add first Server Sent Events API endpoint (#1107)

* Add Server Sent Events API endpoint

* Support both event handlers as a transitory measure

* Fix merge conflicts

beacon_node/beacon_chain/src/events.rs

@@ -1,6 +1,10 @@
+use bus::Bus;
+use parking_lot::Mutex;
 use serde_derive::{Deserialize, Serialize};
+use slog::{error, Logger};
 use std::marker::PhantomData;
-use types::{Attestation, Epoch, EthSpec, Hash256, SignedBeaconBlock};
+use std::sync::Arc;
+use types::{Attestation, Epoch, EthSpec, Hash256, SignedBeaconBlock, SignedBeaconBlockHash};
 pub use websocket_server::WebSocketSender;
 
 pub trait EventHandler<T: EthSpec>: Sized + Send + Sync {
@@ -18,6 +22,80 @@ impl<T: EthSpec> EventHandler<T> for WebSocketSender<T> {
     }
 }
 
+pub struct ServerSentEvents<T: EthSpec> {
+    // Bus<> is itself Sync + Send.  We use Mutex<> here only because of the surrounding code does
+    // not enforce mutability statically (i.e. relies on interior mutability).
+    head_changed_queue: Arc<Mutex<Bus<SignedBeaconBlockHash>>>,
+    log: Logger,
+    _phantom: PhantomData<T>,
+}
+
+impl<T: EthSpec> ServerSentEvents<T> {
+    pub fn new(log: Logger) -> (Self, Arc<Mutex<Bus<SignedBeaconBlockHash>>>) {
+        let bus = Bus::new(T::slots_per_epoch() as usize);
+        let mutex = Mutex::new(bus);
+        let arc = Arc::new(mutex);
+        let this = Self {
+            head_changed_queue: arc.clone(),
+            log: log,
+            _phantom: PhantomData,
+        };
+        (this, arc)
+    }
+}
+
+impl<T: EthSpec> EventHandler<T> for ServerSentEvents<T> {
+    fn register(&self, kind: EventKind<T>) -> Result<(), String> {
+        match kind {
+            EventKind::BeaconHeadChanged {
+                current_head_beacon_block_root,
+                ..
+            } => {
+                let mut guard = self.head_changed_queue.lock();
+                if let Err(_) = guard.try_broadcast(current_head_beacon_block_root.into()) {
+                    error!(
+                        self.log,
+                        "Head change streaming queue full";
+                        "dropped_change" => format!("{}", current_head_beacon_block_root),
+                    );
+                }
+                Ok(())
+            }
+            _ => Ok(()),
+        }
+    }
+}
+
+// An event handler that pushes events to both the websockets handler and the SSE handler.
+// Named after the unix `tee` command.  Meant as a temporary solution before ditching WebSockets
+// completely once SSE functions well enough.
+pub struct TeeEventHandler<E: EthSpec> {
+    websockets_handler: WebSocketSender<E>,
+    sse_handler: ServerSentEvents<E>,
+}
+
+impl<E: EthSpec> TeeEventHandler<E> {
+    pub fn new(
+        log: Logger,
+        websockets_handler: WebSocketSender<E>,
+    ) -> Result<(Self, Arc<Mutex<Bus<SignedBeaconBlockHash>>>), String> {
+        let (sse_handler, bus) = ServerSentEvents::new(log);
+        let result = Self {
+            websockets_handler: websockets_handler,
+            sse_handler: sse_handler,
+        };
+        Ok((result, bus))
+    }
+}
+
+impl<E: EthSpec> EventHandler<E> for TeeEventHandler<E> {
+    fn register(&self, kind: EventKind<E>) -> Result<(), String> {
+        self.websockets_handler.register(kind.clone())?;
+        self.sse_handler.register(kind)?;
+        Ok(())
+    }
+}
+
 impl<T: EthSpec> EventHandler<T> for NullEventHandler<T> {
     fn register(&self, _kind: EventKind<T>) -> Result<(), String> {
         Ok(())
@@ -30,7 +108,7 @@ impl<T: EthSpec> Default for NullEventHandler<T> {
     }
 }
 
-#[derive(Debug, Serialize, Deserialize)]
+#[derive(Debug, Serialize, Deserialize, Clone)]
 #[serde(
     bound = "T: EthSpec",
     rename_all = "snake_case",