Allow HTTP API to return SSZ blocks (#2209)

## Issue Addressed

Implements https://github.com/ethereum/eth2.0-APIs/pull/125

## Proposed Changes

Optionally return SSZ bytes from the `beacon/blocks` endpoint.

beacon_node/http_api/src/lib.rs

@@ -872,11 +872,35 @@ pub fn serve<T: BeaconChainTypes>(
         .and(chain_filter.clone());
 
     // GET beacon/blocks/{block_id}
-    let get_beacon_block = beacon_blocks_path.clone().and(warp::path::end()).and_then(
-        |block_id: BlockId, chain: Arc<BeaconChain<T>>| {
-            blocking_json_task(move || block_id.block(&chain).map(api_types::GenericResponse::from))
-        },
-    );
+    let get_beacon_block = beacon_blocks_path
+        .clone()
+        .and(warp::path::end())
+        .and(warp::header::optional::<api_types::Accept>("accept"))
+        .and_then(
+            |block_id: BlockId,
+             chain: Arc<BeaconChain<T>>,
+             accept_header: Option<api_types::Accept>| {
+                blocking_task(move || {
+                    let block = block_id.block(&chain)?;
+                    match accept_header {
+                        Some(api_types::Accept::Ssz) => Response::builder()
+                            .status(200)
+                            .header("Content-Type", "application/octet-stream")
+                            .body(block.as_ssz_bytes().into())
+                            .map_err(|e| {
+                                warp_utils::reject::custom_server_error(format!(
+                                    "failed to create response: {}",
+                                    e
+                                ))
+                            }),
+                        _ => Ok(
+                            warp::reply::json(&api_types::GenericResponseRef::from(&block))
+                                .into_response(),
+                        ),
+                    }
+                })
+            },
+        );
 
     // GET beacon/blocks/{block_id}/root
     let get_beacon_block_root = beacon_blocks_path

beacon_node/http_api/tests/tests.rs

@@ -955,16 +955,18 @@ impl ApiTester {
 
     pub async fn test_beacon_blocks(self) -> Self {
         for block_id in self.interesting_block_ids() {
-            let result = self
+            let expected = self.get_block(block_id);
+
+            let json_result = self
                 .client
                 .get_beacon_blocks(block_id)
                 .await
                 .unwrap()
                 .map(|res| res.data);
+            assert_eq!(json_result, expected, "{:?}", block_id);
 
-            let expected = self.get_block(block_id);
-
-            assert_eq!(result, expected, "{:?}", block_id);
+            let ssz_result = self.client.get_beacon_blocks_ssz(block_id).await.unwrap();
+            assert_eq!(ssz_result, expected, "{:?}", block_id);
         }
 
         self

common/eth2/src/lib.rs

@@ -491,6 +491,27 @@ impl BeaconNodeHttpClient {
         self.get_opt(path).await
     }
 
+    /// `GET beacon/blocks` as SSZ
+    ///
+    /// Returns `Ok(None)` on a 404 error.
+    pub async fn get_beacon_blocks_ssz<T: EthSpec>(
+        &self,
+        block_id: BlockId,
+    ) -> Result<Option<SignedBeaconBlock<T>>, Error> {
+        let mut path = self.eth_path()?;
+
+        path.path_segments_mut()
+            .map_err(|()| Error::InvalidUrl(self.server.clone()))?
+            .push("beacon")
+            .push("blocks")
+            .push(&block_id.to_string());
+
+        self.get_bytes_opt_accept_header(path, Accept::Ssz)
+            .await?
+            .map(|bytes| SignedBeaconBlock::from_ssz_bytes(&bytes).map_err(Error::InvalidSsz))
+            .transpose()
+    }
+
     /// `GET beacon/blocks/{block_id}/root`
     ///
     /// Returns `Ok(None)` on a 404 error.