Add Beacon API endpoint to download blobs by block ID

beacon_node/http_api/src/block_id.rs

@@ -4,7 +4,7 @@ use eth2::types::BlockId as CoreBlockId;
 use std::fmt;
 use std::str::FromStr;
 use std::sync::Arc;
-use types::{Hash256, SignedBeaconBlock, SignedBlindedBeaconBlock, Slot};
+use types::{BlobsSidecar, Hash256, SignedBeaconBlock, SignedBlindedBeaconBlock, Slot};
 
 /// Wraps `eth2::types::BlockId` and provides a simple way to obtain a block or root for a given
 /// `BlockId`.
@@ -211,6 +211,51 @@ impl BlockId {
             }
         }
     }
+
+    /// Return the `BlobsSidecar` identified by `self`.
+    pub async fn blobs_sidecar<T: BeaconChainTypes>(
+        &self,
+        chain: &BeaconChain<T>,
+    ) -> Result<(Arc<BlobsSidecar<T::EthSpec>>), warp::Rejection> {
+        let root = match &self.0 {
+            CoreBlockId::Head => {
+                let (cached_head, _execution_status) = chain
+                    .canonical_head
+                    .head_and_execution_status()
+                    .map_err(warp_utils::reject::beacon_chain_error)?;
+                cached_head.head_block_root()
+            }
+            CoreBlockId::Slot(slot) => {
+                let maybe_block_root = chain
+                    .block_root_at_slot(*slot, WhenSlotSkipped::None)
+                    .ok()
+                    .flatten();
+                match maybe_block_root {
+                    Some(block_root) => block_root,
+                    None => {
+                        return Err(warp_utils::reject::custom_not_found(format!(
+                            "Block root for slot {} not found",
+                            slot
+                        )))
+                    }
+                }
+            }
+            _ => self.root(chain)?.0,
+        };
+
+        match chain.store.get_blobs(&root) {
+            Ok(Some(blob)) => Ok((Arc::new(blob))),
+            Ok(None) => Err(warp_utils::reject::custom_not_found(format!(
+                "Blob with block root {} is not in the store",
+                root
+            ))),
+            // should we use `warp_utils::reject::beacon_chain_error` instead?
+            Err(e) => Err(warp_utils::reject::custom_not_found(format!(
+                "Error fetching blob with block root {}: {:?}",
+                root, e
+            ))),
+        }
+    }
 }
 
 impl FromStr for BlockId {

beacon_node/http_api/src/lib.rs

@@ -3324,6 +3324,41 @@ pub fn serve<T: BeaconChainTypes>(
             )))
         });
 
+    // GET lighthouse/beacon/blob_sidecars/{block_id}
+    let get_blob_sidecars = warp::path("lighthouse")
+        .and(warp::path("beacon"))
+        .and(warp::path("blob_sidecars"))
+        .and(block_id_or_err)
+        .and(warp::path::end())
+        .and(chain_filter.clone())
+        .and(warp::header::optional::<api_types::Accept>("accept"))
+        .and_then(
+            |block_id: BlockId,
+             chain: Arc<BeaconChain<T>>,
+             accept_header: Option<api_types::Accept>| {
+                async move {
+                    let blobs_sidecar = block_id.blobs_sidecar(&chain).await?;
+
+                    match accept_header {
+                        Some(api_types::Accept::Ssz) => Response::builder()
+                            .status(200)
+                            .header("Content-Type", "application/octet-stream")
+                            .body(blobs_sidecar.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::GenericResponse::from(
+                            blobs_sidecar,
+                        ))
+                        .into_response()),
+                    }
+                }
+            },
+        );
+
     let get_events = eth_v1
         .and(warp::path("events"))
         .and(warp::path::end())