Implement SSZ responses in the light client REST APIs

Other changes:

* More DRY encoding of the Nimbus content type preference.
* Switch if/elif/else to exhaustive case statements to guard
  the code better from future changes.

beacon_chain/rpc/rest_beacon_api.nim

@@ -785,22 +785,18 @@ proc installBeaconApiHandlers*(router: var RestRouter, node: BeaconNode) =
       return RestApiResponse.jsonError(
         Http404, BlockNotFoundError, "v1 API supports only phase 0 blocks")
 
-    let contentType =
-      block:
-        let res = preferredContentType(jsonMediaType,
-                                       sszMediaType)
-        if res.isErr():
-          return RestApiResponse.jsonError(Http406, ContentNotAcceptableError)
-        res.get()
+    let responseType = request.pickResponseType().valueOr:
+      return RestApiResponse.jsonError(Http406, ContentNotAcceptableError)
 
     return
-      if contentType == sszMediaType:
+      case responseType
+      of sszResponseType:
         var data: seq[byte]
         if not node.dag.getBlockSSZ(bid, data):
           return RestApiResponse.jsonError(Http404, BlockNotFoundError)
 
         RestApiResponse.response(data, Http200, $sszMediaType)
-      elif contentType == jsonMediaType:
+      of jsonResponseType:
         let bdata = node.dag.getForkedBlock(bid).valueOr:
           return RestApiResponse.jsonError(Http404, BlockNotFoundError)
 
@@ -811,8 +807,6 @@ proc installBeaconApiHandlers*(router: var RestRouter, node: BeaconNode) =
           # issue..
           RestApiResponse.jsonError(
             Http404, BlockNotFoundError, "v1 API supports only phase 0 blocks")
-      else:
-        RestApiResponse.jsonError(Http500, InvalidAcceptError)
 
   # https://ethereum.github.io/beacon-APIs/#/Beacon/getBlockV2
   router.api(MethodGet, "/eth/v2/beacon/blocks/{block_id}") do (
@@ -824,15 +818,12 @@ proc installBeaconApiHandlers*(router: var RestRouter, node: BeaconNode) =
       bid = node.getBlockId(blockIdent).valueOr:
         return RestApiResponse.jsonError(Http404, BlockNotFoundError)
 
-    let contentType =
-      block:
-        let res = preferredContentType(jsonMediaType,
-                                       sszMediaType)
-        if res.isErr():
-          return RestApiResponse.jsonError(Http406, ContentNotAcceptableError)
-        res.get()
+    let responseType = request.pickResponseType().valueOr:
+      return RestApiResponse.jsonError(Http406, ContentNotAcceptableError)
+
     return
-      if contentType == sszMediaType:
+      case responseType
+      of sszResponseType:
         var data: seq[byte]
         if not node.dag.getBlockSSZ(bid, data):
           return RestApiResponse.jsonError(Http404, BlockNotFoundError)
@@ -843,7 +834,7 @@ proc installBeaconApiHandlers*(router: var RestRouter, node: BeaconNode) =
 
         RestApiResponse.response(data, Http200, $sszMediaType,
                                  headers = headers)
-      elif contentType == jsonMediaType:
+      of jsonResponseType:
         let bdata = node.dag.getForkedBlock(bid).valueOr:
           return RestApiResponse.jsonError(Http404, BlockNotFoundError)
 
@@ -856,8 +847,6 @@ proc installBeaconApiHandlers*(router: var RestRouter, node: BeaconNode) =
           node.getBlockOptimistic(bdata),
           headers
         )
-      else:
-        RestApiResponse.jsonError(Http500, InvalidAcceptError)
 
   # https://ethereum.github.io/beacon-APIs/#/Beacon/getBlockRoot
   router.api(MethodGet, "/eth/v1/beacon/blocks/{block_id}/root") do (

beacon_chain/rpc/rest_debug_api.nim

@@ -30,23 +30,15 @@ proc installDebugApiHandlers*(router: var RestRouter, node: BeaconNode) =
           return RestApiResponse.jsonError(Http404, StateNotFoundError,
                                            $bres.error())
         bres.get()
-    let contentType =
-      block:
-        let res = preferredContentType(jsonMediaType,
-                                       sszMediaType)
-        if res.isErr():
-          return RestApiResponse.jsonError(Http406, ContentNotAcceptableError)
-        res.get()
+
+    let responseType = request.pickResponseType().valueOr:
+      return RestApiResponse.jsonError(Http406, ContentNotAcceptableError)
+
     node.withStateForBlockSlotId(bslot):
       return
         case state.kind
         of BeaconStateFork.Phase0:
-          if contentType == sszMediaType:
-            RestApiResponse.sszResponse(state.phase0Data.data, [])
-          elif contentType == jsonMediaType:
-            RestApiResponse.jsonResponse(state.phase0Data.data)
-          else:
-            RestApiResponse.jsonError(Http500, InvalidAcceptError)
+          responseType.okResponse state.phase0Data.data
         of BeaconStateFork.Altair, BeaconStateFork.Bellatrix:
           RestApiResponse.jsonError(Http404, StateNotFoundError)
     return RestApiResponse.jsonError(Http404, StateNotFoundError)
@@ -65,26 +57,22 @@ proc installDebugApiHandlers*(router: var RestRouter, node: BeaconNode) =
           return RestApiResponse.jsonError(Http404, StateNotFoundError,
                                            $bres.error())
         bres.get()
-    let contentType =
-      block:
-        let res = preferredContentType(jsonMediaType,
-                                       sszMediaType)
-        if res.isErr():
-          return RestApiResponse.jsonError(Http406, ContentNotAcceptableError)
-        res.get()
+
+    let responseType = request.pickResponseType().valueOr:
+      return RestApiResponse.jsonError(Http406, ContentNotAcceptableError)
+
     node.withStateForBlockSlotId(bslot):
       return
-        if contentType == jsonMediaType:
+        case responseType
+        of jsonResponseType:
           RestApiResponse.jsonResponseState(
             state,
             node.getStateOptimistic(state)
           )
-        elif contentType == sszMediaType:
+        of sszResponseType:
           let headers = [("eth-consensus-version", state.kind.toString())]
           withState(state):
             RestApiResponse.sszResponse(state.data, headers)
-        else:
-          RestApiResponse.jsonError(Http500, InvalidAcceptError)
     return RestApiResponse.jsonError(Http404, StateNotFoundError)
 
   # https://ethereum.github.io/beacon-APIs/#/Debug/getDebugChainHeads

beacon_chain/rpc/rest_light_client_api.nim

@@ -13,6 +13,11 @@ import ../beacon_node,
 
 logScope: topics = "rest_light_client"
 
+const
+  # TODO: This needs to be specified in the spec
+  # https://github.com/ethereum/beacon-APIs/pull/181#issuecomment-1172877455
+  MAX_CLIENT_UPDATES = 10000
+
 proc installLightClientApiHandlers*(router: var RestRouter, node: BeaconNode) =
   # https://github.com/ethereum/beacon-APIs/pull/181
   router.api(MethodGet,
@@ -25,9 +30,13 @@ proc installLightClientApiHandlers*(router: var RestRouter, node: BeaconNode) =
                                          $block_root.error())
       block_root.get()
 
-    let bootstrap = node.dag.getLightClientBootstrap(vroot)
+    let
+      responseType = request.pickResponseType().valueOr:
+        return RestApiResponse.jsonError(Http406, ContentNotAcceptableError)
+      bootstrap = node.dag.getLightClientBootstrap(vroot)
+
     if bootstrap.isOk:
-      return RestApiResponse.jsonResponse(bootstrap)
+      return responseType.okResponse bootstrap.get
     else:
       return RestApiResponse.jsonError(Http404, LCBootstrapUnavailable)
 
@@ -53,7 +62,10 @@ proc installLightClientApiHandlers*(router: var RestRouter, node: BeaconNode) =
         return RestApiResponse.jsonError(Http400, InvalidCountError,
                                          $rcount.error())
       rcount.get()
+
     let
+      responseType = request.pickResponseType().valueOr:
+        return RestApiResponse.jsonError(Http406, ContentNotAcceptableError)
       headPeriod = node.dag.head.slot.sync_committee_period
        # Limit number of updates in response
       maxSupportedCount =
@@ -69,16 +81,26 @@ proc installLightClientApiHandlers*(router: var RestRouter, node: BeaconNode) =
       let update = node.dag.getLightClientUpdateForPeriod(period)
       if update.isSome:
         updates.add update.get
-    return RestApiResponse.jsonResponse(updates)
+
+    return
+      case responseType
+      of jsonResponseType:
+        RestApiResponse.jsonResponse(updates)
+      of sszResponseType:
+        RestApiResponse.sszResponse(updates.asSszList(MAX_CLIENT_UPDATES))
 
   # https://github.com/ethereum/beacon-APIs/pull/181
   router.api(MethodGet,
              "/eth/v0/beacon/light_client/finality_update") do (
     ) -> RestApiResponse:
     doAssert node.dag.lcDataStore.serve
-    let finality_update = node.dag.getLightClientFinalityUpdate()
+    let
+      responseType = request.pickResponseType().valueOr:
+        return RestApiResponse.jsonError(Http406, ContentNotAcceptableError)
+      finality_update = node.dag.getLightClientFinalityUpdate()
+
     if finality_update.isSome:
-      return RestApiResponse.jsonResponse(finality_update)
+      return responseType.okResponse finality_update.get
     else:
       return RestApiResponse.jsonError(Http404, LCFinUpdateUnavailable)
 
@@ -87,8 +109,12 @@ proc installLightClientApiHandlers*(router: var RestRouter, node: BeaconNode) =
              "/eth/v0/beacon/light_client/optimistic_update") do (
     ) -> RestApiResponse:
     doAssert node.dag.lcDataStore.serve
-    let optimistic_update = node.dag.getLightClientOptimisticUpdate()
+    let
+      responseType = request.pickResponseType().valueOr:
+        return RestApiResponse.jsonError(Http406, ContentNotAcceptableError)
+      optimistic_update = node.dag.getLightClientOptimisticUpdate()
+
     if optimistic_update.isSome:
-      return RestApiResponse.jsonResponse(optimistic_update)
+      return responseType.okResponse optimistic_update.get
     else:
       return RestApiResponse.jsonError(Http404, LCOptUpdateUnavailable)

beacon_chain/rpc/rest_utils.nim

@@ -310,3 +310,40 @@ const
   jsonMediaType* = MediaType.init("application/json")
   sszMediaType* = MediaType.init("application/octet-stream")
   textEventStreamMediaType* = MediaType.init("text/event-stream")
+
+type
+  ResponseContentType* = enum
+    # Please note that the order of content types here determines
+    # the order of preference for the returned result by Nimbus.
+    # This Nimbus preference is used when the request doesn't
+    # explicitly state another preference.
+    jsonResponseType = "application/json"
+    sszResponseType = "application/octet-stream"
+
+proc pickResponseType*(request: HttpRequestRef): Result[ResponseContentType, void] =
+  proc mediaTypesSeq: seq[MediaType] {.compileTime.} =
+    for t in enumStrValuesArray ResponseContentType:
+      result.add MediaType.init(t)
+
+  const mediaTypes = mediaTypesSeq()
+
+  let pick = try: request.preferredContentType(mediaTypes)
+             except ValueError:
+               # TODO: Fix this API in Chronos.
+               # Mixing Result with Exceptions is awkward to use.
+               return err()
+  if pick.isErr:
+    return err()
+
+  for idx, mediaType in mediaTypes:
+    if pick.get == mediaType:
+      return ok ResponseContentType(idx)
+
+proc okResponse*(
+    responseType: ResponseContentType,
+    value: auto): RestApiResponse =
+  case responseType
+  of jsonResponseType:
+    RestApiResponse.jsonResponse value
+  of sszResponseType:
+    RestApiResponse.sszResponse value

beacon_chain/spec/eth2_apis/eth2_rest_serialization.nim

@@ -131,6 +131,12 @@ type
 
 {.push raises: [Defect].}
 
+template asSszList*[T](x: seq[T], limit: static Limit): untyped =
+  # XXX This works around generic instantiation problems
+  type E = typeof x[0]
+  const L = limit
+  List[E, L](x)
+
 proc writeValue*(writer: var JsonWriter[RestJson],
                  epochFlags: EpochParticipationFlags)
                 {.raises: [IOError, Defect].} =
@@ -448,7 +454,7 @@ proc jsonErrorList*(t: typedesc[RestApiResponse],
   RestApiResponse.error(status, data, "application/json")
 
 proc sszResponse*(t: typedesc[RestApiResponse], data: auto,
-                  headers: openArray[tuple[key: string, value: string]]
+                  headers: openArray[tuple[key: string, value: string]] = []
                  ): RestApiResponse =
   let res =
     block:

beacon_chain/spec/eth2_apis/rest_debug_calls.nim

@@ -94,8 +94,9 @@ proc getStateV2*(client: RestClientRef, state_id: StateIdent,
       of "application/json":
         let state =
           block:
-            let res = newClone(decodeBytes(GetStateV2Response, resp.data,
-                                  resp.contentType))
+            let res = newClone(decodeBytes(GetStateV2Response,
+                                           resp.data,
+                                           resp.contentType))
             if res[].isErr():
               raise newException(RestError, $res[].error())
             newClone(res[].get())