widen allowed specs for validator client

The validator client was only able to connect to beacon nodes exposing
the exact same set of spec constants that are locally known via their
config/spec REST API. However, that set of spec constants is dynamic.
As the validator client only requires a subset of relevant constants,
this may lead to compatible specs being rejected. This patch widens the
allowed specs by only verifying that the required set of constants are
present in the spec response, ignoring any spec constants that are not
locally known, and ignoring missing spec constants that are locally
known but not included by the remote beacon node when not relevant for
operation of the validator client.

beacon_chain/spec/eth2_apis/eth2_rest_serialization.nim

@@ -71,6 +71,12 @@ type
     GetBlockV2Response |
     GetStateV2Response
 
+  # These types may be extended with additional fields in the future.
+  # Locally unknown fields are silently ignored when decoding them.
+  ExtensibleDecodeTypes* =
+    GetSpecResponse |
+    GetSpecVCResponse
+
   SszDecodeTypes* =
     GetPhase0StateSszResponse |
     GetAltairStateSszResponse |
@@ -957,10 +963,11 @@ proc encodeBytes*[T: EncodeArrays](value: T,
 
 proc decodeBytes*[T: DecodeTypes](t: typedesc[T], value: openarray[byte],
                                   contentType: string): RestResult[T] =
+  const isExtensibleType = t is ExtensibleDecodeTypes
   case contentType
   of "application/json":
     try:
-      ok RestJson.decode(value, T)
+      ok RestJson.decode(value, T, allowUnknownFields = isExtensibleType)
     except SerializationError as exc:
       err("Serialization error")
   else:

beacon_chain/spec/eth2_apis/rest_config_calls.nim

@@ -20,6 +20,10 @@ proc getSpec*(): RestResponse[GetSpecResponse] {.
      rest, endpoint: "/eth/v1/config/spec", meth: MethodGet.}
   ## https://ethereum.github.io/beacon-APIs/#/Config/getSpec
 
+proc getSpecVC*(): RestResponse[GetSpecVCResponse] {.
+     rest, endpoint: "/eth/v1/config/spec", meth: MethodGet.}
+  ## https://ethereum.github.io/beacon-APIs/#/Config/getSpec
+
 proc getDepositContract*(): RestResponse[GetDepositContractResponse] {.
      rest, endpoint: "/eth/v1/config/deposit_contract", meth: MethodGet.}
   ## https://ethereum.github.io/beacon-APIs/#/Config/getDepositContract

beacon_chain/spec/eth2_apis/rest_types.nim

@@ -306,6 +306,39 @@ type
     DOMAIN_SYNC_COMMITTEE*: DomainType
     DOMAIN_SYNC_COMMITTEE_SELECTION_PROOF*: DomainType
 
+  # The `RestSpec` is a dynamic dictionary that includes version-specific spec
+  # constants. New versions may introduce new constants, and remove old ones.
+  # The Nimbus validator client fetches the remote spec to determine whether it
+  # is connected to a compatible beacon node. For this purpose, it only needs to
+  # verify a small set of relevant spec constants. To avoid rejecting a remote
+  # spec that includes all of those relevant spec constants, but that does not
+  # include all of the locally known spec constants, a separate type is defined
+  # that includes just the spec constants relevant for the validator client.
+  # Extra spec constants are silently ignored.
+  RestSpecVC* = object
+    # /!\ Keep in sync with `validator_client/api.nim` > `checkCompatible`.
+    MAX_VALIDATORS_PER_COMMITTEE*: uint64
+    SLOTS_PER_EPOCH*: uint64
+    SECONDS_PER_SLOT*: uint64
+    EPOCHS_PER_ETH1_VOTING_PERIOD*: uint64
+    SLOTS_PER_HISTORICAL_ROOT*: uint64
+    EPOCHS_PER_HISTORICAL_VECTOR*: uint64
+    EPOCHS_PER_SLASHINGS_VECTOR*: uint64
+    HISTORICAL_ROOTS_LIMIT*: uint64
+    VALIDATOR_REGISTRY_LIMIT*: uint64
+    MAX_PROPOSER_SLASHINGS*: uint64
+    MAX_ATTESTER_SLASHINGS*: uint64
+    MAX_ATTESTATIONS*: uint64
+    MAX_DEPOSITS*: uint64
+    MAX_VOLUNTARY_EXITS*: uint64
+    DOMAIN_BEACON_PROPOSER*: DomainType
+    DOMAIN_BEACON_ATTESTER*: DomainType
+    DOMAIN_RANDAO*: DomainType
+    DOMAIN_DEPOSIT*: DomainType
+    DOMAIN_VOLUNTARY_EXIT*: DomainType
+    DOMAIN_SELECTION_PROOF*: DomainType
+    DOMAIN_AGGREGATE_AND_PROOF*: DomainType
+
   RestDepositContract* = object
     chain_id*: string
     address*: string
@@ -371,6 +404,7 @@ type
   GetProposerDutiesResponse* = DataRootEnclosedObject[seq[RestProposerDuty]]
   GetSyncCommitteeDutiesResponse* = DataEnclosedObject[seq[RestSyncCommitteeDuty]]
   GetSpecResponse* = DataEnclosedObject[RestSpec]
+  GetSpecVCResponse* = DataEnclosedObject[RestSpecVC]
   GetStateFinalityCheckpointsResponse* = DataEnclosedObject[RestBeaconStatesFinalityCheckpoints]
   GetStateForkResponse* = DataEnclosedObject[Fork]
   GetStateRootResponse* = DataEnclosedObject[Eth2Digest]

beacon_chain/validator_client/api.nim

@@ -16,7 +16,7 @@ proc checkCompatible*(vc: ValidatorClientRef,
   let info =
     try:
       debug "Requesting beacon node network configuration"
-      let res = await node.client.getSpec()
+      let res = await node.client.getSpecVC()
       res.data.data
     except CancelledError as exc:
       error "Configuration request was interrupted"
@@ -55,6 +55,7 @@ proc checkCompatible*(vc: ValidatorClientRef,
 
   let genesisFlag = (genesis != vc.beaconGenesis)
   let configFlag =
+    # /!\ Keep in sync with `spec/eth2_apis/rest_types.nim` > `RestSpecVC`.
     info.MAX_VALIDATORS_PER_COMMITTEE != MAX_VALIDATORS_PER_COMMITTEE or
     info.SLOTS_PER_EPOCH != SLOTS_PER_EPOCH or
     info.SECONDS_PER_SLOT != SECONDS_PER_SLOT or

beacon_chain/validator_client/common.nim

@@ -69,7 +69,7 @@ type
   BeaconNodeServer* = object
     client*: RestClientRef
     endpoint*: string
-    config*: Option[RestSpec]
+    config*: Option[RestSpecVC]
     ident*: Option[string]
     genesis*: Option[RestGenesis]
     syncInfo*: Option[RestSyncInfo]