Add validation on gossip of LC updates as per spec (#2528)

fluffy/network/beacon/beacon_db.nim

@@ -314,6 +314,12 @@ proc createGetHandler*(db: BeaconDb): DbGetHandler =
         if len(updates) == 0:
           Opt.none(seq[byte])
         else:
+          # Note that this might not return all of the requested updates.
+          # This might seem faulty/tricky as it is also used in handleOffer to
+          # check if an offer should be accepted.
+          # But it is actually fine as this will occur only when the node is
+          # synced and it would not be able to verify the older updates in the
+          # range anyhow.
           Opt.some(SSZ.encode(updates))
       of lightClientFinalityUpdate:
         # TODO:

fluffy/network/beacon/beacon_network.nim

@@ -225,9 +225,23 @@ proc new*(
     trustedBlockRoot: trustedBlockRoot,
   )
 
+proc lightClientVerifier(
+    processor: ref LightClientProcessor, obj: SomeForkedLightClientObject
+): Future[Result[void, VerifierError]] {.async: (raises: [CancelledError], raw: true).} =
+  let resfut = Future[Result[void, VerifierError]].Raising([CancelledError]).init(
+      "lightClientVerifier"
+    )
+  processor[].addObject(MsgSource.gossip, obj, resfut)
+  resfut
+
+proc updateVerifier*(
+    processor: ref LightClientProcessor, obj: ForkedLightClientUpdate
+): auto =
+  processor.lightClientVerifier(obj)
+
 proc validateContent(
     n: BeaconNetwork, content: seq[byte], contentKey: ContentKeyByteList
-): Result[void, string] =
+): Future[Result[void, string]] {.async: (raises: [CancelledError]).} =
   let key = contentKey.decode().valueOr:
     return err("Error decoding content key")
 
@@ -277,15 +291,23 @@ proc validateContent(
       else:
         err("No LC data before Altair")
   of lightClientUpdate:
-    let decodingResult = decodeLightClientUpdatesByRange(n.forkDigests, content)
-    if decodingResult.isOk:
-      # TODO:
-      # Currently only verifying if the content can be decoded.
-      # Eventually only new updates that can be verified because the local
-      # node is synced should be accepted.
+    let updates = decodeLightClientUpdatesByRange(n.forkDigests, content).valueOr:
+      return err("Error decoding content: " & error)
+
+    # Only new updates can be verified as they get applied by the LC processor,
+    # so verification works only by being part of the sync process.
+    # This means that no backfill is possible, for that we need updates that
+    # get provided with a proof against historical_summaries, see also:
+    # https://github.com/ethereum/portal-network-specs/issues/305
+    # It is however a little more tricky, even updates that we do not have
+    # applied yet may fail here if the list of updates does not contain first
+    # the next update that is required currently for the sync.
+    for update in updates:
+      let res = await n.processor.updateVerifier(update)
+      if res.isErr():
+        return err("Error verifying LC updates: " & $res.error)
+
     ok()
-    else:
-      err("Error decoding content: " & decodingResult.error)
   of lightClientFinalityUpdate:
     let update = decodeLightClientFinalityUpdateForked(n.forkDigests, content).valueOr:
       return err("Error decoding content: " & error)
@@ -317,7 +339,7 @@ proc validateContent(
   for i, contentItem in contentItems:
     let
       contentKey = contentKeys[i]
-      validation = n.validateContent(contentItem, contentKey)
+      validation = await n.validateContent(contentItem, contentKey)
     if validation.isOk():
       let contentIdOpt = n.portalProtocol.toContentId(contentKey)
       if contentIdOpt.isNone():

fluffy/tools/portal_bridge/portal_bridge_beacon.nim

@@ -294,6 +294,16 @@ proc runBeacon*(config: PortalBridgeConf) {.raises: [CatchableError].} =
       backfillAmount: uint64,
       trustedBlockRoot: Option[TrustedDigest],
   ) {.async.} =
+    # TODO:
+    # It can get tricky when we need to bootstrap the beacon network with
+    # a portal_bridge:
+    # - Either a very recent bootstrap needs to be taken so that no updates are
+    # required for the nodes to sync.
+    # - Or the bridge needs to be tuned together with the selected bootstrap to
+    # provide the right amount of backfill updates.
+    # - Or the above point could get automatically implemented here based on the
+    # provided trusted-block-root
+
     # Bootstrap backfill, currently just one bootstrap selected by
     # trusted-block-root, could become a selected list, or some other way.
     if trustedBlockRoot.isSome():
@@ -306,6 +316,10 @@ proc runBeacon*(config: PortalBridgeConf) {.raises: [CatchableError].} =
 
       await portalRpcClient.close()
 
+    # Add some seconds delay to allow the bootstrap to be gossiped around.
+    # Without the bootstrap, following updates will not get accepted.
+    await sleepAsync(5.seconds)
+
     # Updates backfill, selected by backfillAmount
     # Might want to alter this to default backfill to the
     # `MIN_EPOCHS_FOR_BLOCK_REQUESTS`.