[FIX Fix bug in pivot block selector

src/main/scala/io/iohk/ethereum/blockchain/sync/PivotBlockSelector.scala

@@ -109,13 +109,15 @@ class PivotBlockSelector(
       timeout: Cancellable,
       headers: Map[ByteString, BlockHeaderWithVotes]
   ): Unit = {
-    val BlockHeaderWithVotes(mostPopularBlockHeader, updatedVotes) = headers.mostVotedHeader
+    // most voted header can return empty if we asked one peer and it returned us non expected block. Then headers map is empty
+    // so there is no most voted header
+    val maybeBlockHeaderWithVotes = headers.mostVotedHeader
     // All peers responded - consensus reached
-    if (peersToAsk.isEmpty && updatedVotes >= minPeersToChoosePivotBlock) {
+    if (peersToAsk.isEmpty && maybeBlockHeaderWithVotes.exists(hWv => hWv.votes >= minPeersToChoosePivotBlock)) {
       timeout.cancel()
-      sendResponseAndCleanup(mostPopularBlockHeader)
+      sendResponseAndCleanup(maybeBlockHeaderWithVotes.get.header)
       // Consensus could not be reached - ask additional peer if available
-    } else if (!isPossibleToReachConsensus(peersToAsk.size, updatedVotes)) {
+    } else if (!isPossibleToReachConsensus(peersToAsk.size, maybeBlockHeaderWithVotes.map(_.votes).getOrElse(0))) {
       timeout.cancel()
       if (waitingPeers.nonEmpty) { // There are more peers to ask
         val newTimeout = scheduler.scheduleOnce(peerResponseTimeout, self, ElectionPivotBlockTimeout)
@@ -206,11 +208,11 @@ object PivotBlockSelector {
   case class BlockHeaderWithVotes(header: BlockHeader, votes: Int = 1) {
     def vote: BlockHeaderWithVotes = copy(votes = votes + 1)
   }
-
+  import cats.implicits._
   implicit class SortableHeadersMap(headers: Map[ByteString, BlockHeaderWithVotes]) {
-    def mostVotedHeader: BlockHeaderWithVotes = headers.maxBy { case (_, headerWithVotes) =>
-      headerWithVotes.votes
-    }._2
+    def mostVotedHeader: Option[BlockHeaderWithVotes] = {
+      headers.toList.maximumByOption { case (_, headerWithVotes) => headerWithVotes.votes }.map(_._2)
+    }
   }
 
   case class ElectionDetails(participants: List[Peer], expectedPivotBlock: BigInt) {

src/test/scala/io/iohk/ethereum/blockchain/sync/PivotBlockSelectorSpec.scala

@@ -188,6 +188,37 @@ class PivotBlockSelectorSpec
     )
   }
 
+  it should "handle case when one peer responded with wrong block header" in new TestSetup {
+    override def minPeersToChoosePivotBlock: Int = 1
+
+    updateHandshakedPeers(HandshakedPeers(singlePeer))
+
+    pivotBlockSelector ! SelectPivotBlock
+
+    peerMessageBus.expectMsgAllOf(
+      Subscribe(MessageClassifier(Set(BlockHeaders.code), PeerSelector.WithId(peer1.id)))
+    )
+
+    etcPeerManager.expectMsgAllOf(
+      EtcPeerManagerActor.SendMessage(GetBlockHeaders(Left(expectedPivotBlock), 1, 0, reverse = false), peer1.id)
+    )
+
+    // peer responds with block header number
+    pivotBlockSelector ! MessageFromPeer(
+      BlockHeaders(Seq(pivotBlockHeader.copy(number = expectedPivotBlock + 1))),
+      peer1.id
+    )
+
+    peerMessageBus.expectMsgAllOf(
+      Unsubscribe(MessageClassifier(Set(BlockHeaders.code), PeerSelector.WithId(peer1.id))),
+      Unsubscribe()
+    )
+    time.advance(syncConfig.syncRetryInterval)
+
+    fastSync.expectNoMessage() // consensus not reached - process have to be repeated
+    peerMessageBus.expectNoMessage()
+  }
+
   it should "not ask additional peers if not needed" in new TestSetup {
     override val minPeersToChoosePivotBlock = 2
     override val peersToChoosePivotBlockMargin = 1