Publish clean state when joining (PublishStart), see #2871

* The failure in JoinTwoClustersSpec was due to missing publishing
  of cluster events when clearing current state when joining
* This fix is in the right direction, but joining clusters like this
  will need some design thought, creating ticket 2873 for that

akka-cluster/src/main/scala/akka/cluster/ClusterDaemon.scala

@@ -191,7 +191,7 @@ private[cluster] final class ClusterCoreDaemon(publisher: ActorRef) extends Acto
 
   // note that self is not initially member,
   // and the Gossip is not versioned for this 'Node' yet
-  var latestGossip: Gossip = Gossip()
+  var latestGossip: Gossip = Gossip.empty
 
   var stats = ClusterStats()
 
@@ -283,7 +283,7 @@ private[cluster] final class ClusterCoreDaemon(publisher: ActorRef) extends Acto
   def join(address: Address): Unit = {
     if (!latestGossip.members.exists(_.address == address)) {
       // wipe our state since a node that joins a cluster must be empty
-      latestGossip = Gossip()
+      latestGossip = Gossip.empty
       // wipe the failure detector since we are starting fresh and shouldn't care about the past
       failureDetector.reset()
       // wipe the publisher since we are starting fresh
@@ -377,7 +377,7 @@ private[cluster] final class ClusterCoreDaemon(publisher: ActorRef) extends Acto
   def removing(address: Address): Unit = {
     log.info("Cluster Node [{}] - Node has been REMOVED by the leader - shutting down...", selfAddress)
     // just cleaning up the gossip state
-    latestGossip = Gossip()
+    latestGossip = Gossip.empty
     publish(latestGossip)
     context.become(removed)
     // make sure the final (removed) state is published

akka-cluster/src/main/scala/akka/cluster/ClusterEvent.scala

@@ -238,8 +238,8 @@ object ClusterEvent {
 private[cluster] final class ClusterDomainEventPublisher extends Actor with ActorLogging {
   import InternalClusterAction._
 
-  var latestGossip: Gossip = Gossip()
-  var latestConvergedGossip: Gossip = Gossip()
+  var latestGossip: Gossip = Gossip.empty
+  var latestConvergedGossip: Gossip = Gossip.empty
   var memberEvents: immutable.Seq[MemberEvent] = immutable.Seq.empty
 
   def receive = {
@@ -313,7 +313,11 @@ private[cluster] final class ClusterDomainEventPublisher extends Actor with Acto
 
   def publish(event: AnyRef): Unit = eventStream publish event
 
-  def publishStart(): Unit = clearState()
+  def publishStart(): Unit =
+    if ((latestGossip ne Gossip.empty) || (latestConvergedGossip ne Gossip.empty)) {
+      clearState()
+      publishCurrentClusterState(None)
+    }
 
   def publishDone(receiver: ActorRef): Unit = {
     clearState()
@@ -321,7 +325,7 @@ private[cluster] final class ClusterDomainEventPublisher extends Actor with Acto
   }
 
   def clearState(): Unit = {
-    latestGossip = Gossip()
-    latestConvergedGossip = Gossip()
+    latestGossip = Gossip.empty
+    latestConvergedGossip = Gossip.empty
   }
 }

akka-cluster/src/main/scala/akka/cluster/Gossip.scala

@@ -13,6 +13,10 @@ import MemberStatus._
  */
 private[cluster] object Gossip {
   val emptyMembers: immutable.SortedSet[Member] = immutable.SortedSet.empty
+  val empty: Gossip = new Gossip(Gossip.emptyMembers)
+
+  def apply(members: immutable.SortedSet[Member]) =
+    if (members.isEmpty) empty else empty.copy(members = members)
 }
 
 /**
@@ -49,8 +53,8 @@ private[cluster] object Gossip {
  * removed node telling it to shut itself down.
  */
 private[cluster] case class Gossip(
+  members: immutable.SortedSet[Member], // sorted set of members with their status, sorted by address
   overview: GossipOverview = GossipOverview(),
-  members: immutable.SortedSet[Member] = Gossip.emptyMembers, // sorted set of members with their status, sorted by address
   version: VectorClock = VectorClock()) // vector clock version
   extends ClusterMessage // is a serializable cluster message
   with Versioned[Gossip] {
@@ -128,7 +132,7 @@ private[cluster] case class Gossip(
     // 4. fresh seen table
     val mergedSeen = Map.empty[Address, VectorClock]
 
-    Gossip(GossipOverview(mergedSeen, mergedUnreachable), mergedMembers, mergedVClock)
+    Gossip(mergedMembers, GossipOverview(mergedSeen, mergedUnreachable), mergedVClock)
   }
 
   /**

akka-cluster/src/test/scala/akka/cluster/ClusterDomainEventPublisherSpec.scala

@@ -134,5 +134,17 @@ class ClusterDomainEventPublisherSpec extends AkkaSpec
       expectMsg(MemberUp(c2))
       expectMsgType[SeenChanged]
     }
+
+    "publish clean state when PublishStart" in {
+      publisher ! PublishChanges(g3)
+      expectMsg(MemberUp(b1))
+      expectMsg(MemberUp(c2))
+      expectMsgType[SeenChanged]
+
+      publisher ! PublishStart
+      expectMsgType[CurrentClusterState] must be(CurrentClusterState())
+
+    }
+
   }
 }

akka-cluster/src/test/scala/akka/cluster/GossipSpec.scala

@@ -29,7 +29,7 @@ class GossipSpec extends WordSpec with MustMatchers {
   "A Gossip" must {
 
     "reach convergence when it's empty" in {
-      Gossip().convergence must be(true)
+      Gossip.empty.convergence must be(true)
     }
 
     "merge members by status priority" in {