don't quarantine back, #21450

* Don't quarantine the other system when receiving the Quarantined message,
  since that will result cluster member removal and can result in
  forming two separate clusters (cluster split).
* Instead, the downing strategy should act on ThisActorSystemQuarantinedEvent, e.g.
  use it as a STONITH signal.

akka-cluster/src/multi-jvm/scala/akka/cluster/SurviveNetworkInstabilitySpec.scala

@@ -366,18 +366,11 @@ abstract class SurviveNetworkInstabilitySpec
       }
 
       runOn(side2: _*) {
-        if (RARP(system).provider.remoteSettings.Artery.Enabled) {
+        // side2 comes back but stays unreachable
-          // with artery the other side stays quarantined
-          val expected = (side2 map address).toSet
-          clusterView.members.map(_.address) should ===(expected)
-
-        } else {
-          // with the old remoting side2 comes back but stays unreachable
         val expected = ((side2 ++ side1) map address).toSet
         clusterView.members.map(_.address) should ===(expected)
         assertUnreachable(side1: _*)
       }
-      }
 
       enterBarrier("after-7")
       assertCanTalk(side1AfterJoin: _*)

akka-remote/src/main/scala/akka/remote/artery/ArteryTransport.scala

@@ -623,10 +623,12 @@ private[remote] class ArteryTransport(_system: ExtendedActorSystem, _provider: R
             }
 
           case Quarantined(from, to) if to == localAddress ⇒
+            // Don't quarantine the other system here, since that will result cluster member removal
+            // and can result in forming two separate clusters (cluster split).
+            // Instead, the downing strategy should act on ThisActorSystemQuarantinedEvent, e.g.
+            // use it as a STONITH signal.
             val lifecycleEvent = ThisActorSystemQuarantinedEvent(localAddress.address, from.address)
             publishLifecycleEvent(lifecycleEvent)
-            // quarantine the other system from here
-            association(from.address).quarantine(lifecycleEvent.toString, Some(from.uid))
 
           case _: ActorSystemTerminating ⇒
             inboundEnvelope.sender match {