Merge pull request #25344 from akka/wip-25342-ReplicatorDeltaSpec-patriknw

Harden ReplicatorDeltaSpec, #25342

akka-distributed-data/src/main/scala/akka/cluster/ddata/Replicator.scala

@@ -847,6 +847,7 @@ object Replicator {
           override def zero: DeltaReplicatedData = this
           override def delta: Option[ReplicatedDelta] = None
           override def resetDelta: ReplicatedData = this
+          override def toString: String = "NoDeltaPlaceholder"
         }
     }
     case object DeltaNack extends ReplicatorMessage with DeadLetterSuppression

akka-distributed-data/src/multi-jvm/scala/akka/cluster/ddata/ReplicatorDeltaSpec.scala

@@ -331,6 +331,13 @@ class ReplicatorDeltaSpec extends MultiNodeSpec(ReplicatorDeltaSpec) with STMult
         // Thereafter delta can be propagated and applied again.
         deltaReplicator.tell(Update(KeyHigh, Highest(0), writeAll)(_.incr(100)), p1.ref)
         p1.expectMsgType[UpdateSuccess[_]]
+        // Flush the deltaPropagation buffer, otherwise it will contain
+        // NoDeltaPlaceholder from previous updates and the incr(4) delta will also
+        // be folded into NoDeltaPlaceholder and not propagated as delta. A few DeltaPropagationTick
+        // are needed to send to all and flush buffer.
+        roles.foreach { _ ⇒
+          deltaReplicator ! Replicator.Internal.DeltaPropagationTick
+        }
         deltaReplicator.tell(Update(KeyHigh, Highest(0), WriteLocal)(_.incr(4)), p1.ref)
         p1.expectMsgType[UpdateSuccess[_]]
       }