Allow cluster formation with mixed protocols

cluster/src/main/scala/org/apache/pekko/cluster/ClusterDaemon.scala

@@ -365,6 +365,8 @@ private[cluster] class ClusterCoreDaemon(publisher: ActorRef, joinConfigCompatCh
   val statsEnabled = PublishStatsInterval.isFinite
   var gossipStats = GossipStats()
 
+  val acceptedProtocols = context.system.settings.config.getStringList("pekko.remote.accept-protocol-names")
+
   var seedNodes = SeedNodes
   var seedNodeProcess: Option[ActorRef] = None
   var seedNodeProcessCounter = 0 // for unique names
@@ -701,10 +703,10 @@ private[cluster] class ClusterCoreDaemon(publisher: ActorRef, joinConfigCompatCh
    * which will reply with a `Welcome` message.
    */
   def join(address: Address): Unit = {
-    if (address.protocol != selfAddress.protocol)
+    if (!acceptedProtocols.contains(address.protocol))
       logWarning(
-        "Trying to join member with wrong protocol, but was ignored, expected [{}] but was [{}]",
-        selfAddress.protocol,
+        "Trying to join member with wrong protocol, but was ignored, expected any of {} but was [{}]",
+        acceptedProtocols,
         address.protocol)
     else if (address.system != selfAddress.system)
       logWarning(
@@ -750,10 +752,10 @@ private[cluster] class ClusterCoreDaemon(publisher: ActorRef, joinConfigCompatCh
   def joining(joiningNode: UniqueAddress, roles: Set[String], appVersion: Version): Unit = {
     if (!preparingForShutdown) {
       val selfStatus = latestGossip.member(selfUniqueAddress).status
-      if (joiningNode.address.protocol != selfAddress.protocol)
+      if (!acceptedProtocols.contains(joiningNode.address.protocol))
         logWarning(
-          "Member with wrong protocol tried to join, but was ignored, expected [{}] but was [{}]",
-          selfAddress.protocol,
+          "Member with wrong protocol tried to join, but was ignored, expected any of {} but was [{}]",
+          acceptedProtocols,
           joiningNode.address.protocol)
       else if (joiningNode.address.system != selfAddress.system)
         logWarning(

cluster/src/test/scala/org/apache/pekko/cluster/MixedProtocolClusterSpec.scala

@@ -0,0 +1,92 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.pekko.cluster
+
+import com.typesafe.config.{ Config, ConfigFactory }
+
+import org.apache.pekko.testkit.{ LongRunningTest, PekkoSpec }
+
+object MixedProtocolClusterSpec {
+
+  val baseConfig: Config =
+    ConfigFactory.parseString("""
+     pekko.actor.provider = "cluster"
+     pekko.coordinated-shutdown.terminate-actor-system = on
+
+     pekko.remote.classic.netty.tcp.port = 0
+     pekko.remote.artery.canonical.port = 0
+     pekko.remote.artery.advanced.aeron.idle-cpu-level = 3
+     pekko.remote.accept-protocol-names = ["pekko", "akka"]
+
+     pekko.cluster.jmx.multi-mbeans-in-same-jvm = on
+     pekko.cluster.configuration-compatibility-check.enforce-on-join = off
+     """)
+
+  val configWithPekko: Config =
+    ConfigFactory.parseString("""
+      pekko.remote.protocol-name = "pekko"
+    """).withFallback(baseConfig)
+
+  val configWithAkka: Config =
+    ConfigFactory.parseString("""
+      pekko.remote.protocol-name = "akka"
+    """).withFallback(baseConfig)
+}
+
+class MixedProtocolClusterSpec extends PekkoSpec with ClusterTestKit {
+
+  import MixedProtocolClusterSpec._
+
+  "A node using the akka protocol" must {
+
+    "be allowed to join a cluster with a node using the pekko protocol" taggedAs LongRunningTest in {
+
+      val clusterTestUtil = new ClusterTestUtil(system.name)
+      // start the first node with the "pekko" protocol
+      clusterTestUtil.newActorSystem(configWithPekko)
+
+      // have a node using the "akka" protocol join
+      val joiningNode = clusterTestUtil.newActorSystem(configWithAkka)
+      clusterTestUtil.formCluster()
+
+      try {
+        awaitCond(clusterTestUtil.isMemberUp(joiningNode), message = "awaiting joining node to be 'Up'")
+      } finally {
+        clusterTestUtil.shutdownAll()
+      }
+    }
+
+    "allow a node using the pekko protocol to join the cluster" taggedAs LongRunningTest in {
+
+      val clusterTestUtil = new ClusterTestUtil(system.name)
+
+      // create the first node with the "akka" protocol
+      clusterTestUtil.newActorSystem(configWithAkka)
+
+      // have a node using the "pekko" protocol join
+      val joiningNode = clusterTestUtil.newActorSystem(configWithPekko)
+      clusterTestUtil.formCluster()
+
+      try {
+        awaitCond(clusterTestUtil.isMemberUp(joiningNode), message = "awaiting joining node to be 'Up'")
+      } finally {
+        clusterTestUtil.shutdownAll()
+      }
+    }
+  }
+}