Support disable of join compat config check, #26719

akka-cluster/src/main/resources/reference.conf

@@ -296,6 +296,10 @@ akka {
       # to 'know' if its allowed to join.
       enforce-on-join = on
 
+      # Add named entry to this section with fully qualified class name of the JoinConfigCompatChecker
+      # to enable.
+      # Checkers defined in reference.conf can be disabled by application by using empty string value
+      # for the named entry.
       checkers {
         akka-cluster = "akka.cluster.JoinConfigCompatCheckCluster"
       }

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

@@ -191,7 +191,16 @@ final class ClusterSettings(val config: Config, val systemName: String) {
   val ByPassConfigCompatCheck: Boolean = !cc.getBoolean("configuration-compatibility-check.enforce-on-join")
   val ConfigCompatCheckers: Set[String] = {
     import scala.collection.JavaConverters._
-    cc.getConfig("configuration-compatibility-check.checkers").root.unwrapped.values().asScala.map(_.toString).toSet
+    cc.getConfig("configuration-compatibility-check.checkers")
+      .root
+      .unwrapped
+      .values()
+      .asScala
+      .iterator
+      .collect {
+        case s if s.toString.trim.nonEmpty => s.toString
+      }
+      .toSet
   }
 
   val SensitiveConfigPaths = {

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

@@ -285,6 +285,7 @@ class JoinConfigCompatCheckerSpec extends AkkaSpec with ClusterTestKit {
       }
 
     }
+
   }
 
   "A First Node" must {
@@ -595,6 +596,33 @@ class JoinConfigCompatCheckerSpec extends AkkaSpec with ClusterTestKit {
         clusterTestUtil.shutdownAll()
       }
     }
+
+    "be allowed to disable a check" taggedAs LongRunningTest in {
+
+      // this config has sensitive properties that are not compatible with the cluster
+      // the cluster will ignore them, because they are on the sensitive-config-path
+      // the cluster won't let it be leaked back to the joining node neither which will fail the join attempt.
+      val joinNodeConfig =
+        ConfigFactory.parseString("""
+            akka.cluster {
+              configuration-compatibility-check {
+                checkers {
+                  # disable what is defined in reference.conf
+                  akka-cluster = ""
+                  akka-cluster-test = ""
+                }
+              }
+            }
+          """)
+
+      val clusterTestUtil = new ClusterTestUtil(system.name)
+      try {
+        val sys = clusterTestUtil.newActorSystem(joinNodeConfig.withFallback(configWithChecker))
+        Cluster(sys).settings.ConfigCompatCheckers should ===(Set.empty)
+      } finally {
+        clusterTestUtil.shutdownAll()
+      }
+    }
   }
 }