[Backport 2.x] Handle exception for the updation of a replication rule

src/main/kotlin/org/opensearch/replication/action/autofollow/TransportAutoFollowClusterManagerNodeAction.kt

@@ -111,6 +111,7 @@ class TransportAutoFollowClusterManagerNodeAction @Inject constructor(transportS
         } catch(e: ResourceAlreadyExistsException) {
             // Log and bail as task is already running
             log.warn("Task already started for '$clusterAlias:$patternName'", e)
+            throw OpenSearchException("Exisiting autofollow replication rule cannot be recreated/updated", e)
         } catch (e: Exception) {
             log.error("Failed to start auto follow task for cluster '$clusterAlias:$patternName'", e)
             throw OpenSearchException(AUTOFOLLOW_EXCEPTION_GENERIC_STRING)

src/test/kotlin/org/opensearch/replication/integ/rest/UpdateAutoFollowPatternIT.kt

@@ -309,6 +309,32 @@ class UpdateAutoFollowPatternIT: MultiClusterRestTestCase() {
             followerClient.deleteAutoFollowPattern(connectionAlias, indexPatternName)
         }.doesNotThrowAnyException()
 
+    }
+    fun `test updation of auto follow pattern`() {
+        val followerClient = getClientForCluster(FOLLOWER)
+        createConnectionBetweenClusters(FOLLOWER, LEADER, connectionAlias)
+        followerClient.updateAutoFollowPattern(connectionAlias, indexPatternName, indexPattern)
+        val indexPattern1 = "log*"
+        //Re-create the same replication rule
+        Assertions.assertThatThrownBy {
+            followerClient.updateAutoFollowPattern(connectionAlias, indexPatternName, indexPattern)
+        }.isInstanceOf(ResponseException::class.java)
+                .hasMessageContaining("autofollow replication rule cannot be recreated/updated")
+
+        //Update the replication rule with different indexpattern
+        Assertions.assertThatThrownBy {
+            followerClient.updateAutoFollowPattern(connectionAlias, indexPatternName, indexPattern1)
+        }.isInstanceOf(ResponseException::class.java)
+                .hasMessageContaining("autofollow replication rule cannot be recreated/updated")
+
+        //Create a new replication rule with same indexpattern but unique rule name
+        Assertions.assertThatCode {
+            followerClient.updateAutoFollowPattern(connectionAlias, "unique-rule", indexPattern1)
+        }.doesNotThrowAnyException()
+
+        followerClient.deleteAutoFollowPattern(connectionAlias, indexPatternName)
+        followerClient.deleteAutoFollowPattern(connectionAlias, "unique-rule")
+
     }
 
     fun `test removing autofollow pattern stop autofollow task`() {