KAFKA-4349; Handle 'PreparingRebalance' and 'AwaitingSync' states in consumer group describe

The edge case where consumer group state is `PreparingRebalance` or `AwaitingSync` will be separately handled as the group assignment is not yet determined.

Author: Vahid Hashemian <vahidhashemian@us.ibm.com>

Reviewers: Jason Gustafson <jason@confluent.io>

Closes #2070 from vahidhashemian/KAFKA-4349

core/src/main/scala/kafka/admin/AdminClient.scala

@@ -149,9 +149,15 @@ class AdminClient(val time: Time,
 
     Errors.forCode(metadata.errorCode()).maybeThrow()
     val consumers = metadata.members.map { consumer =>
+      ConsumerSummary(consumer.memberId, consumer.clientId, consumer.clientHost, metadata.state match {
+        case "Stable" =>
           val assignment = ConsumerProtocol.deserializeAssignment(ByteBuffer.wrap(Utils.readBytes(consumer.memberAssignment)))
-      ConsumerSummary(consumer.memberId, consumer.clientId, consumer.clientHost, assignment.partitions.toList)
+          assignment.partitions.toList
+        case _ =>
+          List()
+      })
     }.toList
+
     ConsumerGroupSummary(metadata.state, metadata.protocol, Some(consumers), coordinator)
   }
 

core/src/main/scala/kafka/admin/ConsumerGroupCommand.scala

@@ -73,20 +73,23 @@ object ConsumerGroupCommand extends Logging {
           case None =>
             printError(s"The consumer group '$groupId' does not exist.")
           case Some(assignments) =>
-            if (assignments.isEmpty)
             state match {
               case Some("Dead") =>
                 printError(s"Consumer group '$groupId' does not exist.")
               case Some("Empty") =>
                 printError(s"Consumer group '$groupId' has no active members.")
-                case Some(_) =>
+              case Some("PreparingRebalance") | Some("AwaitingSync") =>
-                  printError(s"Consumer group '$groupId' is rebalancing.")
+                System.err.println(s"Warning: Consumer group '$groupId' is rebalancing.")
+                printAssignment(assignments, !opts.useOldConsumer)
+              case Some("Stable") =>
+                printAssignment(assignments, !opts.useOldConsumer)
+              case Some(other) =>
+                // the control should never reach here
+                throw new KafkaException(s"Expected a valid consumer group state, but found '$other'.")
               case None =>
                 // the control should never reach here
                 throw new KafkaException("Expected a valid consumer group state, but none found.")
             }
-            else
-              printAssignment(assignments, !opts.useOldConsumer)
         }
       }
       else if (opts.options.has(opts.deleteOpt)) {