Forward compatibility for primary context handoff on 6.x (#25545)

Companion PR for #25468

Relates to #25355

core/src/main/java/org/elasticsearch/action/support/replication/TransportReplicationAction.java

@@ -21,6 +21,7 @@
 
 import org.apache.logging.log4j.message.ParameterizedMessage;
 import org.elasticsearch.ElasticsearchException;
+import org.elasticsearch.Version;
 import org.elasticsearch.action.ActionListener;
 import org.elasticsearch.action.ActionListenerResponseHandler;
 import org.elasticsearch.action.UnavailableShardsException;
@@ -293,6 +294,19 @@ public void onResponse(PrimaryShardReference primaryShardReference) {
                     final ShardRouting primary = primaryShardReference.routingEntry();
                     assert primary.relocating() : "indexShard is marked as relocated but routing isn't" + primary;
                     DiscoveryNode relocatingNode = clusterService.state().nodes().get(primary.relocatingNodeId());
+                    if (relocatingNode != null && relocatingNode.getVersion().major > Version.CURRENT.major) {
+                        // ES 6.x requires a primary context hand-off during primary relocation which is not implemented on ES 5.x,
+                        // otherwise it might not be aware of a replica that finished recovery and became activated on the master before
+                        // the new primary became in charge of replicating operations, as the cluster state with that in-sync information
+                        // might not be applied yet on the primary relocation target before it would be in charge of replicating operations.
+                        // This would mean that the new primary could advance the global checkpoint too quickly, not taking into account
+                        // the newly in-sync replica.
+                        // ES 6.x detects that the primary is relocating from a 5.x node, and activates the primary mode of the global
+                        // checkpoint tracker only after activation of the relocation target, which means, however, that requests cannot
+                        // be handled as long as the relocation target shard has not been activated.
+                        throw new ReplicationOperation.RetryOnPrimaryException(request.shardId(),
+                            "waiting for 6.x primary to be activated");
+                    }
                     transportService.sendRequest(relocatingNode, transportPrimaryAction,
                         new ConcreteShardRequest<>(request, primary.allocationId().getRelocationId()),
                         transportOptions,