Allow subclasses to control whether primary action is forced

Today this requires overriding and reimplementing registerRequestHandlers() but
it's clearer to highlight the difference like this

Author: DaveCTurner

server/src/main/java/org/elasticsearch/action/resync/TransportResyncReplicationAction.java

@@ -64,18 +64,9 @@ public TransportResyncReplicationAction(Settings settings, TransportService tran
     }
 
     @Override
-    protected void registerRequestHandlers(String actionName, TransportService transportService, Supplier<ResyncReplicationRequest> request,
-                                           Supplier<ResyncReplicationRequest> replicaRequest, String executor) {
-        transportService.registerRequestHandler(actionName, request, ThreadPool.Names.SAME, this::handleOperationRequest);
+    protected boolean forcePrimaryActionExecution() {
         // we should never reject resync because of thread pool capacity on primary
-        transportService.registerRequestHandler(transportPrimaryAction,
-            () -> new ConcreteShardRequest<>(request),
-            executor, true, true,
-            this::handlePrimaryRequest);
-        transportService.registerRequestHandler(transportReplicaAction,
-            () -> new ConcreteReplicaRequest<>(replicaRequest),
-            executor, true, true,
-            this::handleReplicaRequest);
+        return true;
     }
 
     @Override

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

@@ -72,7 +72,6 @@
 import org.elasticsearch.transport.TransportChannel;
 import org.elasticsearch.transport.TransportException;
 import org.elasticsearch.transport.TransportRequest;
-import org.elasticsearch.transport.TransportRequestHandler;
 import org.elasticsearch.transport.TransportRequestOptions;
 import org.elasticsearch.transport.TransportResponse;
 import org.elasticsearch.transport.TransportResponse.Empty;
@@ -145,21 +144,21 @@ protected TransportReplicationAction(Settings settings, String actionName, Trans
 
         this.transportPrimaryAction = actionName + "[p]";
         this.transportReplicaAction = actionName + "[r]";
-        registerRequestHandlers(actionName, transportService, request, replicaRequest, executor);
+
+        transportService.registerRequestHandler(actionName, request, ThreadPool.Names.SAME, this::handleOperationRequest);
+        transportService.registerRequestHandler(transportPrimaryAction, () -> new ConcreteShardRequest<>(request), executor, true,
+            forcePrimaryActionExecution(), this::handlePrimaryRequest);
+        // we must never reject on because of thread pool capacity on replicas
+        transportService.registerRequestHandler(
+            transportReplicaAction, () -> new ConcreteReplicaRequest<>(replicaRequest), executor, true, true, this::handleReplicaRequest);
 
         this.transportOptions = transportOptions(settings);
 
         this.syncGlobalCheckpointAfterOperation = syncGlobalCheckpointAfterOperation;
     }
 
-    protected void registerRequestHandlers(String actionName, TransportService transportService, Supplier<Request> request,
-                                           Supplier<ReplicaRequest> replicaRequest, String executor) {
-        transportService.registerRequestHandler(actionName, request, ThreadPool.Names.SAME, this::handleOperationRequest);
-        transportService.registerRequestHandler(transportPrimaryAction, () -> new ConcreteShardRequest<>(request), executor,
-            this::handlePrimaryRequest);
-        // we must never reject on because of thread pool capacity on replicas
-        transportService.registerRequestHandler(
-            transportReplicaAction, () -> new ConcreteReplicaRequest<>(replicaRequest), executor, true, true, this::handleReplicaRequest);
+    protected boolean forcePrimaryActionExecution() {
+        return false;
     }
 
     @Override