kvscheduler: keep node in the graph during recreate

This patch fixes issues reported in ligato#1418.
During recreate (Delete+Create) the node would be completely
removed from the graph by Delete and all the associated flags
(metadata) were therefore lost.
When the subsequent Create fails, the scheduler needs to access
the flags, which in this case are undefined, causing the scheduler
to panic by dereferencing nil pointer.
This patch ensures that the node and its flags are preserved
during re-creation.

An in-progress refactor of the scheduling algorithm
will approach value recreation in a much cleaner
way...

Signed-off-by: Milan Lenco <milan.lenco@pantheon.tech>

plugins/kvscheduler/txn_exec.go

@@ -255,7 +255,9 @@ func (s *Scheduler) applyValue(args *applyValueArgs) (executed kvs.RecordedTxnOp
 }
 
 // applyDelete removes value.
-func (s *Scheduler) applyDelete(node graph.NodeRW, txnOp *kvs.RecordedTxnOp, args *applyValueArgs, pending bool) (executed kvs.RecordedTxnOps, err error) {
+func (s *Scheduler) applyDelete(node graph.NodeRW, txnOp *kvs.RecordedTxnOp, args *applyValueArgs,
+	pending bool) (executed kvs.RecordedTxnOps, err error) {
+
 	if s.logGraphWalk {
 		endLog := s.logNodeVisit("applyDelete", args)
 		defer endLog()
@@ -290,7 +292,9 @@ func (s *Scheduler) applyDelete(node graph.NodeRW, txnOp *kvs.RecordedTxnOp, arg
 				// removed by request
 				txnOp.NewState = kvs.ValueState_REMOVED
 				if args.isDerived {
-					args.graphW.DeleteNode(args.kv.key)
+					if !txnOp.IsRecreate {
+						args.graphW.DeleteNode(args.kv.key)
+					}
 				} else {
 					s.updateNodeState(node, txnOp.NewState, args)
 				}
@@ -517,7 +521,7 @@ func (s *Scheduler) applyUpdate(node graph.NodeRW, txnOp *kvs.RecordedTxnOp, arg
 	descriptor := s.registry.GetDescriptorForKey(args.kv.key)
 	handler := newDescriptorHandler(descriptor)
 	if !args.dryRun && args.kv.origin == kvs.FromNB {
-		err = handler.validate(node.GetKey(), node.GetValue())
+		err = handler.validate(node.GetKey(), args.kv.value)
 		if err != nil {
 			node.SetValue(args.kv.value) // save the invalid value
 			node.SetFlags(&UnavailValueFlag{})