scheduler: merge Reserve and Unreserve plugins

Previously, separate interfaces were defined for Reserve and Unreserve
plugins. However, in nearly all cases, a plugin that allocates a
resource using Reserve will likely want to register itself for Unreserve
as well in order to free the allocated resource at the end of a failed
scheduling/binding cycle. Having separate plugins for Reserve and
Unreserve also adds unnecessary config toil. To that end, this patch
aims to merge the two plugins into a single interface called a
ReservePlugin that requires implementing both the Reserve and Unreserve
methods.

staging/src/k8s.io/kube-scheduler/config/v1beta1/types.go

@@ -175,7 +175,8 @@ type Plugins struct {
 	// Score is a list of plugins that should be invoked when ranking nodes that have passed the filtering phase.
 	Score *PluginSet `json:"score,omitempty"`
 
-	// Reserve is a list of plugins invoked when reserving a node to run the pod.
+	// Reserve is a list of plugins invoked when reserving/unreserving resources
+	// after a node is assigned to run the pod.
 	Reserve *PluginSet `json:"reserve,omitempty"`
 
 	// Permit is a list of plugins that control binding of a Pod. These plugins can prevent or delay binding of a Pod.
@@ -190,9 +191,6 @@ type Plugins struct {
 
 	// PostBind is a list of plugins that should be invoked after a pod is successfully bound.
 	PostBind *PluginSet `json:"postBind,omitempty"`
-
-	// Unreserve is a list of plugins invoked when a pod that was previously reserved is rejected in a later phase.
-	Unreserve *PluginSet `json:"unreserve,omitempty"`
 }
 
 // PluginSet specifies enabled and disabled plugins for an extension point.

staging/src/k8s.io/kube-scheduler/config/v1beta1/zz_generated.deepcopy.go

@@ -449,11 +449,6 @@ func (in *Plugins) DeepCopyInto(out *Plugins) {
 		*out = new(PluginSet)
 		(*in).DeepCopyInto(*out)
 	}
-	if in.Unreserve != nil {
-		in, out := &in.Unreserve, &out.Unreserve
-		*out = new(PluginSet)
-		(*in).DeepCopyInto(*out)
-	}
 	return
 }