metadata: use resource max and end on registration

Ensure the registered resource type does not conflict
with existing resource types or over the max.

Signed-off-by: Derek McGowan <derek@mcg.dev>

metadata/db.go

@@ -280,6 +280,12 @@ func (m *DB) RegisterMutationCallback(fn func(bool)) {
 // - Collectible Resources must track whether the resource is active and/or
 //   lease membership.
 func (m *DB) RegisterCollectibleResource(t gc.ResourceType, c Collector) {
+	if t < resourceEnd {
+		panic("cannot re-register metadata resource")
+	} else if t >= gc.ResourceMax {
+		panic("resource type greater than max")
+	}
+
 	m.wlock.Lock()
 	defer m.wlock.Unlock()
 
@@ -287,16 +293,11 @@ func (m *DB) RegisterCollectibleResource(t gc.ResourceType, c Collector) {
 		m.collectors = map[gc.ResourceType]Collector{}
 	}
 
-	switch t {
-	case ResourceContainer:
-		panic("cannot re-register metadata resource")
-	default:
 	if _, ok := m.collectors[t]; ok {
 		panic("cannot register collectible type twice")
 	}
 	m.collectors[t] = c
 }
-}
 
 // GCStats holds the duration for the different phases of the garbage collector
 type GCStats struct {

metadata/gc.go

@@ -44,6 +44,8 @@ const (
 	ResourceLease
 	// ResourceIngest specifies a content ingest
 	ResourceIngest
+	// resourceEnd is the end of specified resource types
+	resourceEnd
 )
 
 const (