pkg/controller/garbagecollector: simplify mutexes.

Similar to #29598, we can rely on the zero-value construction behavior
to embed `sync.Mutex` into parent structs.

pkg/controller/garbagecollector/garbagecollector.go

@@ -67,7 +67,7 @@ func (s objectReference) String() string {
 type node struct {
 	identity objectReference
 	// dependents will be read by the orphan() routine, we need to protect it with a lock.
-	dependentsLock *sync.RWMutex
+	dependentsLock sync.RWMutex
 	dependents     map[*node]struct{}
 	// When processing an Update event, we need to compare the updated
 	// ownerReferences with the owners recorded in the graph.
@@ -151,7 +151,6 @@ func (p *Propagator) addDependentToOwners(n *node, owners []metatypes.OwnerRefer
 					OwnerReference: owner,
 					Namespace:      n.identity.Namespace,
 				},
-				dependentsLock: &sync.RWMutex{},
 				dependents: make(map[*node]struct{}),
 			}
 			p.uidToNode.Write(ownerNode)
@@ -378,7 +377,6 @@ func (p *Propagator) processEvent() {
 				},
 				Namespace: accessor.GetNamespace(),
 			},
-			dependentsLock: &sync.RWMutex{},
 			dependents: make(map[*node]struct{}),
 			owners:     accessor.GetOwnerReferences(),
 		}

pkg/controller/garbagecollector/garbagecollector_test.go

@@ -301,7 +301,7 @@ func TestDependentsRace(t *testing.T) {
 	}
 
 	const updates = 100
-	owner := &node{dependentsLock: &sync.RWMutex{}, dependents: make(map[*node]struct{})}
+	owner := &node{dependents: make(map[*node]struct{})}
 	ownerUID := types.UID("owner")
 	gc.propagator.uidToNode.Write(owner)
 	go func() {