grpcsync: use context.AfterFunc to close buffer after context canceled in CallbackSerializer (#8489)

[The current minimum supported Go version is now
1.23](https://github.com/grpc/grpc-go/blob/62ec29fd9b3f9ea3cea6dc08a31e837aa92678b7/go.mod#L3).
`context.AfterFunc` is available for all of grpc-go's latest version
users. Thus we can do this pending TODO.

`context.AfterFunc` would invoke the given function for both _immediate_
context cancelation and timer-based context cancelation (`WithTimeout`,
`WithDeadline`). So I think this change is safe.

RELEASE NOTES: N/A

Author: turfaa

internal/grpcsync/callback_serializer.go

@@ -80,25 +80,11 @@ func (cs *CallbackSerializer) ScheduleOr(f func(ctx context.Context), onFailure
 func (cs *CallbackSerializer) run(ctx context.Context) {
 	defer close(cs.done)
 
-	// TODO: when Go 1.21 is the oldest supported version, this loop and Close
-	// can be replaced with:
-	//
-	// context.AfterFunc(ctx, cs.callbacks.Close)
-	for ctx.Err() == nil {
-		select {
-		case <-ctx.Done():
-			// Do nothing here. Next iteration of the for loop will not happen,
-			// since ctx.Err() would be non-nil.
-		case cb := <-cs.callbacks.Get():
-			cs.callbacks.Load()
-			cb.(func(context.Context))(ctx)
-		}
-	}
-
-	// Close the buffer to prevent new callbacks from being added.
-	cs.callbacks.Close()
+	// Close the buffer when the context is canceled
+	// to prevent new callbacks from being added.
+	context.AfterFunc(ctx, cs.callbacks.Close)
 
-	// Run all pending callbacks.
+	// Run all callbacks.
 	for cb := range cs.callbacks.Get() {
 		cs.callbacks.Load()
 		cb.(func(context.Context))(ctx)

xds/internal/clients/internal/syncutil/callback_serializer.go

@@ -80,25 +80,11 @@ func (cs *CallbackSerializer) ScheduleOr(f func(ctx context.Context), onFailure
 func (cs *CallbackSerializer) run(ctx context.Context) {
 	defer close(cs.done)
 
-	// TODO: when Go 1.21 is the oldest supported version, this loop and Close
-	// can be replaced with:
-	//
-	// context.AfterFunc(ctx, cs.callbacks.Close)
-	for ctx.Err() == nil {
-		select {
-		case <-ctx.Done():
-			// Do nothing here. Next iteration of the for loop will not happen,
-			// since ctx.Err() would be non-nil.
-		case cb := <-cs.callbacks.Get():
-			cs.callbacks.Load()
-			cb.(func(context.Context))(ctx)
-		}
-	}
-
-	// Close the buffer to prevent new callbacks from being added.
-	cs.callbacks.Close()
+	// Close the buffer when the context is canceled
+	// to prevent new callbacks from being added.
+	context.AfterFunc(ctx, cs.callbacks.Close)
 
-	// Run all pending callbacks.
+	// Run all callbacks.
 	for cb := range cs.callbacks.Get() {
 		cs.callbacks.Load()
 		cb.(func(context.Context))(ctx)