unsubscribe and closures

[paleozogt]

Unsubscribe is broken with closures. This code shows the problem:

makeHandler := func(tag string) func(msg string) {
	return func(msg string) {
		fmt.Printf("%s %s\n", tag, msg)
	}
}

var bus EventBus.Bus = EventBus.New()

handler1 := makeHandler("handler1")
fmt.Printf("handler1 pointer %x\n", reflect.ValueOf(handler1).Pointer())
bus.Subscribe("foo", handler1)

handler2 := makeHandler("handler2")
fmt.Printf("handler2 pointer %x\n", reflect.ValueOf(handler2).Pointer())
bus.Subscribe("foo", handler2)

bus.Publish("foo", "A")
bus.Unsubscribe("foo", handler2)
bus.Publish("foo", "B")

Here's the output:

handler1 pointer 11ac100
handler2 pointer 11ac100
handler1 A
handler2 A
handler2 B

Note that even though we removed handler2, it still got an the B event.

What's happening is that EventBus uses reflect.ValueOf().Pointer() for Unsubscribe. However, the pointer to a function isn't enough to distinguish one closure from another. You can see the problem in the above output where it shows that the pointer values for handler1 and handler2 are the same.

[paleozogt]

Some ideas for fixing it:

• Have clients pass in an explicit id to Subscribe/Unsubscribe along with the func

  bus.Subscribe("foo", "handler2", handler2)
  bus.Unsubscribe("foo", "handler2")
  

• Subscribe could return a handle that could be used for Unsubscribe

  h1 := bus.Subscribe("foo", handler2)
  bus.Unsubscribe(h1)
  

• Have clients pass in a struct that wraps the function, guaranteeing a unique pointer

  h1 := Handler{ Func: handler1 }
  bus.Subscribe("foo", &h1)
  bus.Unsubscribe("foo", &h1)
  

[LubosD]

I've just hit this bug. As I show in this playground, you can just use plain == to compare reflect.Values containing lambdas and it seems to work correcly.