Essentially an atomic version of:

	val, exists := map.Load(key)
	if !exists {
		val, _ = map.LoadOrStore(key, func())
	}
	
	
	... = val

CC @bcmills

We can't change the existing LoadOrStore method but we could in principle add a new method.

@ianlancetaylor if I understand correctly the proposal is to add a special case where if value interface{} is a func() interface{} (say) then use it as a callback. Now that isn't backward compatible as-is, but if we added a new type type LoadFunc func() interface{} and only looked for values of that named type then it would be.

To me, overloading this API by shoehorning a callback into the value interface{} seems worse than adding a new method.

Either way, though, I think this has a sharp edge. The callback function cannot block any other Map method, including concurrent LoadOrStores (that would be antithetical to sync.Map). (To put it another way, it is not possible to implement that code snippet which includes a user-provided callback atomically without introducing unacceptable blocking.) It follows that the callbacks may be called multiple times concurrently. I think users may find that surprising.

This seems like a clear duplicate of the recently declined #44159.

Duplicate of #44159.