use a synchronization mechanism such as a lock or channel communication to establish a relative ordering.

https://golang.org/ref/mem#tmp_6

Goroutine destruction

The exit of a goroutine is not guaranteed to happen before any event in the program. For example, in this program:

var a string

func hello() {
	go func() { a = "hello" }()
	print(a)
}

the assignment to a is not followed by any synchronization event, so it is not guaranteed to be observed by any other goroutine. In fact, an aggressive compiler might delete the entire go statement.

If the effects of a goroutine must be observed by another goroutine, use a synchronization mechanism such as a lock or channel communication to establish a relative ordering.

Channel communication

Channel communication is the main method of synchronization between goroutines. Each send on a particular channel is matched to a corresponding receive from that channel, usually in a different goroutine.