1package nosync
2
3// Once is an object that will perform exactly one action.
4type Once struct {
5	doing bool
6	done  bool
7}
8
9// Do calls the function f if and only if Do is being called for the
10// first time for this instance of Once. In other words, given
11// 	var once Once
12// if once.Do(f) is called multiple times, only the first call will invoke f,
13// even if f has a different value in each invocation.  A new instance of
14// Once is required for each function to execute.
15//
16// Do is intended for initialization that must be run exactly once.  Since f
17// is niladic, it may be necessary to use a function literal to capture the
18// arguments to a function to be invoked by Do:
19// 	config.once.Do(func() { config.init(filename) })
20//
21// If f causes Do to be called, it will panic.
22//
23// If f panics, Do considers it to have returned; future calls of Do return
24// without calling f.
25//
26func (o *Once) Do(f func()) {
27	if o.done {
28		return
29	}
30	if o.doing {
31		panic("nosync: Do called within f")
32	}
33	o.doing = true
34	defer func() {
35		o.doing = false
36		o.done = true
37	}()
38	f()
39}
40