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package nosync
// Once is an object that will perform exactly one action.
type Once struct {
doing bool
done bool
}
// Do calls the function f if and only if Do is being called for the
// first time for this instance of Once. In other words, given
// var once Once
// if once.Do(f) is called multiple times, only the first call will invoke f,
// even if f has a different value in each invocation. A new instance of
// Once is required for each function to execute.
//
// Do is intended for initialization that must be run exactly once. Since f
// is niladic, it may be necessary to use a function literal to capture the
// arguments to a function to be invoked by Do:
// config.once.Do(func() { config.init(filename) })
//
// If f causes Do to be called, it will panic.
//
// If f panics, Do considers it to have returned; future calls of Do return
// without calling f.
//
func (o *Once) Do(f func()) {
if o.done {
return
}
if o.doing {
panic("nosync: Do called within f")
}
o.doing = true
defer func() {
o.doing = false
o.done = true
}()
f()
}
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