src/sync/oncefunc.go

// Copyright 2022 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package sync

// OnceFunc returns a function that invokes f only once. The returned function
// may be called concurrently.
//
// If f panics, the returned function will panic with the same value on every call.
func OnceFunc(f func()) func() {
	var (
		once  Once
		valid bool
		p     any
	)
	// Construct the inner closure just once to reduce costs on the fast path.
	g := func() {
		defer func() {
			p = recover()
			if !valid {
				// Re-panic immediately so on the first call the user gets a
				// complete stack trace into f.
				panic(p)
			}
		}()
		f()
		f = nil      // Do not keep f alive after invoking it.
		valid = true // Set only if f does not panic.
	}
	return func() {
		once.Do(g)
		if !valid {
			panic(p)
		}
	}
}

// OnceValue returns a function that invokes f only once and returns the value
// returned by f. The returned function may be called concurrently.
//
// If f panics, the returned function will panic with the same value on every call.
func OnceValue[T any](f func() T) func() T {
	var (
		once   Once
		valid  bool
		p      any
		result T
	)
	g := func() {
		defer func() {
			p = recover()
			if !valid {
				panic(p)
			}
		}()
		result = f()
		f = nil
		valid = true
	}
	return func() T {
		once.Do(g)
		if !valid {
			panic(p)
		}
		return result
	}
}

// OnceValues returns a function that invokes f only once and returns the values
// returned by f. The returned function may be called concurrently.
//
// If f panics, the returned function will panic with the same value on every call.
func OnceValues[T1, T2 any](f func() (T1, T2)) func() (T1, T2) {
	var (
		once  Once
		valid bool
		p     any
		r1    T1
		r2    T2
	)
	g := func() {
		defer func() {
			p = recover()
			if !valid {
				panic(p)
			}
		}()
		r1, r2 = f()
		f = nil
		valid = true
	}
	return func() (T1, T2) {
		once.Do(g)
		if !valid {
			panic(p)
		}
		return r1, r2
	}
}