// 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() {
// Use a struct so that there's a single heap allocation.
d := struct {
f func()
once Once
valid bool
p any
}{
f: f,
}
return func() {
d.once.Do(func() {
defer func() {
d.f = nil // Do not keep f alive after invoking it.
d.p = recover()
if !d.valid {
// Re-panic immediately so on the first
// call the user gets a complete stack
// trace into f.
panic(d.p)
}
}()
d.f()
d.valid = true // Set only if f does not panic.
})
if !d.valid {
panic(d.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 {
// Use a struct so that there's a single heap allocation.
d := struct {
f func() T
once Once
valid bool
p any
result T
}{
f: f,
}
return func() T {
d.once.Do(func() {
defer func() {
d.f = nil
d.p = recover()
if !d.valid {
panic(d.p)
}
}()
d.result = d.f()
d.valid = true
})
if !d.valid {
panic(d.p)
}
return d.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) {
// Use a struct so that there's a single heap allocation.
d := struct {
f func() (T1, T2)
once Once
valid bool
p any
r1 T1
r2 T2
}{
f: f,
}
return func() (T1, T2) {
d.once.Do(func() {
defer func() {
d.f = nil
d.p = recover()
if !d.valid {
panic(d.p)
}
}()
d.r1, d.r2 = d.f()
d.valid = true
})
if !d.valid {
panic(d.p)
}
return d.r1, d.r2
}
}
