sync: add implementation from upstream Go for OnceFunc, OnceValue, and OnceValues

src/sync/oncefunc.go

@@ -0,0 +1,97 @@
+// 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()
+		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()
+		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()
+		valid = true
+	}
+	return func() (T1, T2) {
+		once.Do(g)
+		if !valid {
+			panic(p)
+		}
+		return r1, r2
+	}
+}

src/sync/oncefunc_test.go

@@ -0,0 +1,159 @@
+// 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_test
+
+import (
+	"sync"
+	"testing"
+)
+
+// We assume that the Once.Do tests have already covered parallelism.
+
+func TestOnceFunc(t *testing.T) {
+	calls := 0
+	f := sync.OnceFunc(func() { calls++ })
+	allocs := testing.AllocsPerRun(10, f)
+	if calls != 1 {
+		t.Errorf("want calls==1, got %d", calls)
+	}
+	if allocs != 0 {
+		t.Errorf("want 0 allocations per call, got %v", allocs)
+	}
+}
+
+func TestOnceValue(t *testing.T) {
+	calls := 0
+	f := sync.OnceValue(func() int {
+		calls++
+		return calls
+	})
+	allocs := testing.AllocsPerRun(10, func() { f() })
+	value := f()
+	if calls != 1 {
+		t.Errorf("want calls==1, got %d", calls)
+	}
+	if value != 1 {
+		t.Errorf("want value==1, got %d", value)
+	}
+	if allocs != 0 {
+		t.Errorf("want 0 allocations per call, got %v", allocs)
+	}
+}
+
+func TestOnceValues(t *testing.T) {
+	calls := 0
+	f := sync.OnceValues(func() (int, int) {
+		calls++
+		return calls, calls + 1
+	})
+	allocs := testing.AllocsPerRun(10, func() { f() })
+	v1, v2 := f()
+	if calls != 1 {
+		t.Errorf("want calls==1, got %d", calls)
+	}
+	if v1 != 1 || v2 != 2 {
+		t.Errorf("want v1==1 and v2==2, got %d and %d", v1, v2)
+	}
+	if allocs != 0 {
+		t.Errorf("want 0 allocations per call, got %v", allocs)
+	}
+}
+
+// TODO: need to implement more complete panic handling for these tests.
+// func testOncePanicX(t *testing.T, calls *int, f func()) {
+// 	testOncePanicWith(t, calls, f, func(label string, p any) {
+// 		if p != "x" {
+// 			t.Fatalf("%s: want panic %v, got %v", label, "x", p)
+// 		}
+// 	})
+// }
+
+// func testOncePanicWith(t *testing.T, calls *int, f func(), check func(label string, p any)) {
+// 	// Check that the each call to f panics with the same value, but the
+// 	// underlying function is only called once.
+// 	for _, label := range []string{"first time", "second time"} {
+// 		var p any
+// 		panicked := true
+// 		func() {
+// 			defer func() {
+// 				p = recover()
+// 			}()
+// 			f()
+// 			panicked = false
+// 		}()
+// 		if !panicked {
+// 			t.Fatalf("%s: f did not panic", label)
+// 		}
+// 		check(label, p)
+// 	}
+// 	if *calls != 1 {
+// 		t.Errorf("want calls==1, got %d", *calls)
+// 	}
+// }
+
+// func TestOnceFuncPanic(t *testing.T) {
+// 	calls := 0
+// 	f := sync.OnceFunc(func() {
+// 		calls++
+// 		panic("x")
+// 	})
+// 	testOncePanicX(t, &calls, f)
+// }
+
+// func TestOnceValuePanic(t *testing.T) {
+// 	calls := 0
+// 	f := sync.OnceValue(func() int {
+// 		calls++
+// 		panic("x")
+// 	})
+// 	testOncePanicX(t, &calls, func() { f() })
+// }
+
+// func TestOnceValuesPanic(t *testing.T) {
+// 	calls := 0
+// 	f := sync.OnceValues(func() (int, int) {
+// 		calls++
+// 		panic("x")
+// 	})
+// 	testOncePanicX(t, &calls, func() { f() })
+// }
+//
+// func TestOnceFuncPanicNil(t *testing.T) {
+// 	calls := 0
+// 	f := sync.OnceFunc(func() {
+// 		calls++
+// 		panic(nil)
+// 	})
+// 	testOncePanicWith(t, &calls, f, func(label string, p any) {
+// 		switch p.(type) {
+// 		case nil, *runtime.PanicNilError:
+// 			return
+// 		}
+// 		t.Fatalf("%s: want nil panic, got %v", label, p)
+// 	})
+// }
+//
+// func TestOnceFuncGoexit(t *testing.T) {
+// 	// If f calls Goexit, the results are unspecified. But check that f doesn't
+// 	// get called twice.
+// 	calls := 0
+// 	f := sync.OnceFunc(func() {
+// 		calls++
+// 		runtime.Goexit()
+// 	})
+// 	var wg sync.WaitGroup
+// 	for i := 0; i < 2; i++ {
+// 		wg.Add(1)
+// 		go func() {
+// 			defer wg.Done()
+// 			defer func() { recover() }()
+// 			f()
+// 		}()
+// 		wg.Wait()
+// 	}
+// 	if calls != 1 {
+// 		t.Errorf("want calls==1, got %d", calls)
+// 	}
+// }