1 files changed, 265 insertions, 0 deletions
diff --git a/src/sync/oncefunc_test.go b/src/sync/oncefunc_test.go
new file mode 100644
index 0000000..3c523a5
--- /dev/null
+++ b/src/sync/oncefunc_test.go
@@ -0,0 +1,265 @@
+// 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 (
+	"bytes"
+	"runtime"
+	"runtime/debug"
+	"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)
+	}
+}
+
+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)
+	}
+}
+
+func TestOnceFuncPanicTraceback(t *testing.T) {
+	// Test that on the first invocation of a OnceFunc, the stack trace goes all
+	// the way to the origin of the panic.
+	f := sync.OnceFunc(onceFuncPanic)
+
+	defer func() {
+		if p := recover(); p != "x" {
+			t.Fatalf("want panic %v, got %v", "x", p)
+		}
+		stack := debug.Stack()
+		want := "sync_test.onceFuncPanic"
+		if !bytes.Contains(stack, []byte(want)) {
+			t.Fatalf("want stack containing %v, got:\n%s", want, string(stack))
+		}
+	}()
+	f()
+}
+
+func onceFuncPanic() {
+	panic("x")
+}
+
+var (
+	onceFunc = sync.OnceFunc(func() {})
+
+	onceFuncOnce sync.Once
+)
+
+func doOnceFunc() {
+	onceFuncOnce.Do(func() {})
+}
+
+func BenchmarkOnceFunc(b *testing.B) {
+	b.Run("v=Once", func(b *testing.B) {
+		b.ReportAllocs()
+		for i := 0; i < b.N; i++ {
+			// The baseline is direct use of sync.Once.
+			doOnceFunc()
+		}
+	})
+	b.Run("v=Global", func(b *testing.B) {
+		b.ReportAllocs()
+		for i := 0; i < b.N; i++ {
+			// As of 3/2023, the compiler doesn't recognize that onceFunc is
+			// never mutated and is a closure that could be inlined.
+			// Too bad, because this is how OnceFunc will usually be used.
+			onceFunc()
+		}
+	})
+	b.Run("v=Local", func(b *testing.B) {
+		b.ReportAllocs()
+		// As of 3/2023, the compiler *does* recognize this local binding as an
+		// inlinable closure. This is the best case for OnceFunc, but probably
+		// not typical usage.
+		f := sync.OnceFunc(func() {})
+		for i := 0; i < b.N; i++ {
+			f()
+		}
+	})
+}
+
+var (
+	onceValue = sync.OnceValue(func() int { return 42 })
+
+	onceValueOnce  sync.Once
+	onceValueValue int
+)
+
+func doOnceValue() int {
+	onceValueOnce.Do(func() {
+		onceValueValue = 42
+	})
+	return onceValueValue
+}
+
+func BenchmarkOnceValue(b *testing.B) {
+	// See BenchmarkOnceFunc
+	b.Run("v=Once", func(b *testing.B) {
+		b.ReportAllocs()
+		for i := 0; i < b.N; i++ {
+			if want, got := 42, doOnceValue(); want != got {
+				b.Fatalf("want %d, got %d", want, got)
+			}
+		}
+	})
+	b.Run("v=Global", func(b *testing.B) {
+		b.ReportAllocs()
+		for i := 0; i < b.N; i++ {
+			if want, got := 42, onceValue(); want != got {
+				b.Fatalf("want %d, got %d", want, got)
+			}
+		}
+	})
+	b.Run("v=Local", func(b *testing.B) {
+		b.ReportAllocs()
+		onceValue := sync.OnceValue(func() int { return 42 })
+		for i := 0; i < b.N; i++ {
+			if want, got := 42, onceValue(); want != got {
+				b.Fatalf("want %d, got %d", want, got)
+			}
+		}
+	})
+}