Add Pointer[T]

Adds a Pointer[T] type that provide the same functionality as
Go 1.19's `sync/atomic.Pointer[T]` in both, Go 1.18 and 1.19.

In Go 1.19, this uses the standard library's implementation,
and in 1.18 our own custom implementation based on UnsafePointer.

Refs GO-1572
Resolves #115

pointer_go119.go

@@ -0,0 +1,61 @@
+// Copyright (c) 2022 Uber Technologies, Inc.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+
+//go:build go1.19
+// +build go1.19
+
+package atomic
+
+import "sync/atomic"
+
+// Pointer is an atomic pointer of type *T.
+type Pointer[T any] struct {
+	_ nocmp // disallow non-atomic comparison
+	p atomic.Pointer[T]
+}
+
+// NewPointer creates a new Pointer.
+func NewPointer[T any](v *T) *Pointer[T] {
+	var p Pointer[T]
+	if v != nil {
+		p.p.Store(v)
+	}
+	return &p
+}
+
+// Load atomically loads the wrapped value.
+func (p *Pointer[T]) Load() *T {
+	return p.p.Load()
+}
+
+// Store atomically stores the passed value.
+func (p *Pointer[T]) Store(val *T) {
+	p.p.Store(val)
+}
+
+// Swap atomically swaps the wrapped pointer and returns the old value.
+func (p *Pointer[T]) Swap(val *T) (old *T) {
+	return p.p.Swap(val)
+}
+
+// CompareAndSwap is an atomic compare-and-swap.
+func (p *Pointer[T]) CompareAndSwap(old, new *T) (swapped bool) {
+	return p.p.CompareAndSwap(old, new)
+}

pointer_pre_go119.go

@@ -0,0 +1,60 @@
+// Copyright (c) 2022 Uber Technologies, Inc.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+
+//go:build !go1.19
+// +build !go1.19
+
+package atomic
+
+import "unsafe"
+
+type Pointer[T any] struct {
+	_ nocmp // disallow non-atomic comparison
+	p UnsafePointer
+}
+
+// NewPointer creates a new Pointer.
+func NewPointer[T any](v *T) *Pointer[T] {
+	var p Pointer[T]
+	if v != nil {
+		p.p.Store(unsafe.Pointer(v))
+	}
+	return &p
+}
+
+// Load atomically loads the wrapped value.
+func (p *Pointer[T]) Load() *T {
+	return (*T)(p.p.Load())
+}
+
+// Store atomically stores the passed value.
+func (p *Pointer[T]) Store(val *T) {
+	p.p.Store(unsafe.Pointer(val))
+}
+
+// Swap atomically swaps the wrapped pointer and returns the old value.
+func (p *Pointer[T]) Swap(val *T) (old *T) {
+	return (*T)(p.p.Swap(unsafe.Pointer(val)))
+}
+
+// CompareAndSwap is an atomic compare-and-swap.
+func (p *Pointer[T]) CompareAndSwap(old, new *T) (swapped bool) {
+	return p.p.CompareAndSwap(unsafe.Pointer(old), unsafe.Pointer(new))
+}

pointer_test.go

@@ -0,0 +1,71 @@
+package atomic
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestPointer(t *testing.T) {
+	type foo struct{ v int }
+
+	i := foo{42}
+	j := foo{0}
+	k := foo{1}
+
+	tests := []struct {
+		desc      string
+		newAtomic func() *Pointer[foo]
+		initial   *foo
+	}{
+		{
+			desc: "New",
+			newAtomic: func() *Pointer[foo] {
+				return NewPointer(&i)
+			},
+			initial: &i,
+		},
+		{
+			desc: "New/nil",
+			newAtomic: func() *Pointer[foo] {
+				return NewPointer[foo](nil)
+			},
+			initial: nil,
+		},
+		{
+			desc: "zero value",
+			newAtomic: func() *Pointer[foo] {
+				var p Pointer[foo]
+				return &p
+			},
+			initial: nil,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.desc, func(t *testing.T) {
+			t.Run("Load", func(t *testing.T) {
+				atom := tt.newAtomic()
+				require.Equal(t, tt.initial, atom.Load(), "Load should report nil.")
+			})
+
+			t.Run("Swap", func(t *testing.T) {
+				atom := tt.newAtomic()
+				require.Equal(t, tt.initial, atom.Swap(&k), "Swap didn't return the old value.")
+				require.Equal(t, &k, atom.Load(), "Swap didn't set the correct value.")
+			})
+
+			t.Run("CAS", func(t *testing.T) {
+				atom := tt.newAtomic()
+				require.True(t, atom.CompareAndSwap(tt.initial, &j), "CAS didn't report a swap.")
+				require.Equal(t, &j, atom.Load(), "CAS didn't set the correct value.")
+			})
+
+			t.Run("Store", func(t *testing.T) {
+				atom := tt.newAtomic()
+				atom.Store(&i)
+				require.Equal(t, &i, atom.Load(), "Store didn't set the correct value.")
+			})
+		})
+	}
+}