syncs: fix AtomicValue for interface kinds (#11943)

If AtomicValue[T] is used with a T that is an interface kind,
then Store may panic if different concret types are ever stored.

Fix this by always wrapping in a concrete type.
Technically, this is only needed if T is an interface kind,
but there is no harm in doing it also for non-interface kinds.

Updates #cleanup

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

syncs/syncs.go

@@ -23,11 +23,18 @@ func initClosedChan() <-chan struct{} {
 	return ch
 }
 
-// AtomicValue is the generic version of atomic.Value.
+// AtomicValue is the generic version of [atomic.Value].
 type AtomicValue[T any] struct {
 	v atomic.Value
 }
 
+// wrappedValue is used to wrap a value T in a concrete type,
+// otherwise atomic.Value.Store may panic due to mismatching types in interfaces.
+// This wrapping is not necessary for non-interface kinds of T,
+// but there is no harm in wrapping anyways.
+// See https://cs.opensource.google/go/go/+/refs/tags/go1.22.2:src/sync/atomic/value.go;l=78
+type wrappedValue[T any] struct{ v T }
+
 // Load returns the value set by the most recent Store.
 // It returns the zero value for T if the value is empty.
 func (v *AtomicValue[T]) Load() T {
@@ -40,7 +47,7 @@ func (v *AtomicValue[T]) Load() T {
 func (v *AtomicValue[T]) LoadOk() (_ T, ok bool) {
 	x := v.v.Load()
 	if x != nil {
-		return x.(T), true
+		return x.(wrappedValue[T]).v, true
 	}
 	var zero T
 	return zero, false
@@ -48,22 +55,22 @@ func (v *AtomicValue[T]) LoadOk() (_ T, ok bool) {
 
 // Store sets the value of the Value to x.
 func (v *AtomicValue[T]) Store(x T) {
-	v.v.Store(x)
+	v.v.Store(wrappedValue[T]{x})
 }
 
 // Swap stores new into Value and returns the previous value.
 // It returns the zero value for T if the value is empty.
 func (v *AtomicValue[T]) Swap(x T) (old T) {
-	oldV := v.v.Swap(x)
+	oldV := v.v.Swap(wrappedValue[T]{x})
 	if oldV != nil {
-		return oldV.(T)
+		return oldV.(wrappedValue[T]).v
 	}
 	return old
 }
 
 // CompareAndSwap executes the compare-and-swap operation for the Value.
 func (v *AtomicValue[T]) CompareAndSwap(oldV, newV T) (swapped bool) {
-	return v.v.CompareAndSwap(oldV, newV)
+	return v.v.CompareAndSwap(wrappedValue[T]{oldV}, wrappedValue[T]{newV})
 }
 
 // WaitGroupChan is like a sync.WaitGroup, but has a chan that closes

syncs/syncs_test.go

@@ -5,12 +5,67 @@ package syncs
 
 import (
 	"context"
+	"io"
+	"os"
 	"sync"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
 )
 
+func TestAtomicValue(t *testing.T) {
+	{
+		// Always wrapping should not allocate for simple values
+		// because wrappedValue[T] has the same memory layout as T.
+		var v AtomicValue[bool]
+		bools := []bool{true, false}
+		if n := int(testing.AllocsPerRun(1000, func() {
+			for _, b := range bools {
+				v.Store(b)
+			}
+		})); n != 0 {
+			t.Errorf("AllocsPerRun = %d, want 0", n)
+		}
+	}
+
+	{
+		var v AtomicValue[int]
+		got, gotOk := v.LoadOk()
+		if got != 0 || gotOk {
+			t.Fatalf("LoadOk = (%v, %v), want (0, false)", got, gotOk)
+		}
+		v.Store(1)
+		got, gotOk = v.LoadOk()
+		if got != 1 || !gotOk {
+			t.Fatalf("LoadOk = (%v, %v), want (1, true)", got, gotOk)
+		}
+	}
+
+	{
+		var v AtomicValue[error]
+		got, gotOk := v.LoadOk()
+		if got != nil || gotOk {
+			t.Fatalf("LoadOk = (%v, %v), want (nil, false)", got, gotOk)
+		}
+		v.Store(io.EOF)
+		got, gotOk = v.LoadOk()
+		if got != io.EOF || !gotOk {
+			t.Fatalf("LoadOk = (%v, %v), want (EOF, true)", got, gotOk)
+		}
+		err := &os.PathError{}
+		v.Store(err)
+		got, gotOk = v.LoadOk()
+		if got != err || !gotOk {
+			t.Fatalf("LoadOk = (%v, %v), want (%v, true)", got, gotOk, err)
+		}
+		v.Store(nil)
+		got, gotOk = v.LoadOk()
+		if got != nil || !gotOk {
+			t.Fatalf("LoadOk = (%v, %v), want (nil, true)", got, gotOk)
+		}
+	}
+}
+
 func TestWaitGroupChan(t *testing.T) {
 	wg := NewWaitGroupChan()