fix: TypedValue.CompareAndSwap

tailscale/tailscale@84aa7ff

Author: wwqgtxx

common/atomic/value.go

@@ -27,11 +27,16 @@ type tValue[T any] struct {
 }
 
 func (t *TypedValue[T]) Load() T {
+	value, _ := t.LoadOk()
+	return value
+}
+
+func (t *TypedValue[T]) LoadOk() (_ T, ok bool) {
 	value := t.value.Load()
 	if value == nil {
-		return DefaultValue[T]()
+		return DefaultValue[T](), false
 	}
-	return value.(tValue[T]).value
+	return value.(tValue[T]).value, true
 }
 
 func (t *TypedValue[T]) Store(value T) {
@@ -47,7 +52,11 @@ func (t *TypedValue[T]) Swap(new T) T {
 }
 
 func (t *TypedValue[T]) CompareAndSwap(old, new T) bool {
-	return t.value.CompareAndSwap(tValue[T]{old}, tValue[T]{new})
+	return t.value.CompareAndSwap(tValue[T]{old}, tValue[T]{new}) ||
+		// In the edge-case where [atomic.Value.Store] is uninitialized
+		// and trying to compare with the zero value of T,
+		// then compare-and-swap with the nil any value.
+		(any(old) == any(DefaultValue[T]()) && t.value.CompareAndSwap(any(nil), tValue[T]{new}))
 }
 
 func (t *TypedValue[T]) MarshalJSON() ([]byte, error) {

common/atomic/value_test.go

@@ -0,0 +1,77 @@
+package atomic
+
+import (
+	"io"
+	"os"
+	"testing"
+)
+
+func TestTypedValue(t *testing.T) {
+	{
+		// Always wrapping should not allocate for simple values
+		// because tValue[T] has the same memory layout as T.
+		var v TypedValue[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 TypedValue[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 TypedValue[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)
+		}
+	}
+
+	{
+		c1, c2, c3 := make(chan struct{}), make(chan struct{}), make(chan struct{})
+		var v TypedValue[chan struct{}]
+		if v.CompareAndSwap(c1, c2) != false {
+			t.Fatalf("CompareAndSwap = true, want false")
+		}
+		if v.CompareAndSwap(nil, c1) != true {
+			t.Fatalf("CompareAndSwap = false, want true")
+		}
+		if v.CompareAndSwap(c2, c3) != false {
+			t.Fatalf("CompareAndSwap = true, want false")
+		}
+		if v.CompareAndSwap(c1, c2) != true {
+			t.Fatalf("CompareAndSwap = false, want true")
+		}
+	}
+}

go.mod

@@ -25,7 +25,7 @@ require (
 	github.com/metacubex/gopacket v1.1.20-0.20230608035415-7e2f98a3e759
 	github.com/metacubex/quic-go v0.52.1-0.20250522021943-aef454b9e639
 	github.com/metacubex/randv2 v0.2.0
-	github.com/metacubex/sing v0.5.3
+	github.com/metacubex/sing v0.5.4-0.20250605054047-54dc6097da29
 	github.com/metacubex/sing-mux v0.3.2
 	github.com/metacubex/sing-quic v0.0.0-20250523120938-f1a248e5ec7f
 	github.com/metacubex/sing-shadowsocks v0.2.10

go.sum

@@ -116,8 +116,8 @@ github.com/metacubex/quic-go v0.52.1-0.20250522021943-aef454b9e639/go.mod h1:Kc6
 github.com/metacubex/randv2 v0.2.0 h1:uP38uBvV2SxYfLj53kuvAjbND4RUDfFJjwr4UigMiLs=
 github.com/metacubex/randv2 v0.2.0/go.mod h1:kFi2SzrQ5WuneuoLLCMkABtiBu6VRrMrWFqSPyj2cxY=
 github.com/metacubex/sing v0.5.2/go.mod h1:ypf0mjwlZm0sKdQSY+yQvmsbWa0hNPtkeqyRMGgoN+w=
-github.com/metacubex/sing v0.5.3 h1:QWdN16WFKMk06x4nzkc8SvZ7y2x+TLQrpkPoHs+WSVM=
-github.com/metacubex/sing v0.5.3/go.mod h1:ypf0mjwlZm0sKdQSY+yQvmsbWa0hNPtkeqyRMGgoN+w=
+github.com/metacubex/sing v0.5.4-0.20250605054047-54dc6097da29 h1:SD9q025FNTaepuFXFOKDhnGLVu6PQYChBvw2ZYPXeLo=
+github.com/metacubex/sing v0.5.4-0.20250605054047-54dc6097da29/go.mod h1:ypf0mjwlZm0sKdQSY+yQvmsbWa0hNPtkeqyRMGgoN+w=
 github.com/metacubex/sing-mux v0.3.2 h1:nJv52pyRivHcaZJKk2JgxpaVvj1GAXG81scSa9N7ncw=
 github.com/metacubex/sing-mux v0.3.2/go.mod h1:3rt1soewn0O6j89GCLmwAQFsq257u0jf2zQSPhTL3Bw=
 github.com/metacubex/sing-quic v0.0.0-20250523120938-f1a248e5ec7f h1:mP3vIm+9hRFI0C0Vl3pE0NESF/L85FDbuB0tGgUii6I=