sync: implement sync.Swap

src/sync/map.go

@@ -70,3 +70,15 @@ func (m *Map) Range(f func(key, value interface{}) bool) {
 		}
 	}
 }
+
+// Swap replaces the value for the given key, and returns the old value if any.
+func (m *Map) Swap(key, value any) (previous any, loaded bool) {
+	m.lock.Lock()
+	defer m.lock.Unlock()
+	if m.m == nil {
+		m.m = make(map[interface{}]interface{})
+	}
+	previous, loaded = m.m[key]
+	m.m[key] = value
+	return
+}

src/sync/map_test.go

@@ -17,3 +17,22 @@ func TestMapLoadAndDelete(t *testing.T) {
 		t.Errorf("LoadAndDelete returned %v, %v, want nil, false", v, ok)
 	}
 }
+
+func TestMapSwap(t *testing.T) {
+	var sm sync.Map
+	sm.Store("present", "value")
+
+	if v, ok := sm.Swap("present", "value2"); !ok || v != "value" {
+		t.Errorf("Swap returned %v, %v, want value, true", v, ok)
+	}
+	if v, ok := sm.Load("present"); !ok || v != "value2" {
+		t.Errorf("Load after Swap returned %v, %v, want value2, true", v, ok)
+	}
+
+	if v, ok := sm.Swap("new", "foo"); ok || v != nil {
+		t.Errorf("Swap returned %v, %v, want nil, false", v, ok)
+	}
+	if v, ok := sm.Load("present"); !ok || v != "value2" {
+		t.Errorf("Load after Swap returned %v, %v, want foo, true", v, ok)
+	}
+}