sync: implement sync.Map

This is a very simple implementation, just enough to get packages to
compile.

src/sync/map.go

@@ -0,0 +1,44 @@
+package sync
+
+// This file implements just enough of sync.Map to get packages to compile. It
+// is no more efficient than a map with a lock.
+
+type Map struct {
+	lock Mutex
+	m    map[interface{}]interface{}
+}
+
+func (m *Map) Delete(key interface{}) {
+	m.lock.Lock()
+	defer m.lock.Unlock()
+	delete(m.m, key)
+}
+
+func (m *Map) Load(key interface{}) (value interface{}, ok bool) {
+	m.lock.Lock()
+	defer m.lock.Unlock()
+	value, ok = m.m[key]
+	return
+}
+
+func (m *Map) LoadOrStore(key, value interface{}) (actual interface{}, loaded bool) {
+	m.lock.Lock()
+	defer m.lock.Unlock()
+	if m.m == nil {
+		m.m = make(map[interface{}]interface{})
+	}
+	if existing, ok := m.m[key]; ok {
+		return existing, true
+	}
+	m.m[key] = value
+	return value, false
+}
+
+func (m *Map) Store(key, value interface{}) {
+	m.lock.Lock()
+	defer m.lock.Unlock()
+	if m.m == nil {
+		m.m = make(map[interface{}]interface{})
+	}
+	m.m[key] = value
+}