Golang 中的 context 包提供了 Background 方法和 TODO 方法，用来返回一个emptyContext

源代码如下

var (
    background = new(emptyCtx)
    todo       = new(emptyCtx)
)

// Background returns a non-nil, empty Context. It is never canceled, has no
// values, and has no deadline. It is typically used by the main function,
// initialization, and tests, and as the top-level Context for incoming
// requests.
func Background() Context {
    return background
}

// TODO returns a non-nil, empty Context. Code should use context.TODO when
// it's unclear which Context to use or it is not yet available (because the
// surrounding function has not yet been extended to accept a Context
// parameter). TODO is recognized by static analysis tools that determine
// whether Contexts are propagated correctly in a program.
func TODO() Context {
    return todo
}

如上， todo 和background 都是一个emtyCtx实例，为什么要把两者区分开呢，使用效果上有什么不一样的么？