Golang Context 详细介绍
Golang context本文包含对context实现上的分析和使用方式，分析部分源码讲解比价多，可能会比较枯燥，读者可以直接跳过去阅读使用部分。
ps： 作者本着开源分享的精神撰写本篇文章，如果出现任何误差务必留言指正，作者会在第一时间内修正，共同维护一个好的开源生态，谢谢！！！
一、简介
作者所讲的context的包名称是： "golang.org/x/net/context" ,希望读者不要引用错误了。
在godoc中对context的介绍如下：
Package context defines the Context type, which carries deadlines, cancelation signals, and other request-scoped values across API boundaries and between processes.
As of Go 1.7 this package is available in the standard library under the name context. https://golang.org/pkg/context.
Incoming requests to a server should create a Context, and outgoing calls to servers should accept a Context. The chain of function calls between must propagate the Context, optionally replacing it with a modified copy created using WithDeadline, WithTimeout, WithCancel, or WithValue.
Programs that use Contexts should follow these rules to keep interfaces consistent across packages and enable static analysis tools to check context propagation:
Do not store Contexts inside a struct type; instead, pass a Context explicitly to each function that needs it. The Context should be the first parameter, typically named ctx:
func DoSomething(ctx context.Context, arg Arg) error {
// ... use ctx ...
}
Do not pass a nil Context, even if a function permits it. Pass context.TODO if you are unsure about which Context to use.
Use context Values only for request-scoped data that transits processes and APIs, not for passing optional parameters to functions.
The same Context may be passed to functions running in different goroutines; Contexts are safe for simultaneous use by multiple goroutines.
See http://blog.golang.org/context for example code for a server that uses Contexts.
View Code 鉴于作者英文水平有限，在这里不进行对照翻译，以免误导读者。它的第一句已经介绍了它的作用了：一个贯穿API的边界和进程之间的context 类型，可以携带deadlines、cancel signals和其他信息。就如同它的中文翻译一样：上下文。在一个应用服务中会并行运行很多的goroutines或进程， 它们彼此之间或者是从属关系、竞争关系、互斥关系，不同的goroutines和进程进行交互的时候需要进行状态的切换和数据的同步，而这就是context包要支持的功能。
二、解析
context的接口定义如下：
每一个接口都有详细的注释，这里就不重复了。 在context的源码中有以下几个结构体实现了Context Interface：
// A Context carries a deadline, a cancelation signal, and other values across
// API boundaries.Context's methods may be called by multiple goroutines simultaneously.
type Context interface {
// Deadline returns the time when work done on behalf of this context
// should be canceled.Deadline returns ok==false when no deadline is
// set.Successive calls to Deadline return the same results.
Deadline() (deadline time.Time, ok bool)
// Done returns a channel that's closed when work done on behalf of this
// context should be canceled.Done may return nil if this context can
// never be canceled.Successive calls to Done return the same value.
Done()
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