笔记- Go context

context包在Go并发编程代码中经常可见，而在我学习Go的两本书中《Go圣经》《Go实战》都没有看到这个包的介绍，才知道是Go1.7版本中才将其放入到标准库中。

在Go服务中，一个请求过来，我们给分配一个gor处理，在这个请求里，通常还会继续开启gor来访问其他后端服务。而当这个请求结束的时候，我们必须要终止这些额外开启的gor，以免gor泄露。context包正是为了解决这个问题。

Context接口类型

type Context interface {
    // 当Context被取消或者超时，Done()返回一个关闭状态的channel
    Done() <-chan struct{}

    // 表示这个Context为什么被取消
    Err() error
    
    // 返回这个context什么时候被取消
    Deadline() (deadline time.Time, ok bool)
    
    // 返回这个key对应的value 没有则是nil
    Value(key interface{}) interface{}
}

Done()方法返回关闭状态的channel，就表示函数该结束并返回了
Context是并发安全的，可以在多个gor中传递。
Value允许Context在一个请求域范围下绑定数据，这个数据要求并发安全，通常绑定请求id如request id。

Context派生

Context是以树形结构生长，获取一个新的Context需要依赖一个存在的Context，当一个Context被取消后，它派生的其他Context也会被取消。

Background是Context树的根，顶层Context。

// Background returns an empty Context. It is never canceled, has no deadline,
// and has no values. Background is typically used in main, init, and tests,
// and as the top-level Context for incoming requests.
func Background() Context

返回一个空的Context，不会被取消，没有deadline，没有values。Background通常用于main、init、test作为顶层Context。

WithCancel 和 WithTimeout 返回派生的Context。

// WithCancel returns a copy of parent whose Done channel is closed as soon as
// parent.Done is closed or cancel is called.
func WithCancel(parent Context) (ctx Context, cancel CancelFunc)

// A CancelFunc cancels a Context.
type CancelFunc func()

// WithTimeout returns a copy of parent whose Done channel is closed as soon as
// parent.Done is closed, cancel is called, or timeout elapses. The new
// Context's Deadline is the sooner of now+timeout and the parent's deadline, if
// any. If the timer is still running, the cancel function releases its
// resources.
func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc)

WithValue

// WithValue returns a copy of parent whose Value method returns val for key.
func WithValue(parent Context, key interface{}, val interface{}) Context

example

go标准库中的例子，加深理解

WithCancel

利用Context通知产生数据的gor停止，避免gor泄露


// This example demonstrates the use of a cancelable context to prevent a
// goroutine leak. By the end of the example function, the goroutine started
// by gen will return without leaking.
func ExampleWithCancel() {
  // gen generates integers in a separate goroutine and
  // sends them to the returned channel.
  // The callers of gen need to cancel the context once
  // they are done consuming generated integers not to leak
  // the internal goroutine started by gen.
  gen := func(ctx context.Context) <-chan int {
    dst := make(chan int)
    n := 1
    go func() {
      for {
        select {
        case <-ctx.Done():
          return // returning not to leak the goroutine
        case dst <- n:
          n++
        }
      }
    }()
    return dst
  }

  ctx, cancel := context.WithCancel(context.Background())
  defer cancel() // cancel when we are finished consuming integers

  for n := range gen(ctx) {
    fmt.Println(n)
    if n == 5 {
      break
    }
  }
  // Output:
  // 1
  // 2
  // 3
  // 4
  // 5
}

WithDeadline

// This example passes a context with an arbitrary deadline to tell a blocking
// function that it should abandon its work as soon as it gets to it.
func ExampleWithDeadline() {
  d := time.Now().Add(50 * time.Millisecond)
  ctx, cancel := context.WithDeadline(context.Background(), d)

  // Even though ctx will be expired, it is good practice to call its
  // cancellation function in any case. Failure to do so may keep the
  // context and its parent alive longer than necessary.
  defer cancel()

  select {
  case <-time.After(1 * time.Second):
    fmt.Println("overslept")
  case <-ctx.Done():
    fmt.Println(ctx.Err())
  }

  // Output:
  // context deadline exceeded
}

WithTimeout

// This example passes a context with a timeout to tell a blocking function that
// it should abandon its work after the timeout elapses.
func ExampleWithTimeout() {
  // Pass a context with a timeout to tell a blocking function that it
  // should abandon its work after the timeout elapses.
  ctx, cancel := context.WithTimeout(context.Background(), 50*time.Millisecond)
  defer cancel()

  select {
  case <-time.After(1 * time.Second):
    fmt.Println("overslept")
  case <-ctx.Done():
    fmt.Println(ctx.Err()) // prints "context deadline exceeded"
  }

  // Output:
  // context deadline exceeded
}

WithValue

// This example demonstrates how a value can be passed to the context
// and also how to retrieve it if it exists.
func ExampleWithValue() {
  type favContextKey string

  f := func(ctx context.Context, k favContextKey) {
    if v := ctx.Value(k); v != nil {
      fmt.Println("found value:", v)
      return
    }
    fmt.Println("key not found:", k)
  }

  k := favContextKey("language")
  ctx := context.WithValue(context.Background(), k, "Go")

  f(ctx, k)
  f(ctx, favContextKey("color"))

  // Output:
  // found value: Go
  // key not found: color
}

参考：

https://blog.golang.org/context
go标准库