context

一个接口

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.
    // The close of the Done channel may happen asynchronously,
    // after the cancel function returns.
    //
    // WithCancel arranges for Done to be closed when cancel is called;
    // WithDeadline arranges for Done to be closed when the deadline
    // expires; WithTimeout arranges for Done to be closed when the timeout
    // elapses.
    //
    // Done is provided for use in select statements:
    //
    //  // Stream generates values with DoSomething and sends them to out
    //  // until DoSomething returns an error or ctx.Done is closed.
    //  func Stream(ctx context.Context, out chan<- Value) error {
    //      for {
    //          v, err := DoSomething(ctx)
    //          if err != nil {
    //              return err
    //          }
    //          select {
    //          case <-ctx.Done():
    //              return ctx.Err()
    //          case out <- v:
    //          }
    //      }
    //  }
    //
    // See https://blog.golang.org/pipelines for more examples of how to use
    // a Done channel for cancellation.
    Done() <-chan struct{}

    // If Done is not yet closed, Err returns nil.
    // If Done is closed, Err returns a non-nil error explaining why:
    // Canceled if the context was canceled
    // or DeadlineExceeded if the context's deadline passed.
    // After Err returns a non-nil error, successive calls to Err return the same error.
    Err() error

    // Value returns the value associated with this context for key, or nil
    // if no value is associated with key. Successive calls to Value with
    // the same key returns the same result.
    //
    // Use context values only for request-scoped data that transits
    // processes and API boundaries, not for passing optional parameters to
    // functions.
    //
    // A key identifies a specific value in a Context. Functions that wish
    // to store values in Context typically allocate a key in a global
    // variable then use that key as the argument to context.WithValue and
    // Context.Value. A key can be any type that supports equality;
    // packages should define keys as an unexported type to avoid
    // collisions.
    //
    // Packages that define a Context key should provide type-safe accessors
    // for the values stored using that key:
    //
    //     // Package user defines a User type that's stored in Contexts.
    //     package user
    //
    //     import "context"
    //
    //     // User is the type of value stored in the Contexts.
    //     type User struct {...}
    //
    //     // key is an unexported type for keys defined in this package.
    //     // This prevents collisions with keys defined in other packages.
    //     type key int
    //
    //     // userKey is the key for user.User values in Contexts. It is
    //     // unexported; clients use user.NewContext and user.FromContext
    //     // instead of using this key directly.
    //     var userKey key
    //
    //     // NewContext returns a new Context that carries value u.
    //     func NewContext(ctx context.Context, u *User) context.Context {
    //         return context.WithValue(ctx, userKey, u)
    //     }
    //
    //     // FromContext returns the User value stored in ctx, if any.
    //     func FromContext(ctx context.Context) (*User, bool) {
    //         u, ok := ctx.Value(userKey).(*User)
    //         return u, ok
    //     }
    Value(key interface{}) interface{}
}

四个具体实现

emptyCtx

// An emptyCtx is never canceled, has no values, and has no deadline. It is not
// struct{}, since vars of this type must have distinct addresses.
type emptyCtx int

func (*emptyCtx) Deadline() (deadline time.Time, ok bool) {
    return
}

func (*emptyCtx) Done() <-chan struct{} {
    return nil
}

func (*emptyCtx) Err() error {
    return nil
}

func (*emptyCtx) Value(key interface{}) interface{} {
    return nil
}

func (e *emptyCtx) String() string {
    switch e {
    case background:
        return "context.Background"
    case todo:
        return "context.TODO"
    }
    return "unknown empty Context"
}

cancelCtx

// A cancelCtx can be canceled. When canceled, it also cancels any children
// that implement canceler.
type cancelCtx struct {
    Context

    mu       sync.Mutex            // protects following fields 保护字段操作安全
    done     chan struct{}         // created lazily, closed by first cancel call 传递取消信号
    children map[canceler]struct{} // set to nil by the first cancel call 子节点集
    err      error                 // set to non-nil by the first cancel call 错误信息
}

func (c *cancelCtx) Value(key interface{}) interface{} {
    if key == &cancelCtxKey {
        return c
    }
    return c.Context.Value(key)
}

func (c *cancelCtx) Done() <-chan struct{} {
    c.mu.Lock()
    if c.done == nil {
        c.done = make(chan struct{})
    }
    d := c.done
    c.mu.Unlock()
    return d
}

func (c *cancelCtx) Err() error {
    c.mu.Lock()
    err := c.err
    c.mu.Unlock()
    return err
}

timerCtx

// A timerCtx carries a timer and a deadline. It embeds a cancelCtx to
// implement Done and Err. It implements cancel by stopping its timer then
// delegating to cancelCtx.cancel.
type timerCtx struct {
    cancelCtx
    timer *time.Timer // Under cancelCtx.mu.

    deadline time.Time
}

func (c *timerCtx) Deadline() (deadline time.Time, ok bool) {
    return c.deadline, true
}

valueCtx

// A valueCtx carries a key-value pair. It implements Value for that key and
// delegates all other calls to the embedded Context.
type valueCtx struct {
    Context
    key, val interface{}
}

六个接口函数

Background和TODO

Background和TODO接口函数返回的都是new(emptyCtx)

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).
func TODO() Context {
    return todo
}

image-20210522165132122

WithCancel

生产一个带有取消函数的ctx，调用cancel可以取消对应的context

// WithCancel returns a copy of parent with a new Done channel. The returned
// context's Done channel is closed when the returned cancel function is called
// or when the parent context's Done channel is closed, whichever happens first.
//
// Canceling this context releases resources associated with it, so code should
// call cancel as soon as the operations running in this Context complete.
func WithCancel(parent Context) (ctx Context, cancel CancelFunc) {
    c := newCancelCtx(parent)
    propagateCancel(parent, &c)
    return &c, func() { c.cancel(true, Canceled) }
}

WithTimeout和WithDeadline

WithTimeout设置超时时间

WithDeadline设置过期时间

WithValue

给context设置一个键值对信息，若父节点存在键值对"key":"value"子节点设置的键值对"key":"subValue"

    var ctx1 = context.WithValue(ctx, "key", "value")
    log.Printf("value: %s \n", ctx1.Value("key"))

    var ctx2 = context.WithValue(ctx, "key", "subValue")
    log.Printf("value: %s \n", ctx2.Value("key"))
    //2021/05/22 21:17:53 value: value 
    //2021/05/22 21:17:53 value: subValue