text: // Copyright 2014 The Go Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style
  // license that can be found in the LICENSE file.
  
  // +build !go1.9
  
  package context
  
  import "time"
  
  // 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.
  	//
  	// 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 http://blog.golang.org/pipelines for more examples of how to use
  	// a Done channel for cancelation.
  	Done() <-chan struct{}
  
  	// Err returns a non-nil error value after Done is closed. Err returns
  	// Canceled if the context was canceled or DeadlineExceeded if the
  	// context's deadline passed. No other values for Err are defined.
  	// After Done is closed, successive calls to Err return the same value.
  	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 stores using that key:
  	//
  	// 	// Package user defines a User type that's stored in Contexts.
  	// 	package user
  	//
  	// 	import "golang.org/x/net/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 = 0
  	//
  	// 	// 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{}
  }
  
  // A CancelFunc tells an operation to abandon its work.
  // A CancelFunc does not wait for the work to stop.
  // After the first call, subsequent calls to a CancelFunc do nothing.
  type CancelFunc func()
  