1 // Copyright 2016 The Go Authors. All rights reserved.

     2 // Use of this source code is governed by a BSD-style

     3 // license that can be found in the LICENSE file.

     4 

     5 // +build go1.7

     6 

     7 package context

     8 

     9 import (

    10 	"context" // standard library's context, as of Go 1.7

    11 	"time"

    12 )

    13 

    14 var (

    15 	todo       = context.TODO()

    16 	background = context.Background()

    17 )

    18 

    19 // Canceled is the error returned by Context.Err when the context is canceled.

    20 var Canceled = context.Canceled

    21 

    22 // DeadlineExceeded is the error returned by Context.Err when the context's

    23 // deadline passes.

    24 var DeadlineExceeded = context.DeadlineExceeded

    25 

    26 // WithCancel returns a copy of parent with a new Done channel. The returned

    27 // context's Done channel is closed when the returned cancel function is called

    28 // or when the parent context's Done channel is closed, whichever happens first.

    29 //

    30 // Canceling this context releases resources associated with it, so code should

    31 // call cancel as soon as the operations running in this Context complete.

    32 func WithCancel(parent Context) (ctx Context, cancel CancelFunc) {

    33 	ctx, f := context.WithCancel(parent)

    34 	return ctx, CancelFunc(f)

    35 }

    36 

    37 // WithDeadline returns a copy of the parent context with the deadline adjusted

    38 // to be no later than d. If the parent's deadline is already earlier than d,

    39 // WithDeadline(parent, d) is semantically equivalent to parent. The returned

    40 // context's Done channel is closed when the deadline expires, when the returned

    41 // cancel function is called, or when the parent context's Done channel is

    42 // closed, whichever happens first.

    43 //

    44 // Canceling this context releases resources associated with it, so code should

    45 // call cancel as soon as the operations running in this Context complete.

    46 func WithDeadline(parent Context, deadline time.Time) (Context, CancelFunc) {

    47 	ctx, f := context.WithDeadline(parent, deadline)

    48 	return ctx, CancelFunc(f)

    49 }

    50 

    51 // WithTimeout returns WithDeadline(parent, time.Now().Add(timeout)).

    52 //

    53 // Canceling this context releases resources associated with it, so code should

    54 // call cancel as soon as the operations running in this Context complete:

    55 //

    56 // 	func slowOperationWithTimeout(ctx context.Context) (Result, error) {

    57 // 		ctx, cancel := context.WithTimeout(ctx, 100*time.Millisecond)

    58 // 		defer cancel()  // releases resources if slowOperation completes before timeout elapses

    59 // 		return slowOperation(ctx)

    60 // 	}

    61 func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc) {

    62 	return WithDeadline(parent, time.Now().Add(timeout))

    63 }

    64 

    65 // WithValue returns a copy of parent in which the value associated with key is

    66 // val.

    67 //

    68 // Use context Values only for request-scoped data that transits processes and

    69 // APIs, not for passing optional parameters to functions.

    70 func WithValue(parent Context, key interface{}, val interface{}) Context {

    71 	return context.WithValue(parent, key, val)

    72 }