TheRiver | blog

You have reached the world's edge, none but devils play past here

0%

golang context

Posted on Edited on
Symbols count in article: 9.9k Reading time ≈ 9 mins.
author title version
TheRiver golang reflect go version go1.15.2 darwin/amd64

context.Context

1
2
3
4
5
6
7
8
9
10
// A Context carries a deadline, a cancellation signal, and other values across
// API boundaries.
//
// Context's methods may be called by multiple goroutines simultaneously.
type Context interface {
	Deadline() (deadline time.Time, ok bool)
	Done() <-chan struct{}
	Err() error
	Value(key interface{}) interface{}
}
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
var (
	background = new(emptyCtx)
	todo       = new(emptyCtx)
)

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
}

Background()

1
2
3
func Background() Context {
	return background
}

TODO()

1
2
3
func TODO() Context {
	return todo
}

小结

Background()和TODO()都返回一个实现了context.Context接口的类型emptyCtx,通常作为parent的context节点使用。一般都是用Background(),TODO if you are unsure about which Context to use.

WithValue()

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
// WithValue returns a copy of parent in which the value associated with key is
// val.
//
// Use context Values only for request-scoped data that transits processes and
// APIs, not for passing optional parameters to functions.
//
// The provided key must be comparable and should not be of type
// string or any other built-in type to avoid collisions between
// packages using context. Users of WithValue should define their own
// types for keys. To avoid allocating when assigning to an
// interface{}, context keys often have concrete type
// struct{}. Alternatively, exported context key variables' static
// type should be a pointer or interface.
func WithValue(parent Context, key, val interface{}) Context {
	if parent == nil {
		panic("cannot create context from nil parent")
	}
	if key == nil {
		panic("nil key")
	}
	if !reflectlite.TypeOf(key).Comparable() {
		panic("key is not comparable")
	}
	return &valueCtx{parent, key, val}
}

type valueCtx struct {
	Context
	key, val interface{}
}

Value()

1
2
3
4
5
6
func (c *valueCtx) Value(key interface{}) interface{} {
	if c.key == key {
		return c.val
	}
	return c.Context.Value(key)
}

小结

key必须是可比较的类型，并且不能是string或者其他内置类型，通常定义为type xxx struct{}.valueCtx通过嵌入接口来继承接口的方法，也即是实现了该接口，并在此基础上新增了几个方法(Value()…)

demo

1
2
3
4
5
6
7
8
9
10
11
12
13
14
package main

import (
	"context"
	"fmt"
)

type k struct{}

func main() {
	ctx := context.Background()
	valueCtx := context.WithValue(ctx, k{}, "parent")
	fmt.Println(valueCtx.Value(k{})) //parent
}

WithCancel

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
// 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) {
	if parent == nil {
		panic("cannot create context from nil parent")
	}
	c := newCancelCtx(parent)
    propagateCancel(parent, &c)
    //var Canceled = errors.New("context canceled")
	return &c, func() { c.cancel(true, Canceled) }
}

newCancelCtx

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
// newCancelCtx returns an initialized cancelCtx.
func newCancelCtx(parent Context) cancelCtx {
	return cancelCtx{Context: parent}
}

// 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
	done     chan struct{}         // created lazily, closed by first cancel call
	children map[canceler]struct{} // set to nil by the first cancel call
	err      error                 // set to non-nil by the first cancel call
}

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
}

// A canceler is a context type that can be canceled directly. The
// implementations are *cancelCtx and *timerCtx.
type canceler interface {
	cancel(removeFromParent bool, err error)
	Done() <-chan struct{}
}

propagateCancel

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
// cancel closes c.done, cancels each of c's children, and, if
// removeFromParent is true, removes c from its parent's children.
func (c *cancelCtx) cancel(removeFromParent bool, err error) {
	if err == nil {
		panic("context: internal error: missing cancel error")
	}
	c.mu.Lock()
	if c.err != nil {
		c.mu.Unlock()
		return // already canceled
	}
	c.err = err
	if c.done == nil {
        //var closedchan = make(chan struct{})
		c.done = closedchan
	} else {
		close(c.done)
	}
	for child := range c.children {
		// NOTE: acquiring the child's lock while holding parent's lock.
		child.cancel(false, err)
	}
	c.children = nil
	c.mu.Unlock()

	if removeFromParent {
		removeChild(c.Context, c)
	}
}

// removeChild removes a context from its parent.
func removeChild(parent Context, child canceler) {
    p, ok := parentCancelCtx(parent)
    //第2层以下的为真
	if !ok {
		return
	}
	p.mu.Lock()
	if p.children != nil {
		delete(p.children, child)
	}
	p.mu.Unlock()
}


// parentCancelCtx returns the underlying *cancelCtx for parent.
// It does this by looking up parent.Value(&cancelCtxKey) to find
// the innermost enclosing *cancelCtx and then checking whether
// parent.Done() matches that *cancelCtx. (If not, the *cancelCtx
// has been wrapped in a custom implementation providing a
// different done channel, in which case we should not bypass it.)
func parentCancelCtx(parent Context) (*cancelCtx, bool) {
	done := parent.Done()
	if done == closedchan || done == nil {
		return nil, false
    }
    //这里value返回parent,转换成cancelCtx,按说本来就是cancelCtx。
    //对于root的ctx这里类型转换会panic,不过前面if已经拒绝了root的ctx
    //也就是说这里是第2层以下的parent的ctx
	p, ok := parent.Value(&cancelCtxKey).(*cancelCtx)
	if !ok {
		return nil, false
	}
	p.mu.Lock()
	ok = p.done == done
	p.mu.Unlock()
	if !ok {
		return nil, false
	}
	return p, true
}

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

// propagateCancel arranges for child to be canceled when parent is.
func propagateCancel(parent Context, child canceler) {
	done := parent.Done()
	if done == nil {
		return // parent is never canceled
	}

	select {
	case <-done:
		// parent is already canceled
		child.cancel(false, parent.Err())
		return
	default:
	}

	if p, ok := parentCancelCtx(parent); ok {
		p.mu.Lock()
		if p.err != nil {
			// parent has already been canceled
			child.cancel(false, p.err)
		} else {
			if p.children == nil {
				p.children = make(map[canceler]struct{})
			}
			p.children[child] = struct{}{}
		}
		p.mu.Unlock()
	} else {
		atomic.AddInt32(&goroutines, +1)
		go func() {
			select {
			case <-parent.Done():
				child.cancel(false, parent.Err())
			case <-child.Done():
			}
		}()
	}
}

小结

newCancelCtx返回cancelCtx，cancelCtx实现了5个方法：

  • func (c *cancelCtx) Value(key interface{}) interface{}
  • func (c *cancelCtx) Done() <-chan struct{} //生成channel
  • func (c *cancelCtx) Err() error
  • func (c *cancelCtx) String() string
  • func (c *cancelCtx) cancel(removeFromParent bool, err error) //关闭

最重要的是Done(),Value和cancel函数。

  • Done生成一个channel
  • Vaule返回自身的ctx或者调用parent.Value()
  • cancle关闭自身和children的channel

propagateCancel会把children存入map.

由于context可以无限的向下扩展，层级可能很复杂，这里的函数看起来挺绕的，不深究了。

WithTimeout

1
2
3
func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc) {
	return WithDeadline(parent, time.Now().Add(timeout))
}

WithDeadline

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
// WithDeadline returns a copy of the parent context with the deadline adjusted
// to be no later than d. If the parent's deadline is already earlier than d,
// WithDeadline(parent, d) is semantically equivalent to parent. The returned
// context's Done channel is closed when the deadline expires, 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 WithDeadline(parent Context, d time.Time) (Context, CancelFunc) {
	if parent == nil {
		panic("cannot create context from nil parent")
	}
	if cur, ok := parent.Deadline(); ok && cur.Before(d) {
		// The current deadline is already sooner than the new one.
		return WithCancel(parent)
	}
	c := &timerCtx{
		cancelCtx: newCancelCtx(parent),
		deadline:  d,
	}
	propagateCancel(parent, c)
	dur := time.Until(d)
	if dur <= 0 {
		c.cancel(true, DeadlineExceeded) // deadline has already passed
		return c, func() { c.cancel(false, Canceled) }
	}
	c.mu.Lock()
	defer c.mu.Unlock()
	if c.err == nil {
		c.timer = time.AfterFunc(dur, func() {
			c.cancel(true, DeadlineExceeded)
		})
	}
	return c, func() { c.cancel(true, Canceled) }
}

// 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
}

小结

和withcancle差别不大，就是多了计时功能。WithDeadline生成的ctx才有Deadline()函数。

WithDeadline在time.Time时间后异步的关闭channel

CancelFunc

1
type CancelFunc func()

其他

在database/sql中有遇到context的使用，但只有一层，没看出多大用。以后遇到有意义的场景再补充。

层层生成的新的层级的ctx和一个ctx层层传递的区别？

关闭当前的ctx也会把底层的ctx关闭，这就是区别吧？？？

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
package main

import (
	"context"
)

func ctx2(ctx context.Context) {
}

func ctx3(ctx context.Context) {
	ctx2(ctx)
}

func main() {
	ctx, cancle := context.WithCancel(context.Background())
	ctx2, cancle2 := context.WithCancel(ctx)
	_, cancle3 := context.WithCancel(ctx2)

	defer func() {
		cancle()
		cancle2()
		cancle3()
	}()
}

reference

[1]https://draveness.me/golang/docs/part3-runtime/ch06-concurrency/golang-context/

[2]https://golang.org/pkg/context/

----------- ending -----------