1// Copyright 2014 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//go:build !go1.7
6// +build !go1.7
7
8package context
9
10import (
11	"errors"
12	"fmt"
13	"sync"
14	"time"
15)
16
17// An emptyCtx is never canceled, has no values, and has no deadline. It is not
18// struct{}, since vars of this type must have distinct addresses.
19type emptyCtx int
20
21func (*emptyCtx) Deadline() (deadline time.Time, ok bool) {
22	return
23}
24
25func (*emptyCtx) Done() <-chan struct{} {
26	return nil
27}
28
29func (*emptyCtx) Err() error {
30	return nil
31}
32
33func (*emptyCtx) Value(key interface{}) interface{} {
34	return nil
35}
36
37func (e *emptyCtx) String() string {
38	switch e {
39	case background:
40		return "context.Background"
41	case todo:
42		return "context.TODO"
43	}
44	return "unknown empty Context"
45}
46
47var (
48	background = new(emptyCtx)
49	todo       = new(emptyCtx)
50)
51
52// Canceled is the error returned by Context.Err when the context is canceled.
53var Canceled = errors.New("context canceled")
54
55// DeadlineExceeded is the error returned by Context.Err when the context's
56// deadline passes.
57var DeadlineExceeded = errors.New("context deadline exceeded")
58
59// WithCancel returns a copy of parent with a new Done channel. The returned
60// context's Done channel is closed when the returned cancel function is called
61// or when the parent context's Done channel is closed, whichever happens first.
62//
63// Canceling this context releases resources associated with it, so code should
64// call cancel as soon as the operations running in this Context complete.
65func WithCancel(parent Context) (ctx Context, cancel CancelFunc) {
66	c := newCancelCtx(parent)
67	propagateCancel(parent, c)
68	return c, func() { c.cancel(true, Canceled) }
69}
70
71// newCancelCtx returns an initialized cancelCtx.
72func newCancelCtx(parent Context) *cancelCtx {
73	return &cancelCtx{
74		Context: parent,
75		done:    make(chan struct{}),
76	}
77}
78
79// propagateCancel arranges for child to be canceled when parent is.
80func propagateCancel(parent Context, child canceler) {
81	if parent.Done() == nil {
82		return // parent is never canceled
83	}
84	if p, ok := parentCancelCtx(parent); ok {
85		p.mu.Lock()
86		if p.err != nil {
87			// parent has already been canceled
88			child.cancel(false, p.err)
89		} else {
90			if p.children == nil {
91				p.children = make(map[canceler]bool)
92			}
93			p.children[child] = true
94		}
95		p.mu.Unlock()
96	} else {
97		go func() {
98			select {
99			case <-parent.Done():
100				child.cancel(false, parent.Err())
101			case <-child.Done():
102			}
103		}()
104	}
105}
106
107// parentCancelCtx follows a chain of parent references until it finds a
108// *cancelCtx. This function understands how each of the concrete types in this
109// package represents its parent.
110func parentCancelCtx(parent Context) (*cancelCtx, bool) {
111	for {
112		switch c := parent.(type) {
113		case *cancelCtx:
114			return c, true
115		case *timerCtx:
116			return c.cancelCtx, true
117		case *valueCtx:
118			parent = c.Context
119		default:
120			return nil, false
121		}
122	}
123}
124
125// removeChild removes a context from its parent.
126func removeChild(parent Context, child canceler) {
127	p, ok := parentCancelCtx(parent)
128	if !ok {
129		return
130	}
131	p.mu.Lock()
132	if p.children != nil {
133		delete(p.children, child)
134	}
135	p.mu.Unlock()
136}
137
138// A canceler is a context type that can be canceled directly. The
139// implementations are *cancelCtx and *timerCtx.
140type canceler interface {
141	cancel(removeFromParent bool, err error)
142	Done() <-chan struct{}
143}
144
145// A cancelCtx can be canceled. When canceled, it also cancels any children
146// that implement canceler.
147type cancelCtx struct {
148	Context
149
150	done chan struct{} // closed by the first cancel call.
151
152	mu       sync.Mutex
153	children map[canceler]bool // set to nil by the first cancel call
154	err      error             // set to non-nil by the first cancel call
155}
156
157func (c *cancelCtx) Done() <-chan struct{} {
158	return c.done
159}
160
161func (c *cancelCtx) Err() error {
162	c.mu.Lock()
163	defer c.mu.Unlock()
164	return c.err
165}
166
167func (c *cancelCtx) String() string {
168	return fmt.Sprintf("%v.WithCancel", c.Context)
169}
170
171// cancel closes c.done, cancels each of c's children, and, if
172// removeFromParent is true, removes c from its parent's children.
173func (c *cancelCtx) cancel(removeFromParent bool, err error) {
174	if err == nil {
175		panic("context: internal error: missing cancel error")
176	}
177	c.mu.Lock()
178	if c.err != nil {
179		c.mu.Unlock()
180		return // already canceled
181	}
182	c.err = err
183	close(c.done)
184	for child := range c.children {
185		// NOTE: acquiring the child's lock while holding parent's lock.
186		child.cancel(false, err)
187	}
188	c.children = nil
189	c.mu.Unlock()
190
191	if removeFromParent {
192		removeChild(c.Context, c)
193	}
194}
195
196// WithDeadline returns a copy of the parent context with the deadline adjusted
197// to be no later than d. If the parent's deadline is already earlier than d,
198// WithDeadline(parent, d) is semantically equivalent to parent. The returned
199// context's Done channel is closed when the deadline expires, when the returned
200// cancel function is called, or when the parent context's Done channel is
201// closed, whichever happens first.
202//
203// Canceling this context releases resources associated with it, so code should
204// call cancel as soon as the operations running in this Context complete.
205func WithDeadline(parent Context, deadline time.Time) (Context, CancelFunc) {
206	if cur, ok := parent.Deadline(); ok && cur.Before(deadline) {
207		// The current deadline is already sooner than the new one.
208		return WithCancel(parent)
209	}
210	c := &timerCtx{
211		cancelCtx: newCancelCtx(parent),
212		deadline:  deadline,
213	}
214	propagateCancel(parent, c)
215	d := deadline.Sub(time.Now())
216	if d <= 0 {
217		c.cancel(true, DeadlineExceeded) // deadline has already passed
218		return c, func() { c.cancel(true, Canceled) }
219	}
220	c.mu.Lock()
221	defer c.mu.Unlock()
222	if c.err == nil {
223		c.timer = time.AfterFunc(d, func() {
224			c.cancel(true, DeadlineExceeded)
225		})
226	}
227	return c, func() { c.cancel(true, Canceled) }
228}
229
230// A timerCtx carries a timer and a deadline. It embeds a cancelCtx to
231// implement Done and Err. It implements cancel by stopping its timer then
232// delegating to cancelCtx.cancel.
233type timerCtx struct {
234	*cancelCtx
235	timer *time.Timer // Under cancelCtx.mu.
236
237	deadline time.Time
238}
239
240func (c *timerCtx) Deadline() (deadline time.Time, ok bool) {
241	return c.deadline, true
242}
243
244func (c *timerCtx) String() string {
245	return fmt.Sprintf("%v.WithDeadline(%s [%s])", c.cancelCtx.Context, c.deadline, c.deadline.Sub(time.Now()))
246}
247
248func (c *timerCtx) cancel(removeFromParent bool, err error) {
249	c.cancelCtx.cancel(false, err)
250	if removeFromParent {
251		// Remove this timerCtx from its parent cancelCtx's children.
252		removeChild(c.cancelCtx.Context, c)
253	}
254	c.mu.Lock()
255	if c.timer != nil {
256		c.timer.Stop()
257		c.timer = nil
258	}
259	c.mu.Unlock()
260}
261
262// WithTimeout returns WithDeadline(parent, time.Now().Add(timeout)).
263//
264// Canceling this context releases resources associated with it, so code should
265// call cancel as soon as the operations running in this Context complete:
266//
267// 	func slowOperationWithTimeout(ctx context.Context) (Result, error) {
268// 		ctx, cancel := context.WithTimeout(ctx, 100*time.Millisecond)
269// 		defer cancel()  // releases resources if slowOperation completes before timeout elapses
270// 		return slowOperation(ctx)
271// 	}
272func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc) {
273	return WithDeadline(parent, time.Now().Add(timeout))
274}
275
276// WithValue returns a copy of parent in which the value associated with key is
277// val.
278//
279// Use context Values only for request-scoped data that transits processes and
280// APIs, not for passing optional parameters to functions.
281func WithValue(parent Context, key interface{}, val interface{}) Context {
282	return &valueCtx{parent, key, val}
283}
284
285// A valueCtx carries a key-value pair. It implements Value for that key and
286// delegates all other calls to the embedded Context.
287type valueCtx struct {
288	Context
289	key, val interface{}
290}
291
292func (c *valueCtx) String() string {
293	return fmt.Sprintf("%v.WithValue(%#v, %#v)", c.Context, c.key, c.val)
294}
295
296func (c *valueCtx) Value(key interface{}) interface{} {
297	if c.key == key {
298		return c.val
299	}
300	return c.Context.Value(key)
301}
302