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// Package context defines the Context type, which carries deadlines,
6// cancellation signals, and other request-scoped values across API boundaries
7// and between processes.
8//
9// Incoming requests to a server should create a Context, and outgoing
10// calls to servers should accept a Context. The chain of function
11// calls between them must propagate the Context, optionally replacing
12// it with a derived Context created using WithCancel, WithDeadline,
13// WithTimeout, or WithValue. When a Context is canceled, all
14// Contexts derived from it are also canceled.
15//
16// The WithCancel, WithDeadline, and WithTimeout functions take a
17// Context (the parent) and return a derived Context (the child) and a
18// CancelFunc. Calling the CancelFunc cancels the child and its
19// children, removes the parent's reference to the child, and stops
20// any associated timers. Failing to call the CancelFunc leaks the
21// child and its children until the parent is canceled or the timer
22// fires. The go vet tool checks that CancelFuncs are used on all
23// control-flow paths.
24//
25// Programs that use Contexts should follow these rules to keep interfaces
26// consistent across packages and enable static analysis tools to check context
27// propagation:
28//
29// Do not store Contexts inside a struct type; instead, pass a Context
30// explicitly to each function that needs it. The Context should be the first
31// parameter, typically named ctx:
32//
33// func DoSomething(ctx context.Context, arg Arg) error {
34// // ... use ctx ...
35// }
36//
37// Do not pass a nil Context, even if a function permits it. Pass context.TODO
38// if you are unsure about which Context to use.
39//
40// Use context Values only for request-scoped data that transits processes and
41// APIs, not for passing optional parameters to functions.
42//
43// The same Context may be passed to functions running in different goroutines;
44// Contexts are safe for simultaneous use by multiple goroutines.
45//
46// See https://blog.golang.org/context for example code for a server that uses
47// Contexts.
48package context
49
50import (
51 "errors"
52 "internal/reflectlite"
53 "sync"
54 "sync/atomic"
55 "time"
56)
57
58// A Context carries a deadline, a cancellation signal, and other values across
59// API boundaries.
60//
61// Context's methods may be called by multiple goroutines simultaneously.
62type Context interface {
63 // Deadline returns the time when work done on behalf of this context
64 // should be canceled. Deadline returns ok==false when no deadline is
65 // set. Successive calls to Deadline return the same results.
66 Deadline() (deadline time.Time, ok bool)
67
68 // Done returns a channel that's closed when work done on behalf of this
69 // context should be canceled. Done may return nil if this context can
70 // never be canceled. Successive calls to Done return the same value.
71 // The close of the Done channel may happen asynchronously,
72 // after the cancel function returns.
73 //
74 // WithCancel arranges for Done to be closed when cancel is called;
75 // WithDeadline arranges for Done to be closed when the deadline
76 // expires; WithTimeout arranges for Done to be closed when the timeout
77 // elapses.
78 //
79 // Done is provided for use in select statements:
80 //
81 // // Stream generates values with DoSomething and sends them to out
82 // // until DoSomething returns an error or ctx.Done is closed.
83 // func Stream(ctx context.Context, out chan<- Value) error {
84 // for {
85 // v, err := DoSomething(ctx)
86 // if err != nil {
87 // return err
88 // }
89 // select {
90 // case <-ctx.Done():
91 // return ctx.Err()
92 // case out <- v:
93 // }
94 // }
95 // }
96 //
97 // See https://blog.golang.org/pipelines for more examples of how to use
98 // a Done channel for cancellation.
99 Done() <-chan struct{}
100
101 // If Done is not yet closed, Err returns nil.
102 // If Done is closed, Err returns a non-nil error explaining why:
103 // Canceled if the context was canceled
104 // or DeadlineExceeded if the context's deadline passed.
105 // After Err returns a non-nil error, successive calls to Err return the same error.
106 Err() error
107
108 // Value returns the value associated with this context for key, or nil
109 // if no value is associated with key. Successive calls to Value with
110 // the same key returns the same result.
111 //
112 // Use context values only for request-scoped data that transits
113 // processes and API boundaries, not for passing optional parameters to
114 // functions.
115 //
116 // A key identifies a specific value in a Context. Functions that wish
117 // to store values in Context typically allocate a key in a global
118 // variable then use that key as the argument to context.WithValue and
119 // Context.Value. A key can be any type that supports equality;
120 // packages should define keys as an unexported type to avoid
121 // collisions.
122 //
123 // Packages that define a Context key should provide type-safe accessors
124 // for the values stored using that key:
125 //
126 // // Package user defines a User type that's stored in Contexts.
127 // package user
128 //
129 // import "context"
130 //
131 // // User is the type of value stored in the Contexts.
132 // type User struct {...}
133 //
134 // // key is an unexported type for keys defined in this package.
135 // // This prevents collisions with keys defined in other packages.
136 // type key int
137 //
138 // // userKey is the key for user.User values in Contexts. It is
139 // // unexported; clients use user.NewContext and user.FromContext
140 // // instead of using this key directly.
141 // var userKey key
142 //
143 // // NewContext returns a new Context that carries value u.
144 // func NewContext(ctx context.Context, u *User) context.Context {
145 // return context.WithValue(ctx, userKey, u)
146 // }
147 //
148 // // FromContext returns the User value stored in ctx, if any.
149 // func FromContext(ctx context.Context) (*User, bool) {
150 // u, ok := ctx.Value(userKey).(*User)
151 // return u, ok
152 // }
153 Value(key any) any
154}
155
156// Canceled is the error returned by Context.Err when the context is canceled.
157var Canceled = errors.New("context canceled")
158
159// DeadlineExceeded is the error returned by Context.Err when the context's
160// deadline passes.
161var DeadlineExceeded error = deadlineExceededError{}
162
163type deadlineExceededError struct{}
164
165func (deadlineExceededError) Error() string { return "context deadline exceeded" }
166func (deadlineExceededError) Timeout() bool { return true }
167func (deadlineExceededError) Temporary() bool { return true }
168
169// An emptyCtx is never canceled, has no values, and has no deadline. It is not
170// struct{}, since vars of this type must have distinct addresses.
171type emptyCtx int
172
173func (*emptyCtx) Deadline() (deadline time.Time, ok bool) {
174 return
175}
176
177func (*emptyCtx) Done() <-chan struct{} {
178 return nil
179}
180
181func (*emptyCtx) Err() error {
182 return nil
183}
184
185func (*emptyCtx) Value(key any) any {
186 return nil
187}
188
189func (e *emptyCtx) String() string {
190 switch e {
191 case background:
192 return "context.Background"
193 case todo:
194 return "context.TODO"
195 }
196 return "unknown empty Context"
197}
198
199var (
200 background = new(emptyCtx)
201 todo = new(emptyCtx)
202)
203
204// Background returns a non-nil, empty Context. It is never canceled, has no
205// values, and has no deadline. It is typically used by the main function,
206// initialization, and tests, and as the top-level Context for incoming
207// requests.
208func Background() Context {
209 return background
210}
211
212// TODO returns a non-nil, empty Context. Code should use context.TODO when
213// it's unclear which Context to use or it is not yet available (because the
214// surrounding function has not yet been extended to accept a Context
215// parameter).
216func TODO() Context {
217 return todo
218}
219
220// A CancelFunc tells an operation to abandon its work.
221// A CancelFunc does not wait for the work to stop.
222// A CancelFunc may be called by multiple goroutines simultaneously.
223// After the first call, subsequent calls to a CancelFunc do nothing.
224type CancelFunc func()
225
226// WithCancel returns a copy of parent with a new Done channel. The returned
227// context's Done channel is closed when the returned cancel function is called
228// or when the parent context's Done channel is closed, whichever happens first.
229//
230// Canceling this context releases resources associated with it, so code should
231// call cancel as soon as the operations running in this Context complete.
232func WithCancel(parent Context) (ctx Context, cancel CancelFunc) {
233 if parent == nil {
234 panic("cannot create context from nil parent")
235 }
236 c := newCancelCtx(parent)
237 propagateCancel(parent, &c)
238 return &c, func() { c.cancel(true, Canceled) }
239}
240
241// newCancelCtx returns an initialized cancelCtx.
242func newCancelCtx(parent Context) cancelCtx {
243 return cancelCtx{Context: parent}
244}
245
246// goroutines counts the number of goroutines ever created; for testing.
247var goroutines int32
248
249// propagateCancel arranges for child to be canceled when parent is.
250func propagateCancel(parent Context, child canceler) {
251 done := parent.Done()
252 if done == nil {
253 return // parent is never canceled
254 }
255
256 select {
257 case <-done:
258 // parent is already canceled
259 child.cancel(false, parent.Err())
260 return
261 default:
262 }
263
264 if p, ok := parentCancelCtx(parent); ok {
265 p.mu.Lock()
266 if p.err != nil {
267 // parent has already been canceled
268 child.cancel(false, p.err)
269 } else {
270 if p.children == nil {
271 p.children = make(map[canceler]struct{})
272 }
273 p.children[child] = struct{}{}
274 }
275 p.mu.Unlock()
276 } else {
277 atomic.AddInt32(&goroutines, +1)
278 go func() {
279 select {
280 case <-parent.Done():
281 child.cancel(false, parent.Err())
282 case <-child.Done():
283 }
284 }()
285 }
286}
287
288// &cancelCtxKey is the key that a cancelCtx returns itself for.
289var cancelCtxKey int
290
291// parentCancelCtx returns the underlying *cancelCtx for parent.
292// It does this by looking up parent.Value(&cancelCtxKey) to find
293// the innermost enclosing *cancelCtx and then checking whether
294// parent.Done() matches that *cancelCtx. (If not, the *cancelCtx
295// has been wrapped in a custom implementation providing a
296// different done channel, in which case we should not bypass it.)
297func parentCancelCtx(parent Context) (*cancelCtx, bool) {
298 done := parent.Done()
299 if done == closedchan || done == nil {
300 return nil, false
301 }
302 p, ok := parent.Value(&cancelCtxKey).(*cancelCtx)
303 if !ok {
304 return nil, false
305 }
306 pdone, _ := p.done.Load().(chan struct{})
307 if pdone != done {
308 return nil, false
309 }
310 return p, true
311}
312
313// removeChild removes a context from its parent.
314func removeChild(parent Context, child canceler) {
315 p, ok := parentCancelCtx(parent)
316 if !ok {
317 return
318 }
319 p.mu.Lock()
320 if p.children != nil {
321 delete(p.children, child)
322 }
323 p.mu.Unlock()
324}
325
326// A canceler is a context type that can be canceled directly. The
327// implementations are *cancelCtx and *timerCtx.
328type canceler interface {
329 cancel(removeFromParent bool, err error)
330 Done() <-chan struct{}
331}
332
333// closedchan is a reusable closed channel.
334var closedchan = make(chan struct{})
335
336func init() {
337 close(closedchan)
338}
339
340// A cancelCtx can be canceled. When canceled, it also cancels any children
341// that implement canceler.
342type cancelCtx struct {
343 Context
344
345 mu sync.Mutex // protects following fields
346 done atomic.Value // of chan struct{}, created lazily, closed by first cancel call
347 children map[canceler]struct{} // set to nil by the first cancel call
348 err error // set to non-nil by the first cancel call
349}
350
351func (c *cancelCtx) Value(key any) any {
352 if key == &cancelCtxKey {
353 return c
354 }
355 return value(c.Context, key)
356}
357
358func (c *cancelCtx) Done() <-chan struct{} {
359 d := c.done.Load()
360 if d != nil {
361 return d.(chan struct{})
362 }
363 c.mu.Lock()
364 defer c.mu.Unlock()
365 d = c.done.Load()
366 if d == nil {
367 d = make(chan struct{})
368 c.done.Store(d)
369 }
370 return d.(chan struct{})
371}
372
373func (c *cancelCtx) Err() error {
374 c.mu.Lock()
375 err := c.err
376 c.mu.Unlock()
377 return err
378}
379
380type stringer interface {
381 String() string
382}
383
384func contextName(c Context) string {
385 if s, ok := c.(stringer); ok {
386 return s.String()
387 }
388 return reflectlite.TypeOf(c).String()
389}
390
391func (c *cancelCtx) String() string {
392 return contextName(c.Context) + ".WithCancel"
393}
394
395// cancel closes c.done, cancels each of c's children, and, if
396// removeFromParent is true, removes c from its parent's children.
397func (c *cancelCtx) cancel(removeFromParent bool, err error) {
398 if err == nil {
399 panic("context: internal error: missing cancel error")
400 }
401 c.mu.Lock()
402 if c.err != nil {
403 c.mu.Unlock()
404 return // already canceled
405 }
406 c.err = err
407 d, _ := c.done.Load().(chan struct{})
408 if d == nil {
409 c.done.Store(closedchan)
410 } else {
411 close(d)
412 }
413 for child := range c.children {
414 // NOTE: acquiring the child's lock while holding parent's lock.
415 child.cancel(false, err)
416 }
417 c.children = nil
418 c.mu.Unlock()
419
420 if removeFromParent {
421 removeChild(c.Context, c)
422 }
423}
424
425// WithDeadline returns a copy of the parent context with the deadline adjusted
426// to be no later than d. If the parent's deadline is already earlier than d,
427// WithDeadline(parent, d) is semantically equivalent to parent. The returned
428// context's Done channel is closed when the deadline expires, when the returned
429// cancel function is called, or when the parent context's Done channel is
430// closed, whichever happens first.
431//
432// Canceling this context releases resources associated with it, so code should
433// call cancel as soon as the operations running in this Context complete.
434func WithDeadline(parent Context, d time.Time) (Context, CancelFunc) {
435 if parent == nil {
436 panic("cannot create context from nil parent")
437 }
438 if cur, ok := parent.Deadline(); ok && cur.Before(d) {
439 // The current deadline is already sooner than the new one.
440 return WithCancel(parent)
441 }
442 c := &timerCtx{
443 cancelCtx: newCancelCtx(parent),
444 deadline: d,
445 }
446 propagateCancel(parent, c)
447 dur := time.Until(d)
448 if dur <= 0 {
449 c.cancel(true, DeadlineExceeded) // deadline has already passed
450 return c, func() { c.cancel(false, Canceled) }
451 }
452 c.mu.Lock()
453 defer c.mu.Unlock()
454 if c.err == nil {
455 c.timer = time.AfterFunc(dur, func() {
456 c.cancel(true, DeadlineExceeded)
457 })
458 }
459 return c, func() { c.cancel(true, Canceled) }
460}
461
462// A timerCtx carries a timer and a deadline. It embeds a cancelCtx to
463// implement Done and Err. It implements cancel by stopping its timer then
464// delegating to cancelCtx.cancel.
465type timerCtx struct {
466 cancelCtx
467 timer *time.Timer // Under cancelCtx.mu.
468
469 deadline time.Time
470}
471
472func (c *timerCtx) Deadline() (deadline time.Time, ok bool) {
473 return c.deadline, true
474}
475
476func (c *timerCtx) String() string {
477 return contextName(c.cancelCtx.Context) + ".WithDeadline(" +
478 c.deadline.String() + " [" +
479 time.Until(c.deadline).String() + "])"
480}
481
482func (c *timerCtx) cancel(removeFromParent bool, err error) {
483 c.cancelCtx.cancel(false, err)
484 if removeFromParent {
485 // Remove this timerCtx from its parent cancelCtx's children.
486 removeChild(c.cancelCtx.Context, c)
487 }
488 c.mu.Lock()
489 if c.timer != nil {
490 c.timer.Stop()
491 c.timer = nil
492 }
493 c.mu.Unlock()
494}
495
496// WithTimeout returns WithDeadline(parent, time.Now().Add(timeout)).
497//
498// Canceling this context releases resources associated with it, so code should
499// call cancel as soon as the operations running in this Context complete:
500//
501// func slowOperationWithTimeout(ctx context.Context) (Result, error) {
502// ctx, cancel := context.WithTimeout(ctx, 100*time.Millisecond)
503// defer cancel() // releases resources if slowOperation completes before timeout elapses
504// return slowOperation(ctx)
505// }
506func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc) {
507 return WithDeadline(parent, time.Now().Add(timeout))
508}
509
510// WithValue returns a copy of parent in which the value associated with key is
511// val.
512//
513// Use context Values only for request-scoped data that transits processes and
514// APIs, not for passing optional parameters to functions.
515//
516// The provided key must be comparable and should not be of type
517// string or any other built-in type to avoid collisions between
518// packages using context. Users of WithValue should define their own
519// types for keys. To avoid allocating when assigning to an
520// interface{}, context keys often have concrete type
521// struct{}. Alternatively, exported context key variables' static
522// type should be a pointer or interface.
523func WithValue(parent Context, key, val any) Context {
524 if parent == nil {
525 panic("cannot create context from nil parent")
526 }
527 if key == nil {
528 panic("nil key")
529 }
530 if !reflectlite.TypeOf(key).Comparable() {
531 panic("key is not comparable")
532 }
533 return &valueCtx{parent, key, val}
534}
535
536// A valueCtx carries a key-value pair. It implements Value for that key and
537// delegates all other calls to the embedded Context.
538type valueCtx struct {
539 Context
540 key, val any
541}
542
543// stringify tries a bit to stringify v, without using fmt, since we don't
544// want context depending on the unicode tables. This is only used by
545// *valueCtx.String().
546func stringify(v any) string {
547 switch s := v.(type) {
548 case stringer:
549 return s.String()
550 case string:
551 return s
552 }
553 return "<not Stringer>"
554}
555
556func (c *valueCtx) String() string {
557 return contextName(c.Context) + ".WithValue(type " +
558 reflectlite.TypeOf(c.key).String() +
559 ", val " + stringify(c.val) + ")"
560}
561
562func (c *valueCtx) Value(key any) any {
563 if c.key == key {
564 return c.val
565 }
566 return value(c.Context, key)
567}
568
569func value(c Context, key any) any {
570 for {
571 switch ctx := c.(type) {
572 case *valueCtx:
573 if key == ctx.key {
574 return ctx.val
575 }
576 c = ctx.Context
577 case *cancelCtx:
578 if key == &cancelCtxKey {
579 return c
580 }
581 c = ctx.Context
582 case *timerCtx:
583 if key == &cancelCtxKey {
584 return &ctx.cancelCtx
585 }
586 c = ctx.Context
587 case *emptyCtx:
588 return nil
589 default:
590 return c.Value(key)
591 }
592 }
593}
