Source file src/context/context.go
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 // The [WithCancelCause] function returns a [CancelCauseFunc], which 26 // takes an error and records it as the cancellation cause. Calling 27 // [Cause] on the canceled context or any of its children retrieves 28 // the cause. If no cause is specified, Cause(ctx) returns the same 29 // value as ctx.Err(). 30 // 31 // Programs that use Contexts should follow these rules to keep interfaces 32 // consistent across packages and enable static analysis tools to check context 33 // propagation: 34 // 35 // Do not store Contexts inside a struct type; instead, pass a Context 36 // explicitly to each function that needs it. The Context should be the first 37 // parameter, typically named ctx: 38 // 39 // func DoSomething(ctx context.Context, arg Arg) error { 40 // // ... use ctx ... 41 // } 42 // 43 // Do not pass a nil [Context], even if a function permits it. Pass [context.TODO] 44 // if you are unsure about which Context to use. 45 // 46 // Use context Values only for request-scoped data that transits processes and 47 // APIs, not for passing optional parameters to functions. 48 // 49 // The same Context may be passed to functions running in different goroutines; 50 // Contexts are safe for simultaneous use by multiple goroutines. 51 // 52 // See https://blog.golang.org/context for example code for a server that uses 53 // Contexts. 54 package context 55 56 import ( 57 "errors" 58 "internal/reflectlite" 59 "sync" 60 "sync/atomic" 61 "time" 62 ) 63 64 // A Context carries a deadline, a cancellation signal, and other values across 65 // API boundaries. 66 // 67 // Context's methods may be called by multiple goroutines simultaneously. 68 type Context interface { 69 // Deadline returns the time when work done on behalf of this context 70 // should be canceled. Deadline returns ok==false when no deadline is 71 // set. Successive calls to Deadline return the same results. 72 Deadline() (deadline time.Time, ok bool) 73 74 // Done returns a channel that's closed when work done on behalf of this 75 // context should be canceled. Done may return nil if this context can 76 // never be canceled. Successive calls to Done return the same value. 77 // The close of the Done channel may happen asynchronously, 78 // after the cancel function returns. 79 // 80 // WithCancel arranges for Done to be closed when cancel is called; 81 // WithDeadline arranges for Done to be closed when the deadline 82 // expires; WithTimeout arranges for Done to be closed when the timeout 83 // elapses. 84 // 85 // Done is provided for use in select statements: 86 // 87 // // Stream generates values with DoSomething and sends them to out 88 // // until DoSomething returns an error or ctx.Done is closed. 89 // func Stream(ctx context.Context, out chan<- Value) error { 90 // for { 91 // v, err := DoSomething(ctx) 92 // if err != nil { 93 // return err 94 // } 95 // select { 96 // case <-ctx.Done(): 97 // return ctx.Err() 98 // case out <- v: 99 // } 100 // } 101 // } 102 // 103 // See https://blog.golang.org/pipelines for more examples of how to use 104 // a Done channel for cancellation. 105 Done() <-chan struct{} 106 107 // If Done is not yet closed, Err returns nil. 108 // If Done is closed, Err returns a non-nil error explaining why: 109 // Canceled if the context was canceled 110 // or DeadlineExceeded if the context's deadline passed. 111 // After Err returns a non-nil error, successive calls to Err return the same error. 112 Err() error 113 114 // Value returns the value associated with this context for key, or nil 115 // if no value is associated with key. Successive calls to Value with 116 // the same key returns the same result. 117 // 118 // Use context values only for request-scoped data that transits 119 // processes and API boundaries, not for passing optional parameters to 120 // functions. 121 // 122 // A key identifies a specific value in a Context. Functions that wish 123 // to store values in Context typically allocate a key in a global 124 // variable then use that key as the argument to context.WithValue and 125 // Context.Value. A key can be any type that supports equality; 126 // packages should define keys as an unexported type to avoid 127 // collisions. 128 // 129 // Packages that define a Context key should provide type-safe accessors 130 // for the values stored using that key: 131 // 132 // // Package user defines a User type that's stored in Contexts. 133 // package user 134 // 135 // import "context" 136 // 137 // // User is the type of value stored in the Contexts. 138 // type User struct {...} 139 // 140 // // key is an unexported type for keys defined in this package. 141 // // This prevents collisions with keys defined in other packages. 142 // type key int 143 // 144 // // userKey is the key for user.User values in Contexts. It is 145 // // unexported; clients use user.NewContext and user.FromContext 146 // // instead of using this key directly. 147 // var userKey key 148 // 149 // // NewContext returns a new Context that carries value u. 150 // func NewContext(ctx context.Context, u *User) context.Context { 151 // return context.WithValue(ctx, userKey, u) 152 // } 153 // 154 // // FromContext returns the User value stored in ctx, if any. 155 // func FromContext(ctx context.Context) (*User, bool) { 156 // u, ok := ctx.Value(userKey).(*User) 157 // return u, ok 158 // } 159 Value(key any) any 160 } 161 162 // Canceled is the error returned by [Context.Err] when the context is canceled. 163 var Canceled = errors.New("context canceled") 164 165 // DeadlineExceeded is the error returned by [Context.Err] when the context's 166 // deadline passes. 167 var DeadlineExceeded error = deadlineExceededError{} 168 169 type deadlineExceededError struct{} 170 171 func (deadlineExceededError) Error() string { return "context deadline exceeded" } 172 func (deadlineExceededError) Timeout() bool { return true } 173 func (deadlineExceededError) Temporary() bool { return true } 174 175 // An emptyCtx is never canceled, has no values, and has no deadline. 176 // It is the common base of backgroundCtx and todoCtx. 177 type emptyCtx struct{} 178 179 func (emptyCtx) Deadline() (deadline time.Time, ok bool) { 180 return 181 } 182 183 func (emptyCtx) Done() <-chan struct{} { 184 return nil 185 } 186 187 func (emptyCtx) Err() error { 188 return nil 189 } 190 191 func (emptyCtx) Value(key any) any { 192 return nil 193 } 194 195 type backgroundCtx struct{ emptyCtx } 196 197 func (backgroundCtx) String() string { 198 return "context.Background" 199 } 200 201 type todoCtx struct{ emptyCtx } 202 203 func (todoCtx) String() string { 204 return "context.TODO" 205 } 206 207 // Background returns a non-nil, empty [Context]. It is never canceled, has no 208 // values, and has no deadline. It is typically used by the main function, 209 // initialization, and tests, and as the top-level Context for incoming 210 // requests. 211 func Background() Context { 212 return backgroundCtx{} 213 } 214 215 // TODO returns a non-nil, empty [Context]. Code should use context.TODO when 216 // it's unclear which Context to use or it is not yet available (because the 217 // surrounding function has not yet been extended to accept a Context 218 // parameter). 219 func TODO() Context { 220 return todoCtx{} 221 } 222 223 // A CancelFunc tells an operation to abandon its work. 224 // A CancelFunc does not wait for the work to stop. 225 // A CancelFunc may be called by multiple goroutines simultaneously. 226 // After the first call, subsequent calls to a CancelFunc do nothing. 227 type CancelFunc func() 228 229 // WithCancel returns a copy of parent with a new Done channel. The returned 230 // context's Done channel is closed when the returned cancel function is called 231 // or when the parent context's Done channel is closed, whichever happens first. 232 // 233 // Canceling this context releases resources associated with it, so code should 234 // call cancel as soon as the operations running in this [Context] complete. 235 func WithCancel(parent Context) (ctx Context, cancel CancelFunc) { 236 c := withCancel(parent) 237 return c, func() { c.cancel(true, Canceled, nil) } 238 } 239 240 // A CancelCauseFunc behaves like a [CancelFunc] but additionally sets the cancellation cause. 241 // This cause can be retrieved by calling [Cause] on the canceled Context or on 242 // any of its derived Contexts. 243 // 244 // If the context has already been canceled, CancelCauseFunc does not set the cause. 245 // For example, if childContext is derived from parentContext: 246 // - if parentContext is canceled with cause1 before childContext is canceled with cause2, 247 // then Cause(parentContext) == Cause(childContext) == cause1 248 // - if childContext is canceled with cause2 before parentContext is canceled with cause1, 249 // then Cause(parentContext) == cause1 and Cause(childContext) == cause2 250 type CancelCauseFunc func(cause error) 251 252 // WithCancelCause behaves like [WithCancel] but returns a [CancelCauseFunc] instead of a [CancelFunc]. 253 // Calling cancel with a non-nil error (the "cause") records that error in ctx; 254 // it can then be retrieved using Cause(ctx). 255 // Calling cancel with nil sets the cause to Canceled. 256 // 257 // Example use: 258 // 259 // ctx, cancel := context.WithCancelCause(parent) 260 // cancel(myError) 261 // ctx.Err() // returns context.Canceled 262 // context.Cause(ctx) // returns myError 263 func WithCancelCause(parent Context) (ctx Context, cancel CancelCauseFunc) { 264 c := withCancel(parent) 265 return c, func(cause error) { c.cancel(true, Canceled, cause) } 266 } 267 268 func withCancel(parent Context) *cancelCtx { 269 if parent == nil { 270 panic("cannot create context from nil parent") 271 } 272 c := &cancelCtx{} 273 c.propagateCancel(parent, c) 274 return c 275 } 276 277 // Cause returns a non-nil error explaining why c was canceled. 278 // The first cancellation of c or one of its parents sets the cause. 279 // If that cancellation happened via a call to CancelCauseFunc(err), 280 // then [Cause] returns err. 281 // Otherwise Cause(c) returns the same value as c.Err(). 282 // Cause returns nil if c has not been canceled yet. 283 func Cause(c Context) error { 284 if cc, ok := c.Value(&cancelCtxKey).(*cancelCtx); ok { 285 cc.mu.Lock() 286 defer cc.mu.Unlock() 287 return cc.cause 288 } 289 // There is no cancelCtxKey value, so we know that c is 290 // not a descendant of some Context created by WithCancelCause. 291 // Therefore, there is no specific cause to return. 292 // If this is not one of the standard Context types, 293 // it might still have an error even though it won't have a cause. 294 return c.Err() 295 } 296 297 // AfterFunc arranges to call f in its own goroutine after ctx is done 298 // (canceled or timed out). 299 // If ctx is already done, AfterFunc calls f immediately in its own goroutine. 300 // 301 // Multiple calls to AfterFunc on a context operate independently; 302 // one does not replace another. 303 // 304 // Calling the returned stop function stops the association of ctx with f. 305 // It returns true if the call stopped f from being run. 306 // If stop returns false, 307 // either the context is done and f has been started in its own goroutine; 308 // or f was already stopped. 309 // The stop function does not wait for f to complete before returning. 310 // If the caller needs to know whether f is completed, 311 // it must coordinate with f explicitly. 312 // 313 // If ctx has a "AfterFunc(func()) func() bool" method, 314 // AfterFunc will use it to schedule the call. 315 func AfterFunc(ctx Context, f func()) (stop func() bool) { 316 a := &afterFuncCtx{ 317 f: f, 318 } 319 a.cancelCtx.propagateCancel(ctx, a) 320 return func() bool { 321 stopped := false 322 a.once.Do(func() { 323 stopped = true 324 }) 325 if stopped { 326 a.cancel(true, Canceled, nil) 327 } 328 return stopped 329 } 330 } 331 332 type afterFuncer interface { 333 AfterFunc(func()) func() bool 334 } 335 336 type afterFuncCtx struct { 337 cancelCtx 338 once sync.Once // either starts running f or stops f from running 339 f func() 340 } 341 342 func (a *afterFuncCtx) cancel(removeFromParent bool, err, cause error) { 343 a.cancelCtx.cancel(false, err, cause) 344 if removeFromParent { 345 removeChild(a.Context, a) 346 } 347 a.once.Do(func() { 348 go a.f() 349 }) 350 } 351 352 // A stopCtx is used as the parent context of a cancelCtx when 353 // an AfterFunc has been registered with the parent. 354 // It holds the stop function used to unregister the AfterFunc. 355 type stopCtx struct { 356 Context 357 stop func() bool 358 } 359 360 // goroutines counts the number of goroutines ever created; for testing. 361 var goroutines atomic.Int32 362 363 // &cancelCtxKey is the key that a cancelCtx returns itself for. 364 var cancelCtxKey int 365 366 // parentCancelCtx returns the underlying *cancelCtx for parent. 367 // It does this by looking up parent.Value(&cancelCtxKey) to find 368 // the innermost enclosing *cancelCtx and then checking whether 369 // parent.Done() matches that *cancelCtx. (If not, the *cancelCtx 370 // has been wrapped in a custom implementation providing a 371 // different done channel, in which case we should not bypass it.) 372 func parentCancelCtx(parent Context) (*cancelCtx, bool) { 373 done := parent.Done() 374 if done == closedchan || done == nil { 375 return nil, false 376 } 377 p, ok := parent.Value(&cancelCtxKey).(*cancelCtx) 378 if !ok { 379 return nil, false 380 } 381 pdone, _ := p.done.Load().(chan struct{}) 382 if pdone != done { 383 return nil, false 384 } 385 return p, true 386 } 387 388 // removeChild removes a context from its parent. 389 func removeChild(parent Context, child canceler) { 390 if s, ok := parent.(stopCtx); ok { 391 s.stop() 392 return 393 } 394 p, ok := parentCancelCtx(parent) 395 if !ok { 396 return 397 } 398 p.mu.Lock() 399 if p.children != nil { 400 delete(p.children, child) 401 } 402 p.mu.Unlock() 403 } 404 405 // A canceler is a context type that can be canceled directly. The 406 // implementations are *cancelCtx and *timerCtx. 407 type canceler interface { 408 cancel(removeFromParent bool, err, cause error) 409 Done() <-chan struct{} 410 } 411 412 // closedchan is a reusable closed channel. 413 var closedchan = make(chan struct{}) 414 415 func init() { 416 close(closedchan) 417 } 418 419 // A cancelCtx can be canceled. When canceled, it also cancels any children 420 // that implement canceler. 421 type cancelCtx struct { 422 Context 423 424 mu sync.Mutex // protects following fields 425 done atomic.Value // of chan struct{}, created lazily, closed by first cancel call 426 children map[canceler]struct{} // set to nil by the first cancel call 427 err error // set to non-nil by the first cancel call 428 cause error // set to non-nil by the first cancel call 429 } 430 431 func (c *cancelCtx) Value(key any) any { 432 if key == &cancelCtxKey { 433 return c 434 } 435 return value(c.Context, key) 436 } 437 438 func (c *cancelCtx) Done() <-chan struct{} { 439 d := c.done.Load() 440 if d != nil { 441 return d.(chan struct{}) 442 } 443 c.mu.Lock() 444 defer c.mu.Unlock() 445 d = c.done.Load() 446 if d == nil { 447 d = make(chan struct{}) 448 c.done.Store(d) 449 } 450 return d.(chan struct{}) 451 } 452 453 func (c *cancelCtx) Err() error { 454 c.mu.Lock() 455 err := c.err 456 c.mu.Unlock() 457 return err 458 } 459 460 // propagateCancel arranges for child to be canceled when parent is. 461 // It sets the parent context of cancelCtx. 462 func (c *cancelCtx) propagateCancel(parent Context, child canceler) { 463 c.Context = parent 464 465 done := parent.Done() 466 if done == nil { 467 return // parent is never canceled 468 } 469 470 select { 471 case <-done: 472 // parent is already canceled 473 child.cancel(false, parent.Err(), Cause(parent)) 474 return 475 default: 476 } 477 478 if p, ok := parentCancelCtx(parent); ok { 479 // parent is a *cancelCtx, or derives from one. 480 p.mu.Lock() 481 if p.err != nil { 482 // parent has already been canceled 483 child.cancel(false, p.err, p.cause) 484 } else { 485 if p.children == nil { 486 p.children = make(map[canceler]struct{}) 487 } 488 p.children[child] = struct{}{} 489 } 490 p.mu.Unlock() 491 return 492 } 493 494 if a, ok := parent.(afterFuncer); ok { 495 // parent implements an AfterFunc method. 496 c.mu.Lock() 497 stop := a.AfterFunc(func() { 498 child.cancel(false, parent.Err(), Cause(parent)) 499 }) 500 c.Context = stopCtx{ 501 Context: parent, 502 stop: stop, 503 } 504 c.mu.Unlock() 505 return 506 } 507 508 goroutines.Add(1) 509 go func() { 510 select { 511 case <-parent.Done(): 512 child.cancel(false, parent.Err(), Cause(parent)) 513 case <-child.Done(): 514 } 515 }() 516 } 517 518 type stringer interface { 519 String() string 520 } 521 522 func contextName(c Context) string { 523 if s, ok := c.(stringer); ok { 524 return s.String() 525 } 526 return reflectlite.TypeOf(c).String() 527 } 528 529 func (c *cancelCtx) String() string { 530 return contextName(c.Context) + ".WithCancel" 531 } 532 533 // cancel closes c.done, cancels each of c's children, and, if 534 // removeFromParent is true, removes c from its parent's children. 535 // cancel sets c.cause to cause if this is the first time c is canceled. 536 func (c *cancelCtx) cancel(removeFromParent bool, err, cause error) { 537 if err == nil { 538 panic("context: internal error: missing cancel error") 539 } 540 if cause == nil { 541 cause = err 542 } 543 c.mu.Lock() 544 if c.err != nil { 545 c.mu.Unlock() 546 return // already canceled 547 } 548 c.err = err 549 c.cause = cause 550 d, _ := c.done.Load().(chan struct{}) 551 if d == nil { 552 c.done.Store(closedchan) 553 } else { 554 close(d) 555 } 556 for child := range c.children { 557 // NOTE: acquiring the child's lock while holding parent's lock. 558 child.cancel(false, err, cause) 559 } 560 c.children = nil 561 c.mu.Unlock() 562 563 if removeFromParent { 564 removeChild(c.Context, c) 565 } 566 } 567 568 // WithoutCancel returns a copy of parent that is not canceled when parent is canceled. 569 // The returned context returns no Deadline or Err, and its Done channel is nil. 570 // Calling [Cause] on the returned context returns nil. 571 func WithoutCancel(parent Context) Context { 572 if parent == nil { 573 panic("cannot create context from nil parent") 574 } 575 return withoutCancelCtx{parent} 576 } 577 578 type withoutCancelCtx struct { 579 c Context 580 } 581 582 func (withoutCancelCtx) Deadline() (deadline time.Time, ok bool) { 583 return 584 } 585 586 func (withoutCancelCtx) Done() <-chan struct{} { 587 return nil 588 } 589 590 func (withoutCancelCtx) Err() error { 591 return nil 592 } 593 594 func (c withoutCancelCtx) Value(key any) any { 595 return value(c, key) 596 } 597 598 func (c withoutCancelCtx) String() string { 599 return contextName(c.c) + ".WithoutCancel" 600 } 601 602 // WithDeadline returns a copy of the parent context with the deadline adjusted 603 // to be no later than d. If the parent's deadline is already earlier than d, 604 // WithDeadline(parent, d) is semantically equivalent to parent. The returned 605 // [Context.Done] channel is closed when the deadline expires, when the returned 606 // cancel function is called, or when the parent context's Done channel is 607 // closed, whichever happens first. 608 // 609 // Canceling this context releases resources associated with it, so code should 610 // call cancel as soon as the operations running in this [Context] complete. 611 func WithDeadline(parent Context, d time.Time) (Context, CancelFunc) { 612 return WithDeadlineCause(parent, d, nil) 613 } 614 615 // WithDeadlineCause behaves like [WithDeadline] but also sets the cause of the 616 // returned Context when the deadline is exceeded. The returned [CancelFunc] does 617 // not set the cause. 618 func WithDeadlineCause(parent Context, d time.Time, cause error) (Context, CancelFunc) { 619 if parent == nil { 620 panic("cannot create context from nil parent") 621 } 622 if cur, ok := parent.Deadline(); ok && cur.Before(d) { 623 // The current deadline is already sooner than the new one. 624 return WithCancel(parent) 625 } 626 c := &timerCtx{ 627 deadline: d, 628 } 629 c.cancelCtx.propagateCancel(parent, c) 630 dur := time.Until(d) 631 if dur <= 0 { 632 c.cancel(true, DeadlineExceeded, cause) // deadline has already passed 633 return c, func() { c.cancel(false, Canceled, nil) } 634 } 635 c.mu.Lock() 636 defer c.mu.Unlock() 637 if c.err == nil { 638 c.timer = time.AfterFunc(dur, func() { 639 c.cancel(true, DeadlineExceeded, cause) 640 }) 641 } 642 return c, func() { c.cancel(true, Canceled, nil) } 643 } 644 645 // A timerCtx carries a timer and a deadline. It embeds a cancelCtx to 646 // implement Done and Err. It implements cancel by stopping its timer then 647 // delegating to cancelCtx.cancel. 648 type timerCtx struct { 649 cancelCtx 650 timer *time.Timer // Under cancelCtx.mu. 651 652 deadline time.Time 653 } 654 655 func (c *timerCtx) Deadline() (deadline time.Time, ok bool) { 656 return c.deadline, true 657 } 658 659 func (c *timerCtx) String() string { 660 return contextName(c.cancelCtx.Context) + ".WithDeadline(" + 661 c.deadline.String() + " [" + 662 time.Until(c.deadline).String() + "])" 663 } 664 665 func (c *timerCtx) cancel(removeFromParent bool, err, cause error) { 666 c.cancelCtx.cancel(false, err, cause) 667 if removeFromParent { 668 // Remove this timerCtx from its parent cancelCtx's children. 669 removeChild(c.cancelCtx.Context, c) 670 } 671 c.mu.Lock() 672 if c.timer != nil { 673 c.timer.Stop() 674 c.timer = nil 675 } 676 c.mu.Unlock() 677 } 678 679 // WithTimeout returns WithDeadline(parent, time.Now().Add(timeout)). 680 // 681 // Canceling this context releases resources associated with it, so code should 682 // call cancel as soon as the operations running in this [Context] complete: 683 // 684 // func slowOperationWithTimeout(ctx context.Context) (Result, error) { 685 // ctx, cancel := context.WithTimeout(ctx, 100*time.Millisecond) 686 // defer cancel() // releases resources if slowOperation completes before timeout elapses 687 // return slowOperation(ctx) 688 // } 689 func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc) { 690 return WithDeadline(parent, time.Now().Add(timeout)) 691 } 692 693 // WithTimeoutCause behaves like [WithTimeout] but also sets the cause of the 694 // returned Context when the timeout expires. The returned [CancelFunc] does 695 // not set the cause. 696 func WithTimeoutCause(parent Context, timeout time.Duration, cause error) (Context, CancelFunc) { 697 return WithDeadlineCause(parent, time.Now().Add(timeout), cause) 698 } 699 700 // WithValue returns a copy of parent in which the value associated with key is 701 // val. 702 // 703 // Use context Values only for request-scoped data that transits processes and 704 // APIs, not for passing optional parameters to functions. 705 // 706 // The provided key must be comparable and should not be of type 707 // string or any other built-in type to avoid collisions between 708 // packages using context. Users of WithValue should define their own 709 // types for keys. To avoid allocating when assigning to an 710 // interface{}, context keys often have concrete type 711 // struct{}. Alternatively, exported context key variables' static 712 // type should be a pointer or interface. 713 func WithValue(parent Context, key, val any) Context { 714 if parent == nil { 715 panic("cannot create context from nil parent") 716 } 717 if key == nil { 718 panic("nil key") 719 } 720 if !reflectlite.TypeOf(key).Comparable() { 721 panic("key is not comparable") 722 } 723 return &valueCtx{parent, key, val} 724 } 725 726 // A valueCtx carries a key-value pair. It implements Value for that key and 727 // delegates all other calls to the embedded Context. 728 type valueCtx struct { 729 Context 730 key, val any 731 } 732 733 // stringify tries a bit to stringify v, without using fmt, since we don't 734 // want context depending on the unicode tables. This is only used by 735 // *valueCtx.String(). 736 func stringify(v any) string { 737 switch s := v.(type) { 738 case stringer: 739 return s.String() 740 case string: 741 return s 742 case nil: 743 return "<nil>" 744 } 745 return reflectlite.TypeOf(v).String() 746 } 747 748 func (c *valueCtx) String() string { 749 return contextName(c.Context) + ".WithValue(" + 750 stringify(c.key) + ", " + 751 stringify(c.val) + ")" 752 } 753 754 func (c *valueCtx) Value(key any) any { 755 if c.key == key { 756 return c.val 757 } 758 return value(c.Context, key) 759 } 760 761 func value(c Context, key any) any { 762 for { 763 switch ctx := c.(type) { 764 case *valueCtx: 765 if key == ctx.key { 766 return ctx.val 767 } 768 c = ctx.Context 769 case *cancelCtx: 770 if key == &cancelCtxKey { 771 return c 772 } 773 c = ctx.Context 774 case withoutCancelCtx: 775 if key == &cancelCtxKey { 776 // This implements Cause(ctx) == nil 777 // when ctx is created using WithoutCancel. 778 return nil 779 } 780 c = ctx.c 781 case *timerCtx: 782 if key == &cancelCtxKey { 783 return &ctx.cancelCtx 784 } 785 c = ctx.Context 786 case backgroundCtx, todoCtx: 787 return nil 788 default: 789 return c.Value(key) 790 } 791 } 792 } 793