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