// Copyright 2014 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package context defines the Context type, which carries deadlines, // cancellation signals, and other request-scoped values across API boundaries // and between processes. // // Incoming requests to a server should create a Context, and outgoing // calls to servers should accept a Context. The chain of function // calls between them must propagate the Context, optionally replacing // it with a derived Context created using WithCancel, WithDeadline, // WithTimeout, or WithValue. When a Context is canceled, all // Contexts derived from it are also canceled. // // The WithCancel, WithDeadline, and WithTimeout functions take a // Context (the parent) and return a derived Context (the child) and a // CancelFunc. Calling the CancelFunc cancels the child and its // children, removes the parent's reference to the child, and stops // any associated timers. Failing to call the CancelFunc leaks the // child and its children until the parent is canceled or the timer // fires. The go vet tool checks that CancelFuncs are used on all // control-flow paths. // // Programs that use Contexts should follow these rules to keep interfaces // consistent across packages and enable static analysis tools to check context // propagation: // // Do not store Contexts inside a struct type; instead, pass a Context // explicitly to each function that needs it. The Context should be the first // parameter, typically named ctx: // // func DoSomething(ctx context.Context, arg Arg) error { // // ... use ctx ... // } // // Do not pass a nil Context, even if a function permits it. Pass context.TODO // if you are unsure about which Context to use. // // Use context Values only for request-scoped data that transits processes and // APIs, not for passing optional parameters to functions. // // The same Context may be passed to functions running in different goroutines; // Contexts are safe for simultaneous use by multiple goroutines. // // See https://blog.golang.org/context for example code for a server that uses // Contexts.
package context import ( ) // 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 returns the time when work done on behalf of this context // should be canceled. Deadline returns ok==false when no deadline is // set. Successive calls to Deadline return the same results. Deadline() (deadline time.Time, ok bool) // Done returns a channel that's closed when work done on behalf of this // context should be canceled. Done may return nil if this context can // never be canceled. Successive calls to Done return the same value. // The close of the Done channel may happen asynchronously, // after the cancel function returns. // // WithCancel arranges for Done to be closed when cancel is called; // WithDeadline arranges for Done to be closed when the deadline // expires; WithTimeout arranges for Done to be closed when the timeout // elapses. // // Done is provided for use in select statements: // // // Stream generates values with DoSomething and sends them to out // // until DoSomething returns an error or ctx.Done is closed. // func Stream(ctx context.Context, out chan<- Value) error { // for { // v, err := DoSomething(ctx) // if err != nil { // return err // } // select { // case <-ctx.Done(): // return ctx.Err() // case out <- v: // } // } // } // // See https://blog.golang.org/pipelines for more examples of how to use // a Done channel for cancellation. Done() <-chan struct{} // If Done is not yet closed, Err returns nil. // If Done is closed, Err returns a non-nil error explaining why: // Canceled if the context was canceled // or DeadlineExceeded if the context's deadline passed. // After Err returns a non-nil error, successive calls to Err return the same error. Err() error // Value returns the value associated with this context for key, or nil // if no value is associated with key. Successive calls to Value with // the same key returns the same result. // // Use context values only for request-scoped data that transits // processes and API boundaries, not for passing optional parameters to // functions. // // A key identifies a specific value in a Context. Functions that wish // to store values in Context typically allocate a key in a global // variable then use that key as the argument to context.WithValue and // Context.Value. A key can be any type that supports equality; // packages should define keys as an unexported type to avoid // collisions. // // Packages that define a Context key should provide type-safe accessors // for the values stored using that key: // // // Package user defines a User type that's stored in Contexts. // package user // // import "context" // // // User is the type of value stored in the Contexts. // type User struct {...} // // // key is an unexported type for keys defined in this package. // // This prevents collisions with keys defined in other packages. // type key int // // // userKey is the key for user.User values in Contexts. It is // // unexported; clients use user.NewContext and user.FromContext // // instead of using this key directly. // var userKey key // // // NewContext returns a new Context that carries value u. // func NewContext(ctx context.Context, u *User) context.Context { // return context.WithValue(ctx, userKey, u) // } // // // FromContext returns the User value stored in ctx, if any. // func FromContext(ctx context.Context) (*User, bool) { // u, ok := ctx.Value(userKey).(*User) // return u, ok // } Value(key interface{}) interface{} } // Canceled is the error returned by Context.Err when the context is canceled. var Canceled = errors.New("context canceled") // DeadlineExceeded is the error returned by Context.Err when the context's // deadline passes. var DeadlineExceeded error = deadlineExceededError{} type deadlineExceededError struct{} func (deadlineExceededError) () string { return "context deadline exceeded" } func (deadlineExceededError) () bool { return true } func (deadlineExceededError) () bool { return true } // An emptyCtx is never canceled, has no values, and has no deadline. It is not // struct{}, since vars of this type must have distinct addresses. type emptyCtx int func (*emptyCtx) () ( time.Time, bool) { return } func (*emptyCtx) () <-chan struct{} { return nil } func (*emptyCtx) () error { return nil } func (*emptyCtx) ( interface{}) interface{} { return nil } func ( *emptyCtx) () string { switch { case background: return "context.Background" case todo: return "context.TODO" } return "unknown empty Context" } var ( background = new(emptyCtx) todo = new(emptyCtx) ) // Background returns a non-nil, empty Context. It is never canceled, has no // values, and has no deadline. It is typically used by the main function, // initialization, and tests, and as the top-level Context for incoming // requests. func () Context { return background } // TODO returns a non-nil, empty Context. Code should use context.TODO when // it's unclear which Context to use or it is not yet available (because the // surrounding function has not yet been extended to accept a Context // parameter). func () Context { return todo } // A CancelFunc tells an operation to abandon its work. // A CancelFunc does not wait for the work to stop. // A CancelFunc may be called by multiple goroutines simultaneously. // After the first call, subsequent calls to a CancelFunc do nothing. type CancelFunc func() // 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 ( Context) ( Context, CancelFunc) { if == nil { panic("cannot create context from nil parent") } := newCancelCtx() propagateCancel(, &) return &, func() { .cancel(true, Canceled) } } // newCancelCtx returns an initialized cancelCtx. func newCancelCtx( Context) cancelCtx { return cancelCtx{Context: } } // goroutines counts the number of goroutines ever created; for testing. var goroutines int32 // propagateCancel arranges for child to be canceled when parent is. func propagateCancel( Context, canceler) { := .Done() if == nil { return // parent is never canceled } select { case <-: // parent is already canceled .cancel(false, .Err()) return default: } if , := parentCancelCtx(); { .mu.Lock() if .err != nil { // parent has already been canceled .cancel(false, .err) } else { if .children == nil { .children = make(map[canceler]struct{}) } .children[] = struct{}{} } .mu.Unlock() } else { atomic.AddInt32(&goroutines, +1) go func() { select { case <-.Done(): .cancel(false, .Err()) case <-.Done(): } }() } } // &cancelCtxKey is the key that a cancelCtx returns itself for. var cancelCtxKey int // 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( Context) (*cancelCtx, bool) { := .Done() if == closedchan || == nil { return nil, false } , := .Value(&cancelCtxKey).(*cancelCtx) if ! { return nil, false } , := .done.Load().(chan struct{}) if != { return nil, false } return , true } // removeChild removes a context from its parent. func removeChild( Context, canceler) { , := parentCancelCtx() if ! { return } .mu.Lock() if .children != nil { delete(.children, ) } .mu.Unlock() } // 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{} } // closedchan is a reusable closed channel. var closedchan = make(chan struct{}) func init() { close(closedchan) } // 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 atomic.Value // of 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 ( *cancelCtx) ( interface{}) interface{} { if == &cancelCtxKey { return } return .Context.Value() } func ( *cancelCtx) () <-chan struct{} { := .done.Load() if != nil { return .(chan struct{}) } .mu.Lock() defer .mu.Unlock() = .done.Load() if == nil { = make(chan struct{}) .done.Store() } return .(chan struct{}) } func ( *cancelCtx) () error { .mu.Lock() := .err .mu.Unlock() return } type stringer interface { String() string } func contextName( Context) string { if , := .(stringer); { return .String() } return reflectlite.TypeOf().String() } func ( *cancelCtx) () string { return contextName(.Context) + ".WithCancel" } // cancel closes c.done, cancels each of c's children, and, if // removeFromParent is true, removes c from its parent's children. func ( *cancelCtx) ( bool, error) { if == nil { panic("context: internal error: missing cancel error") } .mu.Lock() if .err != nil { .mu.Unlock() return // already canceled } .err = , := .done.Load().(chan struct{}) if == nil { .done.Store(closedchan) } else { close() } for := range .children { // NOTE: acquiring the child's lock while holding parent's lock. .cancel(false, ) } .children = nil .mu.Unlock() if { removeChild(.Context, ) } } // 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 ( Context, time.Time) (Context, CancelFunc) { if == nil { panic("cannot create context from nil parent") } if , := .Deadline(); && .Before() { // The current deadline is already sooner than the new one. return WithCancel() } := &timerCtx{ cancelCtx: newCancelCtx(), deadline: , } propagateCancel(, ) := time.Until() if <= 0 { .cancel(true, DeadlineExceeded) // deadline has already passed return , func() { .cancel(false, Canceled) } } .mu.Lock() defer .mu.Unlock() if .err == nil { .timer = time.AfterFunc(, func() { .cancel(true, DeadlineExceeded) }) } return , func() { .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 ( *timerCtx) () ( time.Time, bool) { return .deadline, true } func ( *timerCtx) () string { return contextName(.cancelCtx.Context) + ".WithDeadline(" + .deadline.String() + " [" + time.Until(.deadline).String() + "])" } func ( *timerCtx) ( bool, error) { .cancelCtx.cancel(false, ) if { // Remove this timerCtx from its parent cancelCtx's children. removeChild(.cancelCtx.Context, ) } .mu.Lock() if .timer != nil { .timer.Stop() .timer = nil } .mu.Unlock() } // WithTimeout returns WithDeadline(parent, time.Now().Add(timeout)). // // Canceling this context releases resources associated with it, so code should // call cancel as soon as the operations running in this Context complete: // // func slowOperationWithTimeout(ctx context.Context) (Result, error) { // ctx, cancel := context.WithTimeout(ctx, 100*time.Millisecond) // defer cancel() // releases resources if slowOperation completes before timeout elapses // return slowOperation(ctx) // } func ( Context, time.Duration) (Context, CancelFunc) { return WithDeadline(, time.Now().Add()) } // 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 ( Context, , interface{}) Context { if == nil { panic("cannot create context from nil parent") } if == nil { panic("nil key") } if !reflectlite.TypeOf().Comparable() { panic("key is not comparable") } return &valueCtx{, , } } // A valueCtx carries a key-value pair. It implements Value for that key and // delegates all other calls to the embedded Context. type valueCtx struct { Context key, val interface{} } // stringify tries a bit to stringify v, without using fmt, since we don't // want context depending on the unicode tables. This is only used by // *valueCtx.String(). func stringify( interface{}) string { switch s := .(type) { case stringer: return .String() case string: return } return "<not Stringer>" } func ( *valueCtx) () string { return contextName(.Context) + ".WithValue(type " + reflectlite.TypeOf(.key).String() + ", val " + stringify(.val) + ")" } func ( *valueCtx) ( interface{}) interface{} { if .key == { return .val } return .Context.Value() }