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// 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,
// cancelation 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 must
// propagate the Context, optionally replacing it with a modified copy created
// using WithDeadline, WithTimeout, WithCancel, or WithValue.
// 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 for example code for a server that uses
// Contexts.
package context // import ""
import "time"
// A Context carries a deadline, a cancelation 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.
// 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 for more examples of how to use
// a Done channel for cancelation.
Done() <-chan struct{}
// Err returns a non-nil error value after Done is closed. Err returns
// Canceled if the context was canceled or DeadlineExceeded if the
// context's deadline passed. No other values for Err are defined.
// After Done is closed, successive calls to Err return the same value.
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 stores using that key:
// // Package user defines a User type that's stored in Contexts.
// package user
// import ""
// // 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 = 0
// // 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{}
// 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 Background() 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). TODO is recognized by static analysis tools that determine
// whether Contexts are propagated correctly in a program.
func TODO() Context {
return todo
// A CancelFunc tells an operation to abandon its work.
// A CancelFunc does not wait for the work to stop.
// After the first call, subsequent calls to a CancelFunc do nothing.
type CancelFunc func()