Source: context.go in package context

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 (
	"errors"
	"internal/reflectlite"
	"sync"
	"sync/atomic"
	"time"
)

A Context carries a deadline, a cancellation signal, and other values across API boundaries. Context's methods may be called by multiple goroutines simultaneously.

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) Error() string   { return "context deadline exceeded" }
func (deadlineExceededError) Timeout() bool   { return true }
func (deadlineExceededError) Temporary() 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) Deadline() (deadline time.Time, ok bool) {
	return
}

func (*emptyCtx) Done() <-chan struct{} {
	return nil
}

func (*emptyCtx) Err() error {
	return nil
}

func (*emptyCtx) Value(key interface{}) interface{} {
	return nil
}

func (e *emptyCtx) String() string {
	switch e {
	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 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).

func TODO() 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 WithCancel(parent Context) (ctx Context, cancel CancelFunc) {
	if parent == nil {
		panic("cannot create context from nil parent")
	}
	c := newCancelCtx(parent)
	propagateCancel(parent, &c)
	return &c, func() { c.cancel(true, Canceled) }
}

newCancelCtx returns an initialized cancelCtx.

func newCancelCtx(parent Context) cancelCtx {
	return cancelCtx{Context: parent}
}

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(parent Context, child canceler) {
	done := parent.Done()
	if done == nil {
		return // parent is never canceled
	}

	select {

parent is already canceled

		child.cancel(false, parent.Err())
		return
	default:
	}

	if p, ok := parentCancelCtx(parent); ok {
		p.mu.Lock()

parent has already been canceled

			child.cancel(false, p.err)
		} else {
			if p.children == nil {
				p.children = make(map[canceler]struct{})
			}
			p.children[child] = struct{}{}
		}
		p.mu.Unlock()
	} else {
		atomic.AddInt32(&goroutines, +1)
		go func() {
			select {
			case <-parent.Done():
				child.cancel(false, parent.Err())
			case <-child.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(parent Context) (*cancelCtx, bool) {
	done := parent.Done()
	if done == closedchan || done == nil {
		return nil, false
	}
	p, ok := parent.Value(&cancelCtxKey).(*cancelCtx)
	if !ok {
		return nil, false
	}
	p.mu.Lock()
	ok = p.done == done
	p.mu.Unlock()
	if !ok {
		return nil, false
	}
	return p, true
}

removeChild removes a context from its parent.

func removeChild(parent Context, child canceler) {
	p, ok := parentCancelCtx(parent)
	if !ok {
		return
	}
	p.mu.Lock()
	if p.children != nil {
		delete(p.children, child)
	}
	p.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     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 (c *cancelCtx) Value(key interface{}) interface{} {
	if key == &cancelCtxKey {
		return c
	}
	return c.Context.Value(key)
}

func (c *cancelCtx) Done() <-chan struct{} {
	c.mu.Lock()
	if c.done == nil {
		c.done = make(chan struct{})
	}
	d := c.done
	c.mu.Unlock()
	return d
}

func (c *cancelCtx) Err() error {
	c.mu.Lock()
	err := c.err
	c.mu.Unlock()
	return err
}

type stringer interface {
	String() string
}

func contextName(c Context) string {
	if s, ok := c.(stringer); ok {
		return s.String()
	}
	return reflectlite.TypeOf(c).String()
}

func (c *cancelCtx) String() string {
	return contextName(c.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 (c *cancelCtx) cancel(removeFromParent bool, err error) {
	if err == nil {
		panic("context: internal error: missing cancel error")
	}
	c.mu.Lock()
	if c.err != nil {
		c.mu.Unlock()
		return // already canceled
	}
	c.err = err
	if c.done == nil {
		c.done = closedchan
	} else {
		close(c.done)
	}

NOTE: acquiring the child's lock while holding parent's lock.

		child.cancel(false, err)
	}
	c.children = nil
	c.mu.Unlock()

	if removeFromParent {
		removeChild(c.Context, c)
	}
}

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 WithDeadline(parent Context, d time.Time) (Context, CancelFunc) {
	if parent == nil {
		panic("cannot create context from nil parent")
	}

The current deadline is already sooner than the new one.

		return WithCancel(parent)
	}
	c := &timerCtx{
		cancelCtx: newCancelCtx(parent),
		deadline:  d,
	}
	propagateCancel(parent, c)
	dur := time.Until(d)
	if dur <= 0 {
		c.cancel(true, DeadlineExceeded) // deadline has already passed
		return c, func() { c.cancel(false, Canceled) }
	}
	c.mu.Lock()
	defer c.mu.Unlock()
	if c.err == nil {
		c.timer = time.AfterFunc(dur, func() {
			c.cancel(true, DeadlineExceeded)
		})
	}
	return c, func() { c.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 (c *timerCtx) Deadline() (deadline time.Time, ok bool) {
	return c.deadline, true
}

func (c *timerCtx) String() string {
	return contextName(c.cancelCtx.Context) + ".WithDeadline(" +
		c.deadline.String() + " [" +
		time.Until(c.deadline).String() + "])"
}

func (c *timerCtx) cancel(removeFromParent bool, err error) {
	c.cancelCtx.cancel(false, err)

Remove this timerCtx from its parent cancelCtx's children.

		removeChild(c.cancelCtx.Context, c)
	}
	c.mu.Lock()
	if c.timer != nil {
		c.timer.Stop()
		c.timer = nil
	}
	c.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 WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc) {
	return WithDeadline(parent, time.Now().Add(timeout))
}

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 WithValue(parent Context, key, val interface{}) Context {
	if parent == nil {
		panic("cannot create context from nil parent")
	}
	if key == nil {
		panic("nil key")
	}
	if !reflectlite.TypeOf(key).Comparable() {
		panic("key is not comparable")
	}
	return &valueCtx{parent, key, val}
}

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(v interface{}) string {
	switch s := v.(type) {
	case stringer:
		return s.String()
	case string:
		return s
	}
	return "<not Stringer>"
}

func (c *valueCtx) String() string {
	return contextName(c.Context) + ".WithValue(type " +
		reflectlite.TypeOf(c.key).String() +
		", val " + stringify(c.val) + ")"
}

func (c *valueCtx) Value(key interface{}) interface{} {
	if c.key == key {
		return c.val
	}
	return c.Context.Value(key)