300 lines
7.9 KiB
Go
300 lines
7.9 KiB
Go
// Copyright 2014 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// +build !go1.7
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package context
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import (
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"errors"
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"fmt"
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"sync"
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"time"
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)
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// An emptyCtx is never canceled, has no values, and has no deadline. It is not
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// struct{}, since vars of this type must have distinct addresses.
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type emptyCtx int
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func (*emptyCtx) Deadline() (deadline time.Time, ok bool) {
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return
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}
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func (*emptyCtx) Done() <-chan struct{} {
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return nil
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}
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func (*emptyCtx) Err() error {
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return nil
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}
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func (*emptyCtx) Value(key interface{}) interface{} {
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return nil
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}
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func (e *emptyCtx) String() string {
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switch e {
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case background:
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return "context.Background"
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case todo:
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return "context.TODO"
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}
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return "unknown empty Context"
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}
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var (
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background = new(emptyCtx)
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todo = new(emptyCtx)
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)
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// Canceled is the error returned by Context.Err when the context is canceled.
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var Canceled = errors.New("context canceled")
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// DeadlineExceeded is the error returned by Context.Err when the context's
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// deadline passes.
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var DeadlineExceeded = errors.New("context deadline exceeded")
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// WithCancel returns a copy of parent with a new Done channel. The returned
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// context's Done channel is closed when the returned cancel function is called
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// or when the parent context's Done channel is closed, whichever happens first.
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//
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// Canceling this context releases resources associated with it, so code should
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// call cancel as soon as the operations running in this Context complete.
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func WithCancel(parent Context) (ctx Context, cancel CancelFunc) {
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c := newCancelCtx(parent)
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propagateCancel(parent, c)
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return c, func() { c.cancel(true, Canceled) }
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}
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// newCancelCtx returns an initialized cancelCtx.
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func newCancelCtx(parent Context) *cancelCtx {
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return &cancelCtx{
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Context: parent,
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done: make(chan struct{}),
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}
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}
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// propagateCancel arranges for child to be canceled when parent is.
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func propagateCancel(parent Context, child canceler) {
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if parent.Done() == nil {
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return // parent is never canceled
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}
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if p, ok := parentCancelCtx(parent); ok {
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p.mu.Lock()
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if p.err != nil {
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// parent has already been canceled
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child.cancel(false, p.err)
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} else {
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if p.children == nil {
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p.children = make(map[canceler]bool)
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}
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p.children[child] = true
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}
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p.mu.Unlock()
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} else {
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go func() {
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select {
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case <-parent.Done():
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child.cancel(false, parent.Err())
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case <-child.Done():
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}
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}()
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}
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}
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// parentCancelCtx follows a chain of parent references until it finds a
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// *cancelCtx. This function understands how each of the concrete types in this
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// package represents its parent.
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func parentCancelCtx(parent Context) (*cancelCtx, bool) {
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for {
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switch c := parent.(type) {
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case *cancelCtx:
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return c, true
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case *timerCtx:
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return c.cancelCtx, true
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case *valueCtx:
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parent = c.Context
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default:
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return nil, false
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}
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}
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}
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// removeChild removes a context from its parent.
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func removeChild(parent Context, child canceler) {
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p, ok := parentCancelCtx(parent)
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if !ok {
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return
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}
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p.mu.Lock()
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if p.children != nil {
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delete(p.children, child)
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}
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p.mu.Unlock()
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}
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// A canceler is a context type that can be canceled directly. The
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// implementations are *cancelCtx and *timerCtx.
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type canceler interface {
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cancel(removeFromParent bool, err error)
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Done() <-chan struct{}
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}
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// A cancelCtx can be canceled. When canceled, it also cancels any children
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// that implement canceler.
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type cancelCtx struct {
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Context
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done chan struct{} // closed by the first cancel call.
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mu sync.Mutex
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children map[canceler]bool // set to nil by the first cancel call
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err error // set to non-nil by the first cancel call
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}
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func (c *cancelCtx) Done() <-chan struct{} {
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return c.done
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}
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func (c *cancelCtx) Err() error {
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c.mu.Lock()
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defer c.mu.Unlock()
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return c.err
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}
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func (c *cancelCtx) String() string {
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return fmt.Sprintf("%v.WithCancel", c.Context)
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}
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// cancel closes c.done, cancels each of c's children, and, if
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// removeFromParent is true, removes c from its parent's children.
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func (c *cancelCtx) cancel(removeFromParent bool, err error) {
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if err == nil {
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panic("context: internal error: missing cancel error")
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}
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c.mu.Lock()
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if c.err != nil {
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c.mu.Unlock()
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return // already canceled
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}
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c.err = err
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close(c.done)
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for child := range c.children {
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// NOTE: acquiring the child's lock while holding parent's lock.
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child.cancel(false, err)
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}
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c.children = nil
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c.mu.Unlock()
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if removeFromParent {
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removeChild(c.Context, c)
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}
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}
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// WithDeadline returns a copy of the parent context with the deadline adjusted
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// to be no later than d. If the parent's deadline is already earlier than d,
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// WithDeadline(parent, d) is semantically equivalent to parent. The returned
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// context's Done channel is closed when the deadline expires, when the returned
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// cancel function is called, or when the parent context's Done channel is
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// closed, whichever happens first.
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//
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// Canceling this context releases resources associated with it, so code should
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// call cancel as soon as the operations running in this Context complete.
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func WithDeadline(parent Context, deadline time.Time) (Context, CancelFunc) {
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if cur, ok := parent.Deadline(); ok && cur.Before(deadline) {
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// The current deadline is already sooner than the new one.
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return WithCancel(parent)
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}
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c := &timerCtx{
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cancelCtx: newCancelCtx(parent),
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deadline: deadline,
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}
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propagateCancel(parent, c)
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d := deadline.Sub(time.Now())
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if d <= 0 {
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c.cancel(true, DeadlineExceeded) // deadline has already passed
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return c, func() { c.cancel(true, Canceled) }
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}
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c.mu.Lock()
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defer c.mu.Unlock()
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if c.err == nil {
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c.timer = time.AfterFunc(d, func() {
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c.cancel(true, DeadlineExceeded)
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})
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}
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return c, func() { c.cancel(true, Canceled) }
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}
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// A timerCtx carries a timer and a deadline. It embeds a cancelCtx to
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// implement Done and Err. It implements cancel by stopping its timer then
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// delegating to cancelCtx.cancel.
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type timerCtx struct {
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*cancelCtx
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timer *time.Timer // Under cancelCtx.mu.
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deadline time.Time
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}
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func (c *timerCtx) Deadline() (deadline time.Time, ok bool) {
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return c.deadline, true
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}
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func (c *timerCtx) String() string {
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return fmt.Sprintf("%v.WithDeadline(%s [%s])", c.cancelCtx.Context, c.deadline, c.deadline.Sub(time.Now()))
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}
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func (c *timerCtx) cancel(removeFromParent bool, err error) {
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c.cancelCtx.cancel(false, err)
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if removeFromParent {
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// Remove this timerCtx from its parent cancelCtx's children.
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removeChild(c.cancelCtx.Context, c)
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}
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c.mu.Lock()
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if c.timer != nil {
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c.timer.Stop()
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c.timer = nil
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}
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c.mu.Unlock()
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}
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// WithTimeout returns WithDeadline(parent, time.Now().Add(timeout)).
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//
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// Canceling this context releases resources associated with it, so code should
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// call cancel as soon as the operations running in this Context complete:
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//
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// func slowOperationWithTimeout(ctx context.Context) (Result, error) {
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// ctx, cancel := context.WithTimeout(ctx, 100*time.Millisecond)
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// defer cancel() // releases resources if slowOperation completes before timeout elapses
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// return slowOperation(ctx)
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// }
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func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc) {
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return WithDeadline(parent, time.Now().Add(timeout))
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}
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// WithValue returns a copy of parent in which the value associated with key is
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// val.
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//
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// Use context Values only for request-scoped data that transits processes and
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// APIs, not for passing optional parameters to functions.
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func WithValue(parent Context, key interface{}, val interface{}) Context {
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return &valueCtx{parent, key, val}
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}
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// A valueCtx carries a key-value pair. It implements Value for that key and
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// delegates all other calls to the embedded Context.
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type valueCtx struct {
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Context
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key, val interface{}
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}
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func (c *valueCtx) String() string {
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return fmt.Sprintf("%v.WithValue(%#v, %#v)", c.Context, c.key, c.val)
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}
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func (c *valueCtx) Value(key interface{}) interface{} {
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if c.key == key {
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return c.val
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}
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return c.Context.Value(key)
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}
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