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    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 // +build !go1.7
    6 
    7 package context
    8 
    9 import (
   10     "errors"
   11     "fmt"
   12     "sync"
   13     "time"
   14 )
   15 
   16 // An emptyCtx is never canceled, has no values, and has no deadline.  It is not
   17 // struct{}, since vars of this type must have distinct addresses.
   18 type emptyCtx int
   19 
   20 func (*emptyCtx) Deadline() (deadline time.Time, ok bool) {
   21     return
   22 }
   23 
   24 func (*emptyCtx) Done() <-chan struct{} {
   25     return nil
   26 }
   27 
   28 func (*emptyCtx) Err() error {
   29     return nil
   30 }
   31 
   32 func (*emptyCtx) Value(key interface{}) interface{} {
   33     return nil
   34 }
   35 
   36 func (e *emptyCtx) String() string {
   37     switch e {
   38     case background:
   39         return "context.Background"
   40     case todo:
   41         return "context.TODO"
   42     }
   43     return "unknown empty Context"
   44 }
   45 
   46 var (
   47     background = new(emptyCtx)
   48     todo       = new(emptyCtx)
   49 )
   50 
   51 // Canceled is the error returned by Context.Err when the context is canceled.
   52 var Canceled = errors.New("context canceled")
   53 
   54 // DeadlineExceeded is the error returned by Context.Err when the context's
   55 // deadline passes.
   56 var DeadlineExceeded = errors.New("context deadline exceeded")
   57 
   58 // WithCancel returns a copy of parent with a new Done channel. The returned
   59 // context's Done channel is closed when the returned cancel function is called
   60 // or when the parent context's Done channel is closed, whichever happens first.
   61 //
   62 // Canceling this context releases resources associated with it, so code should
   63 // call cancel as soon as the operations running in this Context complete.
   64 func WithCancel(parent Context) (ctx Context, cancel CancelFunc) {
   65     c := newCancelCtx(parent)
   66     propagateCancel(parent, c)
   67     return c, func() { c.cancel(true, Canceled) }
   68 }
   69 
   70 // newCancelCtx returns an initialized cancelCtx.
   71 func newCancelCtx(parent Context) *cancelCtx {
   72     return &cancelCtx{
   73         Context: parent,
   74         done:    make(chan struct{}),
   75     }
   76 }
   77 
   78 // propagateCancel arranges for child to be canceled when parent is.
   79 func propagateCancel(parent Context, child canceler) {
   80     if parent.Done() == nil {
   81         return // parent is never canceled
   82     }
   83     if p, ok := parentCancelCtx(parent); ok {
   84         p.mu.Lock()
   85         if p.err != nil {
   86             // parent has already been canceled
   87             child.cancel(false, p.err)
   88         } else {
   89             if p.children == nil {
   90                 p.children = make(map[canceler]bool)
   91             }
   92             p.children[child] = true
   93         }
   94         p.mu.Unlock()
   95     } else {
   96         go func() {
   97             select {
   98             case <-parent.Done():
   99                 child.cancel(false, parent.Err())
  100             case <-child.Done():
  101             }
  102         }()
  103     }
  104 }
  105 
  106 // parentCancelCtx follows a chain of parent references until it finds a
  107 // *cancelCtx.  This function understands how each of the concrete types in this
  108 // package represents its parent.
  109 func parentCancelCtx(parent Context) (*cancelCtx, bool) {
  110     for {
  111         switch c := parent.(type) {
  112         case *cancelCtx:
  113             return c, true
  114         case *timerCtx:
  115             return c.cancelCtx, true
  116         case *valueCtx:
  117             parent = c.Context
  118         default:
  119             return nil, false
  120         }
  121     }
  122 }
  123 
  124 // removeChild removes a context from its parent.
  125 func removeChild(parent Context, child canceler) {
  126     p, ok := parentCancelCtx(parent)
  127     if !ok {
  128         return
  129     }
  130     p.mu.Lock()
  131     if p.children != nil {
  132         delete(p.children, child)
  133     }
  134     p.mu.Unlock()
  135 }
  136 
  137 // A canceler is a context type that can be canceled directly.  The
  138 // implementations are *cancelCtx and *timerCtx.
  139 type canceler interface {
  140     cancel(removeFromParent bool, err error)
  141     Done() <-chan struct{}
  142 }
  143 
  144 // A cancelCtx can be canceled.  When canceled, it also cancels any children
  145 // that implement canceler.
  146 type cancelCtx struct {
  147     Context
  148 
  149     done chan struct{} // closed by the first cancel call.
  150 
  151     mu       sync.Mutex
  152     children map[canceler]bool // set to nil by the first cancel call
  153     err      error             // set to non-nil by the first cancel call
  154 }
  155 
  156 func (c *cancelCtx) Done() <-chan struct{} {
  157     return c.done
  158 }
  159 
  160 func (c *cancelCtx) Err() error {
  161     c.mu.Lock()
  162     defer c.mu.Unlock()
  163     return c.err
  164 }
  165 
  166 func (c *cancelCtx) String() string {
  167     return fmt.Sprintf("%v.WithCancel", c.Context)
  168 }
  169 
  170 // cancel closes c.done, cancels each of c's children, and, if
  171 // removeFromParent is true, removes c from its parent's children.
  172 func (c *cancelCtx) cancel(removeFromParent bool, err error) {
  173     if err == nil {
  174         panic("context: internal error: missing cancel error")
  175     }
  176     c.mu.Lock()
  177     if c.err != nil {
  178         c.mu.Unlock()
  179         return // already canceled
  180     }
  181     c.err = err
  182     close(c.done)
  183     for child := range c.children {
  184         // NOTE: acquiring the child's lock while holding parent's lock.
  185         child.cancel(false, err)
  186     }
  187     c.children = nil
  188     c.mu.Unlock()
  189 
  190     if removeFromParent {
  191         removeChild(c.Context, c)
  192     }
  193 }
  194 
  195 // WithDeadline returns a copy of the parent context with the deadline adjusted
  196 // to be no later than d.  If the parent's deadline is already earlier than d,
  197 // WithDeadline(parent, d) is semantically equivalent to parent.  The returned
  198 // context's Done channel is closed when the deadline expires, when the returned
  199 // cancel function is called, or when the parent context's Done channel is
  200 // closed, whichever happens first.
  201 //
  202 // Canceling this context releases resources associated with it, so code should
  203 // call cancel as soon as the operations running in this Context complete.
  204 func WithDeadline(parent Context, deadline time.Time) (Context, CancelFunc) {
  205     if cur, ok := parent.Deadline(); ok && cur.Before(deadline) {
  206         // The current deadline is already sooner than the new one.
  207         return WithCancel(parent)
  208     }
  209     c := &timerCtx{
  210         cancelCtx: newCancelCtx(parent),
  211         deadline:  deadline,
  212     }
  213     propagateCancel(parent, c)
  214     d := deadline.Sub(time.Now())
  215     if d <= 0 {
  216         c.cancel(true, DeadlineExceeded) // deadline has already passed
  217         return c, func() { c.cancel(true, Canceled) }
  218     }
  219     c.mu.Lock()
  220     defer c.mu.Unlock()
  221     if c.err == nil {
  222         c.timer = time.AfterFunc(d, func() {
  223             c.cancel(true, DeadlineExceeded)
  224         })
  225     }
  226     return c, func() { c.cancel(true, Canceled) }
  227 }
  228 
  229 // A timerCtx carries a timer and a deadline.  It embeds a cancelCtx to
  230 // implement Done and Err.  It implements cancel by stopping its timer then
  231 // delegating to cancelCtx.cancel.
  232 type timerCtx struct {
  233     *cancelCtx
  234     timer *time.Timer // Under cancelCtx.mu.
  235 
  236     deadline time.Time
  237 }
  238 
  239 func (c *timerCtx) Deadline() (deadline time.Time, ok bool) {
  240     return c.deadline, true
  241 }
  242 
  243 func (c *timerCtx) String() string {
  244     return fmt.Sprintf("%v.WithDeadline(%s [%s])", c.cancelCtx.Context, c.deadline, c.deadline.Sub(time.Now()))
  245 }
  246 
  247 func (c *timerCtx) cancel(removeFromParent bool, err error) {
  248     c.cancelCtx.cancel(false, err)
  249     if removeFromParent {
  250         // Remove this timerCtx from its parent cancelCtx's children.
  251         removeChild(c.cancelCtx.Context, c)
  252     }
  253     c.mu.Lock()
  254     if c.timer != nil {
  255         c.timer.Stop()
  256         c.timer = nil
  257     }
  258     c.mu.Unlock()
  259 }
  260 
  261 // WithTimeout returns WithDeadline(parent, time.Now().Add(timeout)).
  262 //
  263 // Canceling this context releases resources associated with it, so code should
  264 // call cancel as soon as the operations running in this Context complete:
  265 //
  266 //  func slowOperationWithTimeout(ctx context.Context) (Result, error) {
  267 //      ctx, cancel := context.WithTimeout(ctx, 100*time.Millisecond)
  268 //      defer cancel()  // releases resources if slowOperation completes before timeout elapses
  269 //      return slowOperation(ctx)
  270 //  }
  271 func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc) {
  272     return WithDeadline(parent, time.Now().Add(timeout))
  273 }
  274 
  275 // WithValue returns a copy of parent in which the value associated with key is
  276 // val.
  277 //
  278 // Use context Values only for request-scoped data that transits processes and
  279 // APIs, not for passing optional parameters to functions.
  280 func WithValue(parent Context, key interface{}, val interface{}) Context {
  281     return &valueCtx{parent, key, val}
  282 }
  283 
  284 // A valueCtx carries a key-value pair.  It implements Value for that key and
  285 // delegates all other calls to the embedded Context.
  286 type valueCtx struct {
  287     Context
  288     key, val interface{}
  289 }
  290 
  291 func (c *valueCtx) String() string {
  292     return fmt.Sprintf("%v.WithValue(%#v, %#v)", c.Context, c.key, c.val)
  293 }
  294 
  295 func (c *valueCtx) Value(key interface{}) interface{} {
  296     if c.key == key {
  297         return c.val
  298     }
  299     return c.Context.Value(key)
  300 }