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Source file src/context/context.go

Documentation: context

     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  // Package context defines the Context type, which carries deadlines,
     6  // cancelation signals, and other request-scoped values across API boundaries
     7  // and between processes.
     8  //
     9  // Incoming requests to a server should create a Context, and outgoing
    10  // calls to servers should accept a Context. The chain of function
    11  // calls between them must propagate the Context, optionally replacing
    12  // it with a derived Context created using WithCancel, WithDeadline,
    13  // WithTimeout, or WithValue. When a Context is canceled, all
    14  // Contexts derived from it are also canceled.
    15  //
    16  // The WithCancel, WithDeadline, and WithTimeout functions take a
    17  // Context (the parent) and return a derived Context (the child) and a
    18  // CancelFunc. Calling the CancelFunc cancels the child and its
    19  // children, removes the parent's reference to the child, and stops
    20  // any associated timers. Failing to call the CancelFunc leaks the
    21  // child and its children until the parent is canceled or the timer
    22  // fires. The go vet tool checks that CancelFuncs are used on all
    23  // control-flow paths.
    24  //
    25  // Programs that use Contexts should follow these rules to keep interfaces
    26  // consistent across packages and enable static analysis tools to check context
    27  // propagation:
    28  //
    29  // Do not store Contexts inside a struct type; instead, pass a Context
    30  // explicitly to each function that needs it. The Context should be the first
    31  // parameter, typically named ctx:
    32  //
    33  // 	func DoSomething(ctx context.Context, arg Arg) error {
    34  // 		// ... use ctx ...
    35  // 	}
    36  //
    37  // Do not pass a nil Context, even if a function permits it. Pass context.TODO
    38  // if you are unsure about which Context to use.
    39  //
    40  // Use context Values only for request-scoped data that transits processes and
    41  // APIs, not for passing optional parameters to functions.
    42  //
    43  // The same Context may be passed to functions running in different goroutines;
    44  // Contexts are safe for simultaneous use by multiple goroutines.
    45  //
    46  // See https://blog.golang.org/context for example code for a server that uses
    47  // Contexts.
    48  package context
    49  
    50  import (
    51  	"errors"
    52  	"fmt"
    53  	"reflect"
    54  	"sync"
    55  	"time"
    56  )
    57  
    58  // A Context carries a deadline, a cancelation signal, and other values across
    59  // API boundaries.
    60  //
    61  // Context's methods may be called by multiple goroutines simultaneously.
    62  type Context interface {
    63  	// Deadline returns the time when work done on behalf of this context
    64  	// should be canceled. Deadline returns ok==false when no deadline is
    65  	// set. Successive calls to Deadline return the same results.
    66  	Deadline() (deadline time.Time, ok bool)
    67  
    68  	// Done returns a channel that's closed when work done on behalf of this
    69  	// context should be canceled. Done may return nil if this context can
    70  	// never be canceled. Successive calls to Done return the same value.
    71  	//
    72  	// WithCancel arranges for Done to be closed when cancel is called;
    73  	// WithDeadline arranges for Done to be closed when the deadline
    74  	// expires; WithTimeout arranges for Done to be closed when the timeout
    75  	// elapses.
    76  	//
    77  	// Done is provided for use in select statements:
    78  	//
    79  	//  // Stream generates values with DoSomething and sends them to out
    80  	//  // until DoSomething returns an error or ctx.Done is closed.
    81  	//  func Stream(ctx context.Context, out chan<- Value) error {
    82  	//  	for {
    83  	//  		v, err := DoSomething(ctx)
    84  	//  		if err != nil {
    85  	//  			return err
    86  	//  		}
    87  	//  		select {
    88  	//  		case <-ctx.Done():
    89  	//  			return ctx.Err()
    90  	//  		case out <- v:
    91  	//  		}
    92  	//  	}
    93  	//  }
    94  	//
    95  	// See https://blog.golang.org/pipelines for more examples of how to use
    96  	// a Done channel for cancelation.
    97  	Done() <-chan struct{}
    98  
    99  	// If Done is not yet closed, Err returns nil.
   100  	// If Done is closed, Err returns a non-nil error explaining why:
   101  	// Canceled if the context was canceled
   102  	// or DeadlineExceeded if the context's deadline passed.
   103  	// After Err returns a non-nil error, successive calls to Err return the same error.
   104  	Err() error
   105  
   106  	// Value returns the value associated with this context for key, or nil
   107  	// if no value is associated with key. Successive calls to Value with
   108  	// the same key returns the same result.
   109  	//
   110  	// Use context values only for request-scoped data that transits
   111  	// processes and API boundaries, not for passing optional parameters to
   112  	// functions.
   113  	//
   114  	// A key identifies a specific value in a Context. Functions that wish
   115  	// to store values in Context typically allocate a key in a global
   116  	// variable then use that key as the argument to context.WithValue and
   117  	// Context.Value. A key can be any type that supports equality;
   118  	// packages should define keys as an unexported type to avoid
   119  	// collisions.
   120  	//
   121  	// Packages that define a Context key should provide type-safe accessors
   122  	// for the values stored using that key:
   123  	//
   124  	// 	// Package user defines a User type that's stored in Contexts.
   125  	// 	package user
   126  	//
   127  	// 	import "context"
   128  	//
   129  	// 	// User is the type of value stored in the Contexts.
   130  	// 	type User struct {...}
   131  	//
   132  	// 	// key is an unexported type for keys defined in this package.
   133  	// 	// This prevents collisions with keys defined in other packages.
   134  	// 	type key int
   135  	//
   136  	// 	// userKey is the key for user.User values in Contexts. It is
   137  	// 	// unexported; clients use user.NewContext and user.FromContext
   138  	// 	// instead of using this key directly.
   139  	// 	var userKey key
   140  	//
   141  	// 	// NewContext returns a new Context that carries value u.
   142  	// 	func NewContext(ctx context.Context, u *User) context.Context {
   143  	// 		return context.WithValue(ctx, userKey, u)
   144  	// 	}
   145  	//
   146  	// 	// FromContext returns the User value stored in ctx, if any.
   147  	// 	func FromContext(ctx context.Context) (*User, bool) {
   148  	// 		u, ok := ctx.Value(userKey).(*User)
   149  	// 		return u, ok
   150  	// 	}
   151  	Value(key interface{}) interface{}
   152  }
   153  
   154  // Canceled is the error returned by Context.Err when the context is canceled.
   155  var Canceled = errors.New("context canceled")
   156  
   157  // DeadlineExceeded is the error returned by Context.Err when the context's
   158  // deadline passes.
   159  var DeadlineExceeded error = deadlineExceededError{}
   160  
   161  type deadlineExceededError struct{}
   162  
   163  func (deadlineExceededError) Error() string   { return "context deadline exceeded" }
   164  func (deadlineExceededError) Timeout() bool   { return true }
   165  func (deadlineExceededError) Temporary() bool { return true }
   166  
   167  // An emptyCtx is never canceled, has no values, and has no deadline. It is not
   168  // struct{}, since vars of this type must have distinct addresses.
   169  type emptyCtx int
   170  
   171  func (*emptyCtx) Deadline() (deadline time.Time, ok bool) {
   172  	return
   173  }
   174  
   175  func (*emptyCtx) Done() <-chan struct{} {
   176  	return nil
   177  }
   178  
   179  func (*emptyCtx) Err() error {
   180  	return nil
   181  }
   182  
   183  func (*emptyCtx) Value(key interface{}) interface{} {
   184  	return nil
   185  }
   186  
   187  func (e *emptyCtx) String() string {
   188  	switch e {
   189  	case background:
   190  		return "context.Background"
   191  	case todo:
   192  		return "context.TODO"
   193  	}
   194  	return "unknown empty Context"
   195  }
   196  
   197  var (
   198  	background = new(emptyCtx)
   199  	todo       = new(emptyCtx)
   200  )
   201  
   202  // Background returns a non-nil, empty Context. It is never canceled, has no
   203  // values, and has no deadline. It is typically used by the main function,
   204  // initialization, and tests, and as the top-level Context for incoming
   205  // requests.
   206  func Background() Context {
   207  	return background
   208  }
   209  
   210  // TODO returns a non-nil, empty Context. Code should use context.TODO when
   211  // it's unclear which Context to use or it is not yet available (because the
   212  // surrounding function has not yet been extended to accept a Context
   213  // parameter).
   214  func TODO() Context {
   215  	return todo
   216  }
   217  
   218  // A CancelFunc tells an operation to abandon its work.
   219  // A CancelFunc does not wait for the work to stop.
   220  // After the first call, subsequent calls to a CancelFunc do nothing.
   221  type CancelFunc func()
   222  
   223  // WithCancel returns a copy of parent with a new Done channel. The returned
   224  // context's Done channel is closed when the returned cancel function is called
   225  // or when the parent context's Done channel is closed, whichever happens first.
   226  //
   227  // Canceling this context releases resources associated with it, so code should
   228  // call cancel as soon as the operations running in this Context complete.
   229  func WithCancel(parent Context) (ctx Context, cancel CancelFunc) {
   230  	c := newCancelCtx(parent)
   231  	propagateCancel(parent, &c)
   232  	return &c, func() { c.cancel(true, Canceled) }
   233  }
   234  
   235  // newCancelCtx returns an initialized cancelCtx.
   236  func newCancelCtx(parent Context) cancelCtx {
   237  	return cancelCtx{Context: parent}
   238  }
   239  
   240  // propagateCancel arranges for child to be canceled when parent is.
   241  func propagateCancel(parent Context, child canceler) {
   242  	if parent.Done() == nil {
   243  		return // parent is never canceled
   244  	}
   245  	if p, ok := parentCancelCtx(parent); ok {
   246  		p.mu.Lock()
   247  		if p.err != nil {
   248  			// parent has already been canceled
   249  			child.cancel(false, p.err)
   250  		} else {
   251  			if p.children == nil {
   252  				p.children = make(map[canceler]struct{})
   253  			}
   254  			p.children[child] = struct{}{}
   255  		}
   256  		p.mu.Unlock()
   257  	} else {
   258  		go func() {
   259  			select {
   260  			case <-parent.Done():
   261  				child.cancel(false, parent.Err())
   262  			case <-child.Done():
   263  			}
   264  		}()
   265  	}
   266  }
   267  
   268  // parentCancelCtx follows a chain of parent references until it finds a
   269  // *cancelCtx. This function understands how each of the concrete types in this
   270  // package represents its parent.
   271  func parentCancelCtx(parent Context) (*cancelCtx, bool) {
   272  	for {
   273  		switch c := parent.(type) {
   274  		case *cancelCtx:
   275  			return c, true
   276  		case *timerCtx:
   277  			return &c.cancelCtx, true
   278  		case *valueCtx:
   279  			parent = c.Context
   280  		default:
   281  			return nil, false
   282  		}
   283  	}
   284  }
   285  
   286  // removeChild removes a context from its parent.
   287  func removeChild(parent Context, child canceler) {
   288  	p, ok := parentCancelCtx(parent)
   289  	if !ok {
   290  		return
   291  	}
   292  	p.mu.Lock()
   293  	if p.children != nil {
   294  		delete(p.children, child)
   295  	}
   296  	p.mu.Unlock()
   297  }
   298  
   299  // A canceler is a context type that can be canceled directly. The
   300  // implementations are *cancelCtx and *timerCtx.
   301  type canceler interface {
   302  	cancel(removeFromParent bool, err error)
   303  	Done() <-chan struct{}
   304  }
   305  
   306  // closedchan is a reusable closed channel.
   307  var closedchan = make(chan struct{})
   308  
   309  func init() {
   310  	close(closedchan)
   311  }
   312  
   313  // A cancelCtx can be canceled. When canceled, it also cancels any children
   314  // that implement canceler.
   315  type cancelCtx struct {
   316  	Context
   317  
   318  	mu       sync.Mutex            // protects following fields
   319  	done     chan struct{}         // created lazily, closed by first cancel call
   320  	children map[canceler]struct{} // set to nil by the first cancel call
   321  	err      error                 // set to non-nil by the first cancel call
   322  }
   323  
   324  func (c *cancelCtx) Done() <-chan struct{} {
   325  	c.mu.Lock()
   326  	if c.done == nil {
   327  		c.done = make(chan struct{})
   328  	}
   329  	d := c.done
   330  	c.mu.Unlock()
   331  	return d
   332  }
   333  
   334  func (c *cancelCtx) Err() error {
   335  	c.mu.Lock()
   336  	err := c.err
   337  	c.mu.Unlock()
   338  	return err
   339  }
   340  
   341  func (c *cancelCtx) String() string {
   342  	return fmt.Sprintf("%v.WithCancel", c.Context)
   343  }
   344  
   345  // cancel closes c.done, cancels each of c's children, and, if
   346  // removeFromParent is true, removes c from its parent's children.
   347  func (c *cancelCtx) cancel(removeFromParent bool, err error) {
   348  	if err == nil {
   349  		panic("context: internal error: missing cancel error")
   350  	}
   351  	c.mu.Lock()
   352  	if c.err != nil {
   353  		c.mu.Unlock()
   354  		return // already canceled
   355  	}
   356  	c.err = err
   357  	if c.done == nil {
   358  		c.done = closedchan
   359  	} else {
   360  		close(c.done)
   361  	}
   362  	for child := range c.children {
   363  		// NOTE: acquiring the child's lock while holding parent's lock.
   364  		child.cancel(false, err)
   365  	}
   366  	c.children = nil
   367  	c.mu.Unlock()
   368  
   369  	if removeFromParent {
   370  		removeChild(c.Context, c)
   371  	}
   372  }
   373  
   374  // WithDeadline returns a copy of the parent context with the deadline adjusted
   375  // to be no later than d. If the parent's deadline is already earlier than d,
   376  // WithDeadline(parent, d) is semantically equivalent to parent. The returned
   377  // context's Done channel is closed when the deadline expires, when the returned
   378  // cancel function is called, or when the parent context's Done channel is
   379  // closed, whichever happens first.
   380  //
   381  // Canceling this context releases resources associated with it, so code should
   382  // call cancel as soon as the operations running in this Context complete.
   383  func WithDeadline(parent Context, d time.Time) (Context, CancelFunc) {
   384  	if cur, ok := parent.Deadline(); ok && cur.Before(d) {
   385  		// The current deadline is already sooner than the new one.
   386  		return WithCancel(parent)
   387  	}
   388  	c := &timerCtx{
   389  		cancelCtx: newCancelCtx(parent),
   390  		deadline:  d,
   391  	}
   392  	propagateCancel(parent, c)
   393  	dur := time.Until(d)
   394  	if dur <= 0 {
   395  		c.cancel(true, DeadlineExceeded) // deadline has already passed
   396  		return c, func() { c.cancel(false, Canceled) }
   397  	}
   398  	c.mu.Lock()
   399  	defer c.mu.Unlock()
   400  	if c.err == nil {
   401  		c.timer = time.AfterFunc(dur, func() {
   402  			c.cancel(true, DeadlineExceeded)
   403  		})
   404  	}
   405  	return c, func() { c.cancel(true, Canceled) }
   406  }
   407  
   408  // A timerCtx carries a timer and a deadline. It embeds a cancelCtx to
   409  // implement Done and Err. It implements cancel by stopping its timer then
   410  // delegating to cancelCtx.cancel.
   411  type timerCtx struct {
   412  	cancelCtx
   413  	timer *time.Timer // Under cancelCtx.mu.
   414  
   415  	deadline time.Time
   416  }
   417  
   418  func (c *timerCtx) Deadline() (deadline time.Time, ok bool) {
   419  	return c.deadline, true
   420  }
   421  
   422  func (c *timerCtx) String() string {
   423  	return fmt.Sprintf("%v.WithDeadline(%s [%s])", c.cancelCtx.Context, c.deadline, time.Until(c.deadline))
   424  }
   425  
   426  func (c *timerCtx) cancel(removeFromParent bool, err error) {
   427  	c.cancelCtx.cancel(false, err)
   428  	if removeFromParent {
   429  		// Remove this timerCtx from its parent cancelCtx's children.
   430  		removeChild(c.cancelCtx.Context, c)
   431  	}
   432  	c.mu.Lock()
   433  	if c.timer != nil {
   434  		c.timer.Stop()
   435  		c.timer = nil
   436  	}
   437  	c.mu.Unlock()
   438  }
   439  
   440  // WithTimeout returns WithDeadline(parent, time.Now().Add(timeout)).
   441  //
   442  // Canceling this context releases resources associated with it, so code should
   443  // call cancel as soon as the operations running in this Context complete:
   444  //
   445  // 	func slowOperationWithTimeout(ctx context.Context) (Result, error) {
   446  // 		ctx, cancel := context.WithTimeout(ctx, 100*time.Millisecond)
   447  // 		defer cancel()  // releases resources if slowOperation completes before timeout elapses
   448  // 		return slowOperation(ctx)
   449  // 	}
   450  func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc) {
   451  	return WithDeadline(parent, time.Now().Add(timeout))
   452  }
   453  
   454  // WithValue returns a copy of parent in which the value associated with key is
   455  // val.
   456  //
   457  // Use context Values only for request-scoped data that transits processes and
   458  // APIs, not for passing optional parameters to functions.
   459  //
   460  // The provided key must be comparable and should not be of type
   461  // string or any other built-in type to avoid collisions between
   462  // packages using context. Users of WithValue should define their own
   463  // types for keys. To avoid allocating when assigning to an
   464  // interface{}, context keys often have concrete type
   465  // struct{}. Alternatively, exported context key variables' static
   466  // type should be a pointer or interface.
   467  func WithValue(parent Context, key, val interface{}) Context {
   468  	if key == nil {
   469  		panic("nil key")
   470  	}
   471  	if !reflect.TypeOf(key).Comparable() {
   472  		panic("key is not comparable")
   473  	}
   474  	return &valueCtx{parent, key, val}
   475  }
   476  
   477  // A valueCtx carries a key-value pair. It implements Value for that key and
   478  // delegates all other calls to the embedded Context.
   479  type valueCtx struct {
   480  	Context
   481  	key, val interface{}
   482  }
   483  
   484  func (c *valueCtx) String() string {
   485  	return fmt.Sprintf("%v.WithValue(%#v, %#v)", c.Context, c.key, c.val)
   486  }
   487  
   488  func (c *valueCtx) Value(key interface{}) interface{} {
   489  	if c.key == key {
   490  		return c.val
   491  	}
   492  	return c.Context.Value(key)
   493  }
   494  

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