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Source file src/context/context.go

Documentation: context

  // 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 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"
  	"fmt"
  	"reflect"
  	"sync"
  	"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 https://blog.golang.org/pipelines for more examples of how to use
  	// a Done channel for cancelation.
  	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 = 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{}
  }
  
  // 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). 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()
  
  // 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) {
  	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}
  }
  
  // propagateCancel arranges for child to be canceled when parent is.
  func propagateCancel(parent Context, child canceler) {
  	if parent.Done() == nil {
  		return // parent is never canceled
  	}
  	if p, ok := parentCancelCtx(parent); ok {
  		p.mu.Lock()
  		if p.err != nil {
  			// 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 {
  		go func() {
  			select {
  			case <-parent.Done():
  				child.cancel(false, parent.Err())
  			case <-child.Done():
  			}
  		}()
  	}
  }
  
  // parentCancelCtx follows a chain of parent references until it finds a
  // *cancelCtx. This function understands how each of the concrete types in this
  // package represents its parent.
  func parentCancelCtx(parent Context) (*cancelCtx, bool) {
  	for {
  		switch c := parent.(type) {
  		case *cancelCtx:
  			return c, true
  		case *timerCtx:
  			return &c.cancelCtx, true
  		case *valueCtx:
  			parent = c.Context
  		default:
  			return nil, false
  		}
  	}
  }
  
  // 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) 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()
  	defer c.mu.Unlock()
  	return c.err
  }
  
  func (c *cancelCtx) String() string {
  	return fmt.Sprintf("%v.WithCancel", c.Context)
  }
  
  // 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)
  	}
  	for child := range c.children {
  		// 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, deadline time.Time) (Context, CancelFunc) {
  	if cur, ok := parent.Deadline(); ok && cur.Before(deadline) {
  		// The current deadline is already sooner than the new one.
  		return WithCancel(parent)
  	}
  	c := &timerCtx{
  		cancelCtx: newCancelCtx(parent),
  		deadline:  deadline,
  	}
  	propagateCancel(parent, c)
  	d := time.Until(deadline)
  	if d <= 0 {
  		c.cancel(true, DeadlineExceeded) // deadline has already passed
  		return c, func() { c.cancel(true, Canceled) }
  	}
  	c.mu.Lock()
  	defer c.mu.Unlock()
  	if c.err == nil {
  		c.timer = time.AfterFunc(d, 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 fmt.Sprintf("%v.WithDeadline(%s [%s])", c.cancelCtx.Context, c.deadline, time.Until(c.deadline))
  }
  
  func (c *timerCtx) cancel(removeFromParent bool, err error) {
  	c.cancelCtx.cancel(false, err)
  	if removeFromParent {
  		// 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 key == nil {
  		panic("nil key")
  	}
  	if !reflect.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{}
  }
  
  func (c *valueCtx) String() string {
  	return fmt.Sprintf("%v.WithValue(%#v, %#v)", c.Context, c.key, c.val)
  }
  
  func (c *valueCtx) Value(key interface{}) interface{} {
  	if c.key == key {
  		return c.val
  	}
  	return c.Context.Value(key)
  }
  

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