Golang context source code analysis

So today we’re gonna read source code of Golang’s context.

Let’s learn how it is used first, check this demo:

package main

import (

func UseContext(ctx context.Context) {
	for {
		select {
		case <-ctx.Done():
			log.Printf("context is done with error %s", ctx.Err())
			log.Printf("nothing just loop...")
			time.Sleep(time.Second * time.Duration(1))

func main() {
	ctx, cancel := context.WithCancel(context.Background())
	go UseContext(ctx)

	time.Sleep(time.Second * time.Duration(1))
	time.Sleep(time.Second * time.Duration(2))

run it:

$ go run main.go 
2020/04/21 21:25:24 nothing just loop...
2020/04/21 21:25:25 nothing just loop...
2020/04/21 21:25:26 context is done with error context canceled

it works like what we expected. context can also be used to carry on some metadata such as user token, database instance, and so on, but how does it implement this? let’s dig out.

first, we jump into the definition of context, it’s an interface:

// A Context carries a deadline, a cancellation 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.
	// 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{}

as it’s an interface, which means that any object implement these methods can use as Context:

  • Deadline() (deadline time.Time, ok bool)
  • Done() <-chan struct{}
  • Err() error
  • Value(key interface{}) interface{}

Let’s start from the simplest one: context.Background():

func Background() Context {
	return background

// but what is background? it's:
var (
	background = new(emptyCtx)
	todo       = new(emptyCtx)

well, it’s an instance of emptyCtx:

// 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

so what’s the context.WithCancel?

// 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) }

// A cancelCtx can be canceled. When canceled, it also cancels any children
// that implement canceler.
type cancelCtx struct {

	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

when I call cancel, it just close done, which has a type of chan struct{}:

// 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")
	if c.err != nil {
		return // already canceled
	c.err = err
	if c.done == nil {
		c.done = closedchan
	} else {
		close(c.done) // NOTE(jiajun): here it is :)
	for child := range c.children {
		// NOTE: acquiring the child's lock while holding parent's lock.
		child.cancel(false, err)
	c.children = nil

	if removeFromParent {
		removeChild(c.Context, c)

well, that’s simple, so does the context.WithDeadline, this is homework for you :)