src/context/example_test.go

// Copyright 2016 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_test

import (
	"context"
	"errors"
	"fmt"
	"net"
	"sync"
	"time"
)

var neverReady = make(chan struct{}) // never closed

// This example demonstrates the use of a cancelable context to prevent a
// goroutine leak. By the end of the example function, the goroutine started
// by gen will return without leaking.
func ExampleWithCancel() {
	// gen generates integers in a separate goroutine and
	// sends them to the returned channel.
	// The callers of gen need to cancel the context once
	// they are done consuming generated integers not to leak
	// the internal goroutine started by gen.
	gen := func(ctx context.Context) <-chan int {
		dst := make(chan int)
		n := 1
		go func() {
			for {
				select {
				case <-ctx.Done():
					return // returning not to leak the goroutine
				case dst <- n:
					n++
				}
			}
		}()
		return dst
	}

	ctx, cancel := context.WithCancel(context.Background())
	defer cancel() // cancel when we are finished consuming integers

	for n := range gen(ctx) {
		fmt.Println(n)
		if n == 5 {
			break
		}
	}
	// Output:
	// 1
	// 2
	// 3
	// 4
	// 5
}

// This example passes a context with an arbitrary deadline to tell a blocking
// function that it should abandon its work as soon as it gets to it.
func ExampleWithDeadline() {
	d := time.Now().Add(shortDuration)
	ctx, cancel := context.WithDeadline(context.Background(), d)

	// Even though ctx will be expired, it is good practice to call its
	// cancellation function in any case. Failure to do so may keep the
	// context and its parent alive longer than necessary.
	defer cancel()

	select {
	case <-neverReady:
		fmt.Println("ready")
	case <-ctx.Done():
		fmt.Println(ctx.Err())
	}

	// Output:
	// context deadline exceeded
}

// This example passes a context with a timeout to tell a blocking function that
// it should abandon its work after the timeout elapses.
func ExampleWithTimeout() {
	// Pass a context with a timeout to tell a blocking function that it
	// should abandon its work after the timeout elapses.
	ctx, cancel := context.WithTimeout(context.Background(), shortDuration)
	defer cancel()

	select {
	case <-neverReady:
		fmt.Println("ready")
	case <-ctx.Done():
		fmt.Println(ctx.Err()) // prints "context deadline exceeded"
	}

	// Output:
	// context deadline exceeded
}

// This example demonstrates how a value can be passed to the context
// and also how to retrieve it if it exists.
func ExampleWithValue() {
	type favContextKey string

	f := func(ctx context.Context, k favContextKey) {
		if v := ctx.Value(k); v != nil {
			fmt.Println("found value:", v)
			return
		}
		fmt.Println("key not found:", k)
	}

	k := favContextKey("language")
	ctx := context.WithValue(context.Background(), k, "Go")

	f(ctx, k)
	f(ctx, favContextKey("color"))

	// Output:
	// found value: Go
	// key not found: color
}

// This example uses AfterFunc to define a function which waits on a sync.Cond,
// stopping the wait when a context is canceled.
func ExampleAfterFunc_cond() {
	waitOnCond := func(ctx context.Context, cond *sync.Cond, conditionMet func() bool) error {
		stopf := context.AfterFunc(ctx, func() {
			// We need to acquire cond.L here to be sure that the Broadcast
			// below won't occur before the call to Wait, which would result
			// in a missed signal (and deadlock).
			cond.L.Lock()
			defer cond.L.Unlock()

			// If multiple goroutines are waiting on cond simultaneously,
			// we need to make sure we wake up exactly this one.
			// That means that we need to Broadcast to all of the goroutines,
			// which will wake them all up.
			//
			// If there are N concurrent calls to waitOnCond, each of the goroutines
			// will spuriously wake up O(N) other goroutines that aren't ready yet,
			// so this will cause the overall CPU cost to be O(N²).
			cond.Broadcast()
		})
		defer stopf()

		// Since the wakeups are using Broadcast instead of Signal, this call to
		// Wait may unblock due to some other goroutine's context being canceled,
		// so to be sure that ctx is actually canceled we need to check it in a loop.
		for !conditionMet() {
			cond.Wait()
			if ctx.Err() != nil {
				return ctx.Err()
			}
		}

		return nil
	}

	cond := sync.NewCond(new(sync.Mutex))

	var wg sync.WaitGroup
	for range 4 {
		wg.Go(func() {
			ctx, cancel := context.WithTimeout(context.Background(), 1*time.Millisecond)
			defer cancel()

			cond.L.Lock()
			defer cond.L.Unlock()

			err := waitOnCond(ctx, cond, func() bool { return false })
			fmt.Println(err)
		})
	}
	wg.Wait()

	// Output:
	// context deadline exceeded
	// context deadline exceeded
	// context deadline exceeded
	// context deadline exceeded
}

// This example uses AfterFunc to define a function which reads from a net.Conn,
// stopping the read when a context is canceled.
func ExampleAfterFunc_connection() {
	readFromConn := func(ctx context.Context, conn net.Conn, b []byte) (n int, err error) {
		stopc := make(chan struct{})
		stop := context.AfterFunc(ctx, func() {
			conn.SetReadDeadline(time.Now())
			close(stopc)
		})
		n, err = conn.Read(b)
		if !stop() {
			// The AfterFunc was started.
			// Wait for it to complete, and reset the Conn's deadline.
			<-stopc
			conn.SetReadDeadline(time.Time{})
			return n, ctx.Err()
		}
		return n, err
	}

	listener, err := net.Listen("tcp", "localhost:0")
	if err != nil {
		fmt.Println(err)
		return
	}
	defer listener.Close()

	conn, err := net.Dial(listener.Addr().Network(), listener.Addr().String())
	if err != nil {
		fmt.Println(err)
		return
	}
	defer conn.Close()

	ctx, cancel := context.WithTimeout(context.Background(), 1*time.Millisecond)
	defer cancel()

	b := make([]byte, 1024)
	_, err = readFromConn(ctx, conn, b)
	fmt.Println(err)

	// Output:
	// context deadline exceeded
}

// This example uses AfterFunc to define a function which combines
// the cancellation signals of two Contexts.
func ExampleAfterFunc_merge() {
	// mergeCancel returns a context that contains the values of ctx,
	// and which is canceled when either ctx or cancelCtx is canceled.
	mergeCancel := func(ctx, cancelCtx context.Context) (context.Context, context.CancelFunc) {
		ctx, cancel := context.WithCancelCause(ctx)
		stop := context.AfterFunc(cancelCtx, func() {
			cancel(context.Cause(cancelCtx))
		})
		return ctx, func() {
			stop()
			cancel(context.Canceled)
		}
	}

	ctx1, cancel1 := context.WithCancelCause(context.Background())
	defer cancel1(errors.New("ctx1 canceled"))

	ctx2, cancel2 := context.WithCancelCause(context.Background())

	mergedCtx, mergedCancel := mergeCancel(ctx1, ctx2)
	defer mergedCancel()

	cancel2(errors.New("ctx2 canceled"))
	<-mergedCtx.Done()
	fmt.Println(context.Cause(mergedCtx))

	// Output:
	// ctx2 canceled
}