context

This example uses AfterFunc to define a function which waits on a sync.Cond, stopping the wait when a context is canceled.

package main

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

func main() {
	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.

package main

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

func main() {
	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.

package main

import (
	"context"
	"errors"
	"fmt"
)

func main() {
	// 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

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.

package main

import (
	"context"
	"fmt"
)

func main() {
	// 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.

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