golang owned goroutine

resource.go

package resource

import (
	"context"
	"fmt"
	"io"
	"runtime"
	"sync/atomic"
	"time"
)

type Resource struct {
	// we embed the inner representation, so all public method is also available in outer one
	*resource
}

// this is inner representation of the resource
type resource struct {
	ctx    context.Context
	cancel context.CancelFunc

	counter int32
}

// compile time check to make sure *Resource implement io.Closer
var _ io.Closer = (*Resource)(nil)

func (r *resource) Close() error { r.cancel(); return nil }

func (r *resource) main() {
	fmt.Println("resource start")
	defer fmt.Println("resource end")

	for {
		select {
		case <-r.ctx.Done():
			return
		case <-time.After(1 * time.Second):
		}

		r.PrintInc()
	}
}

func (i *resource) PrintInc() {
	fmt.Println("Hello", atomic.AddInt32(&i.counter, 1))
}

func NewResource() *Resource {
	ctx, cancel := context.WithCancel(context.Background())
	ret := &Resource{&resource{ctx, cancel, 0}}

	runtime.SetFinalizer(ret, (*Resource).Close)
	go ret.resource.main()

	return ret
}

most golang resources (like files, socket, database connection) will be automatically closed when GC kick-in,
but, the GC cannot collect any object that is referenced by any goroutine
so, the GC itself cannot collect any resource that has a goroutine attached to them,
one solution is to wrap it behind another pointer, so when the outer pointer is collected, it will close the inner goroutine

for example

func main() {
	resource.NewResource()
	// ops, we forgot to defer close it

	time.Sleep(5 * time.Second)
	runtime.GC()
	runtime.GC()
	time.Sleep(1 * time.Second)
}

will output

resource start
Hello 1
Hello 2
Hello 3
Hello 4
Hello 5
resource end