plugin/localbinary: Exit output stream goroutines when plugin closes

This commit ensures that when a plugin instance is closed the goroutines
responsible for streaming stdout and stderr of the called binary will
also exit, preventing a goroutines leak.

Before this commit these goroutines could stay blocked forever if
Close() was called while the binary still had some pending output.

I found this bug after debugging a real world goroutine leak, the
goroutines dump would show thousands of goroutines at:

```
github.com/tsuru/tsuru/vendor/github.com/docker/machine/libmachine/drivers/plugin/localbinary.stream(0xc023013d80, 0xc0000aad20)
	/home/travis/gopath/src/github.com/tsuru/tsuru/vendor/github.com/docker/machine/libmachine/drivers/plugin/localbinary/plugin.go:177 +0x7c
created by github.com/tsuru/tsuru/vendor/github.com/docker/machine/libmachine/drivers/plugin/localbinary.(*Plugin).AttachStream
	/home/travis/gopath/src/github.com/tsuru/tsuru/vendor/github.com/docker/machine/libmachine/drivers/plugin/localbinary/plugin.go:183 +0x67
```

Signed-off-by: Cezar Sa Espinola <cezarsa@gmail.com>

Author: cezarsa

libmachine/drivers/plugin/localbinary/plugin.go

@@ -72,7 +72,7 @@ type Plugin struct {
 	Addr        string
 	MachineName string
 	addrCh      chan string
-	stopCh      chan bool
+	stopCh      chan struct{}
 	timeout     time.Duration
 }
 
@@ -121,7 +121,7 @@ func NewPlugin(driverName string) (*Plugin, error) {
 	log.Debugf("Found binary path at %s", binaryPath)
 
 	return &Plugin{
-		stopCh: make(chan bool),
+		stopCh: make(chan struct{}),
 		addrCh: make(chan string, 1),
 		Executor: &Executor{
 			DriverName: driverName,
@@ -168,19 +168,23 @@ func (lbe *Executor) Close() error {
 	return nil
 }
 
-func stream(scanner *bufio.Scanner, streamOutCh chan<- string) {
+func stream(scanner *bufio.Scanner, streamOutCh chan<- string, stopCh <-chan struct{}) {
 	for scanner.Scan() {
 		line := scanner.Text()
 		if err := scanner.Err(); err != nil {
 			log.Warnf("Scanning stream: %s", err)
 		}
-		streamOutCh <- strings.Trim(line, "\n")
+		select {
+		case streamOutCh <- strings.Trim(line, "\n"):
+		case <-stopCh:
+			return
+		}
 	}
 }
 
 func (lbp *Plugin) AttachStream(scanner *bufio.Scanner) <-chan string {
 	streamOutCh := make(chan string)
-	go stream(scanner, streamOutCh)
+	go stream(scanner, streamOutCh, lbp.stopCh)
 	return streamOutCh
 }
 
@@ -241,6 +245,6 @@ func (lbp *Plugin) Address() (string, error) {
 }
 
 func (lbp *Plugin) Close() error {
-	lbp.stopCh <- true
+	close(lbp.stopCh)
 	return nil
 }

libmachine/drivers/plugin/localbinary/plugin_test.go

@@ -71,10 +71,10 @@ func TestLocalBinaryPluginAddressTimeout(t *testing.T) {
 
 func TestLocalBinaryPluginClose(t *testing.T) {
 	lbp := &Plugin{}
-	lbp.stopCh = make(chan bool, 1)
+	lbp.stopCh = make(chan struct{})
 	go lbp.Close()
-	stopped := <-lbp.stopCh
-	if !stopped {
+	_, isOpen := <-lbp.stopCh
+	if isOpen {
 		t.Fatal("Close did not send a stop message on the proper channel")
 	}
 }
@@ -106,7 +106,7 @@ func TestExecServer(t *testing.T) {
 		MachineName: machineName,
 		Executor:    fe,
 		addrCh:      make(chan string, 1),
-		stopCh:      make(chan bool, 1),
+		stopCh:      make(chan struct{}),
 	}
 
 	finalErr := make(chan error)