packer/plugin: Managed clients for automatic cleanup

packer.go

@@ -8,7 +8,6 @@ import (
 	"log"
 	"os"
 	"os/exec"
-	"time"
 )
 
 func main() {
@@ -41,6 +40,6 @@ func main() {
 	}
 
 	exitCode, _ := env.Cli(os.Args[1:])
-	time.Sleep(1 * time.Second)
+	plugin.CleanupClients()
 	os.Exit(exitCode)
 }

packer/plugin/client.go

@@ -8,15 +8,39 @@ import (
 	"log"
 	"os/exec"
 	"strings"
+	"sync"
 	"time"
 )
 
+// This is a slice of the "managed" clients which are cleaned up when
+// calling Cleanup
+var managedClients = make([]*client, 0, 5)
+
 type client struct {
 	cmd *exec.Cmd
 	exited bool
 	doneLogging bool
 }
 
+// This makes sure all the managed subprocesses are killed and properly
+// logged. This should be called before the parent process running the
+// plugins exits.
+func CleanupClients() {
+	// Kill all the managed clients in parallel and use a WaitGroup
+	// to wait for them all to finish up.
+	var wg sync.WaitGroup
+	for _, client := range managedClients {
+		wg.Add(1)
+
+		go func() {
+			client.Kill()
+			wg.Done()
+		}()
+	}
+
+	wg.Wait()
+}
+
 func NewClient(cmd *exec.Cmd) *client {
 	return &client{
 		cmd,
@@ -25,6 +49,12 @@ func NewClient(cmd *exec.Cmd) *client {
 	}
 }
 
+func NewManagedClient(cmd *exec.Cmd) (result *client) {
+	result = NewClient(cmd)
+	managedClients = append(managedClients, result)
+	return
+}
+
 func (c *client) Exited() bool {
 	return c.exited
 }

packer/plugin/command.go

@@ -52,7 +52,7 @@ func (c *cmdCommand) checkExit(p interface{}, cb func()) {
 //
 // This function guarantees the subprocess will end in a timely manner.
 func Command(cmd *exec.Cmd) (result packer.Command, err error) {
-	cmdClient := NewClient(cmd)
+	cmdClient := NewManagedClient(cmd)
 	address, err := cmdClient.Start()
 	if err != nil {
 		return