Fix data race

We try to read cmd.Process in the thread that handles incoming
signals, but there's no synchronization between that goroutine and the
one that sets cmd.Process to a non-nil value.

This might be overkill, e.g. there may be a way to write this with
less code, but I confirmed that this patch addresses the
race when shutting down the program.

Fixes #158.

cli/exec.go

@@ -161,15 +161,24 @@ func ExecCommand(app *kingpin.Application, input ExecCommandInput) {
 	cmd.Stdout = os.Stdout
 	cmd.Stderr = os.Stderr
 
+	if err := cmd.Start(); err != nil {
+		app.Errorf("%v", err)
+		return
+	}
+	// wait for the command to finish
+	waitCh := make(chan error)
 	go func() {
-		sig := <-input.Signals
-		if cmd.Process != nil {
-			cmd.Process.Signal(sig)
-		}
+		waitCh <- cmd.Wait()
 	}()
 
-	var waitStatus syscall.WaitStatus
-	if err := cmd.Run(); err != nil {
+	select {
+	case sig := <-input.Signals:
+		if err = cmd.Process.Signal(sig); err != nil {
+			app.Errorf("%v", err)
+			return
+		}
+	case err := <-waitCh:
+		var waitStatus syscall.WaitStatus
 		if exitError, ok := err.(*exec.ExitError); ok {
 			waitStatus = exitError.Sys().(syscall.WaitStatus)
 			os.Exit(waitStatus.ExitStatus())

cli/exec_test.go

@@ -13,7 +13,7 @@ func ExampleExecCommand() {
 		{Key: "llamas", Data: []byte(`{"AccessKeyID":"ABC","SecretAccessKey":"XYZ"}`)},
 	})
 
-	app := kingpin.New(`aws-vault`, ``)
+	app := kingpin.New("aws-vault", "")
 	ConfigureGlobals(app)
 	ConfigureExecCommand(app)
 	kingpin.MustParse(app.Parse([]string{