Inject errors into ioutil.ReadDir calls from devices.go

Added ioutilReadDir variable and test following the pattern
established by osLstat.

Signed-off-by: Steve Powell <spowell@pivotal.io>

devices/devices.go

@@ -19,7 +19,8 @@ var (
 
 // Testing dependencies
 var (
-	osLstat = os.Lstat
+	osLstat       = os.Lstat
+	ioutilReadDir = ioutil.ReadDir
 )
 
 type Device struct {
@@ -92,7 +93,7 @@ func GetHostDeviceNodes() ([]*Device, error) {
 }
 
 func getDeviceNodes(path string) ([]*Device, error) {
-	files, err := ioutil.ReadDir(path)
+	files, err := ioutilReadDir(path)
 	if err != nil {
 		return nil, err
 	}

devices/devices_test.go

@@ -19,3 +19,43 @@ func TestGetDeviceLstatFailure(t *testing.T) {
 		t.Fatalf("Unexpected error %v, expected %v", err, testError)
 	}
 }
+
+func TestGetHostDeviceNodesIoutilReadDirFailure(t *testing.T) {
+	testError := errors.New("test error")
+
+	// Override ioutil.ReadDir to inject error.
+	ioutilReadDir = func(dirname string) ([]os.FileInfo, error) {
+		return nil, testError
+	}
+
+	_, err := GetHostDeviceNodes()
+	if err != testError {
+		t.Fatalf("Unexpected error %v, expected %v", err, testError)
+	}
+}
+
+func TestGetHostDeviceNodesIoutilReadDirDeepFailure(t *testing.T) {
+	testError := errors.New("test error")
+	called := false
+
+	// Override ioutil.ReadDir to inject error after the first call.
+	ioutilReadDir = func(dirname string) ([]os.FileInfo, error) {
+		if called {
+			return nil, testError
+		}
+		called = true
+
+		// Provoke a second call.
+		fi, err := os.Lstat("/tmp")
+		if err != nil {
+			t.Fatalf("Unexpected error %v", err)
+		}
+
+		return []os.FileInfo{fi}, nil
+	}
+
+	_, err := GetHostDeviceNodes()
+	if err != testError {
+		t.Fatalf("Unexpected error %v, expected %v", err, testError)
+	}
+}