When finding cameras on linux systems, look at camera ID before path …

…or video* (pion#476)

pkg/driver/camera/camera_linux.go

@@ -81,6 +81,7 @@ func init() {
 // Initialize finds and registers camera devices. This is part of an experimental API.
 func Initialize() {
 	discovered := make(map[string]struct{})
+	discover(discovered, "/dev/v4l/by-id/*")
 	discover(discovered, "/dev/v4l/by-path/*")
 	discover(discovered, "/dev/video*")
 }

pkg/driver/camera/camera_linux_test.go

@@ -57,6 +57,68 @@ func TestDiscover(t *testing.T) {
 		drvs[0].Info().Label,
 		drvs[1].Info().Label,
 	}
+
+	// Returned drivers are unordered. Sort to get static result.
+	sort.Sort(sort.StringSlice(labels))
+
+	expected := longName + LabelSeparator + shortName
+	if label := labels[0]; label != expected {
+		t.Errorf("Expected label: %s, got: %s", expected, label)
+	}
+
+	expectedNoLink := shortName2 + LabelSeparator + shortName2
+	if label := labels[1]; label != expectedNoLink {
+		t.Errorf("Expected label: %s, got: %s", expectedNoLink, label)
+	}
+}
+
+func TestDiscoverByID(t *testing.T) {
+	const (
+		shortName  = "id-unittest-video0"
+		shortName2 = "id-unittest-video1"
+		longName   = "id-unittest-long-device-name:0:1:2:3"
+	)
+
+	dir, err := ioutil.TempDir("", "")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(dir)
+
+	byIdDir := filepath.Join(dir, "v4l", "by-id")
+	if err := os.MkdirAll(byIdDir, 0755); err != nil {
+		t.Fatal(err)
+	}
+	if err := ioutil.WriteFile(filepath.Join(dir, shortName), []byte{}, 0644); err != nil {
+		t.Fatal(err)
+	}
+	if err := ioutil.WriteFile(filepath.Join(dir, shortName2), []byte{}, 0644); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Symlink(
+		filepath.Join(dir, shortName),
+		filepath.Join(byIdDir, longName),
+	); err != nil {
+		t.Fatal(err)
+	}
+
+	discovered := make(map[string]struct{})
+	discover(discovered, filepath.Join(byIdDir, "*"))
+	discover(discovered, filepath.Join(dir, "id-unittest-video*"))
+
+	drvs := driver.GetManager().Query(func(d driver.Driver) bool {
+		// Ignore real cameras.
+		return d.Info().DeviceType == driver.Camera && strings.Contains(d.Info().Label, "id-unittest")
+	})
+	if len(drvs) != 2 {
+		t.Fatalf("Expected 2 driver, got %d drivers", len(drvs))
+	}
+
+	labels := []string{
+		drvs[0].Info().Label,
+		drvs[1].Info().Label,
+	}
+
 	// Returned drivers are unordered. Sort to get static result.
 	sort.Sort(sort.StringSlice(labels))