RSDK-8663: Optimize away gostream concurrency complexity for camera c…

…lients. (viamrobotics#4363)

components/camera/client.go

@@ -118,6 +118,8 @@ func getExtra(ctx context.Context) (*structpb.Struct, error) {
 	return ext, nil
 }
 
+// RSDK-8663: This method signature is depended on by the `camera.serviceServer` optimization that
+// avoids using an image stream just to get a single image.
 func (c *client) Read(ctx context.Context) (image.Image, func(), error) {
 	ctx, span := trace.StartSpan(ctx, "camera::client::Read")
 	defer span.End()

components/camera/server.go

@@ -36,6 +36,12 @@ func NewRPCServiceServer(coll resource.APIResourceCollection[Camera]) interface{
 	return &serviceServer{coll: coll, logger: logger, imgTypes: imgTypes}
 }
 
+// ReadImager is an interface that cameras can implement when they allow for returning a single
+// image object.
+type ReadImager interface {
+	Read(ctx context.Context) (image.Image, func(), error)
+}
+
 // GetImage returns an image from a camera of the underlying robot. If a specific MIME type
 // is requested and is not available, an error is returned.
 func (s *serviceServer) GetImage(
@@ -75,7 +81,20 @@ func (s *serviceServer) GetImage(
 	ext := req.Extra.AsMap()
 	ctx = NewContext(ctx, ext)
 
-	img, release, err := ReadImage(gostream.WithMIMETypeHint(ctx, req.MimeType), cam)
+	var img image.Image
+	var release func()
+	switch castedCam := cam.(type) {
+	case ReadImager:
+		// RSDK-8663: If available, call a method that reads exactly one image. The default
+		// `ReadImage` implementation will otherwise create a gostream `Stream`, call `Next` and
+		// `Close` the stream. However, between `Next` and `Close`, the stream may have pulled a
+		// second image from the underlying camera. This is particularly noticeable on camera
+		// clients. Where a second `GetImage` request can be processed/returned over the
+		// network. Just to be discarded.
+		img, release, err = castedCam.Read(ctx)
+	default:
+		img, release, err = ReadImage(gostream.WithMIMETypeHint(ctx, req.MimeType), cam)
+	}
 	if err != nil {
 		return nil, err
 	}
@@ -84,6 +103,7 @@ func (s *serviceServer) GetImage(
 			release()
 		}
 	}()
+
 	actualMIME, _ := utils.CheckLazyMIMEType(req.MimeType)
 	resp := pb.GetImageResponse{
 		MimeType: actualMIME,

components/camera/server_test.go

@@ -506,3 +506,17 @@ func TestServer(t *testing.T) {
 		test.That(t, err.Error(), test.ShouldContainSubstring, errStreamFailed.Error())
 	})
 }
+
+//nolint
+func TestCameraClientIsReadImager(t *testing.T) {
+	// RSDK-8663: Enforce that a camera client always satisfies the optimized `ReadImager`
+	// interface.
+	cameraClient, err := camera.NewClientFromConn(nil, nil, "", resource.Name{}, nil)
+	test.That(t, err, test.ShouldBeNil)
+
+	if _, isReadImager := cameraClient.(camera.ReadImager); isReadImager {
+		// Success
+	} else {
+		t.Fatalf("Camera client is expected to be a `ReadImager`. Client type: %T", cameraClient)
+	}
+}