Mac: Fix thumbnail rendering memory leak

cyanfish#308

NAPS2.Images.Mac/MacBitmapHelper.cs

@@ -11,7 +11,8 @@ public static NSBitmapImageRep CopyRep(NSBitmapImageRep original)
         var copy = CreateRepForDrawing(w, h);
         using var c = CreateContext(copy, false, true);
         CGRect rect = new CGRect(0, 0, w, h);
-        c.DrawImage(rect, original.AsCGImage(ref rect, null, null));
+        using var cgImage = original.AsCGImage(ref rect, null, null);
+        c.DrawImage(rect, cgImage);
         return copy;
     }
 

NAPS2.Images.Mac/MacImageContext.cs

@@ -34,6 +34,7 @@ protected override IMemoryImage LoadCore(Stream stream, ImageFileFormat format)
             {
                 if (reps.Length > 1)
                 {
+                    image.Dispose();
                     return CreateImage(reps[0]);
                 }
                 return new MacImage(this, image);
@@ -69,6 +70,7 @@ protected override void LoadFramesCore(Action<IMemoryImage> produceImage, Stream
         }
         finally
         {
+            image.Dispose();
             foreach (var rep in reps)
             {
                 rep.Dispose();

NAPS2.Images.Mac/MacImageTransformer.cs

@@ -32,7 +32,8 @@ protected override MacImage PerformTransform(MacImage image, RotationTransform t
         c.ConcatCTM(CGAffineTransform.Multiply(CGAffineTransform.Multiply(t1, t2), t3));
 
         CGRect rect = new CGRect(0, 0, image.Width, image.Height);
-        c.DrawImage(rect, image.Rep.AsCGImage(ref rect, null, null));
+        using var cgImage = image.Rep.AsCGImage(ref rect, null, null);
+        c.DrawImage(rect, cgImage);
         image.Dispose();
         return newImage;
     }
@@ -46,7 +47,8 @@ protected override MacImage PerformTransform(MacImage image, ResizeTransform tra
         using CGBitmapContext c = MacBitmapHelper.CreateContext(newImage);
         CGRect rect = new CGRect(0, 0, transform.Width, transform.Height);
         // TODO: This changes the image size to match the original which we probably don't want.
-        c.DrawImage(rect, image.Rep.AsCGImage(ref rect, null, null));
+        using var cgImage = image.Rep.AsCGImage(ref rect, null, null);
+        c.DrawImage(rect, cgImage);
         image.Dispose();
         return newImage;
     }

NAPS2.Lib/Images/ThumbnailRenderQueue.cs

@@ -99,7 +99,7 @@ private void RenderThumbnails()
                     {
                         var thumb = worker != null
                             ? RenderThumbnailWithWorker(worker, imageToRender, thumbnailSize)
-                            : _thumbnailRenderer.Render(imageToRender, thumbnailSize);
+                            : _thumbnailRenderer.Render(imageToRender, thumbnailSize).Result;
 
                         if (!ThumbnailStillNeedsRendering(next, thumbnailSize))
                         {

NAPS2.Sdk/Images/ThumbnailRenderer.cs

@@ -15,21 +15,27 @@ public ThumbnailRenderer(ImageContext imageContext)
         _imageContext = imageContext;
     }
 
-    public IMemoryImage Render(ProcessedImage processedImage, int outputSize)
+    public Task<IMemoryImage> Render(ProcessedImage processedImage, int outputSize)
     {
-        var image = _imageContext.RenderWithoutTransforms(processedImage);
-        var transformList = processedImage.TransformState.Transforms;
-        if (!processedImage.TransformState.IsEmpty)
+        // On Mac it's really important this runs as a task, as Mac memory management depends on autorelease pools which
+        // in Xamarin operate at the task level. Otherwise a long-running thumbnail render thread will leak memory like
+        // a sieve.
+        return Task.Run(() =>
         {
-            // When we have additional transformations, performing them on a large original image may be quite slow.
-            // On the other hand, scaling the image to the thumbnail size first can result in transforms losing detail.
-            // As a middle ground we scale to an "oversampled" size first.
-            double oversampledSize = outputSize * OVERSAMPLE;
-            double scaleFactor = Math.Min(oversampledSize / image.Height, oversampledSize / image.Width);
-            scaleFactor = Math.Min(scaleFactor, 1);
-            transformList = transformList.Insert(0, new ScaleTransform(scaleFactor));
-        }
-        transformList = transformList.Add(new ThumbnailTransform(outputSize));
-        return _imageContext.PerformAllTransforms(image, transformList);
+            var image = _imageContext.RenderWithoutTransforms(processedImage);
+            var transformList = processedImage.TransformState.Transforms;
+            if (!processedImage.TransformState.IsEmpty)
+            {
+                // When we have additional transformations, performing them on a large original image may be quite slow.
+                // On the other hand, scaling the image to the thumbnail size first can result in transforms losing detail.
+                // As a middle ground we scale to an "oversampled" size first.
+                double oversampledSize = outputSize * OVERSAMPLE;
+                double scaleFactor = Math.Min(oversampledSize / image.Height, oversampledSize / image.Width);
+                scaleFactor = Math.Min(scaleFactor, 1);
+                transformList = transformList.Insert(0, new ScaleTransform(scaleFactor));
+            }
+            transformList = transformList.Add(new ThumbnailTransform(outputSize));
+            return _imageContext.PerformAllTransforms(image, transformList);
+        });
     }
 }

NAPS2.Sdk/Remoting/Worker/WorkerServiceImpl.cs

@@ -209,7 +209,7 @@ public override async Task<SendMapiEmailResponse> SendMapiEmail(SendMapiEmailReq
         }
     }
 
-    public override Task<RenderThumbnailResponse> RenderThumbnail(RenderThumbnailRequest request,
+    public override async Task<RenderThumbnailResponse> RenderThumbnail(RenderThumbnailRequest request,
         ServerCallContext context)
     {
         using var callRef = StartCall();
@@ -221,16 +221,16 @@ public override Task<RenderThumbnailResponse> RenderThumbnail(RenderThumbnailReq
             };
             using var image =
                 ImageSerializer.Deserialize(_scanningContext, request.Image, deserializeOptions);
-            var thumbnail = _thumbnailRenderer.Render(image, request.Size);
+            var thumbnail = await _thumbnailRenderer.Render(image, request.Size);
             var stream = thumbnail.SaveToMemoryStream(ImageFileFormat.Png);
-            return Task.FromResult(new RenderThumbnailResponse
+            return new RenderThumbnailResponse
             {
                 Thumbnail = ByteString.FromStream(stream)
-            });
+            };
         }
         catch (Exception e)
         {
-            return Task.FromResult(new RenderThumbnailResponse { Error = RemotingHelper.ToError(e) });
+            return new RenderThumbnailResponse { Error = RemotingHelper.ToError(e) };
         }
     }