Make PngEncoder threadsafe. Fix #24

src/ImageSharp/Formats/Png/PngEncoderCore.cs

@@ -60,6 +60,26 @@ namespace ImageSharp.Formats
         /// </summary>
         private int bytesPerPixel;
 
+        /// <summary>
+        /// The buffer for the sub filter
+        /// </summary>
+        private byte[] sub;
+
+        /// <summary>
+        /// The buffer for the up filter
+        /// </summary>
+        private byte[] up;
+
+        /// <summary>
+        /// The buffer for the average filter
+        /// </summary>
+        private byte[] average;
+
+        /// <summary>
+        /// The buffer for the paeth filter
+        /// </summary>
+        private byte[] paeth;
+
         /// <summary>
         /// Gets or sets the quality of output for images.
         /// </summary>
@@ -356,54 +376,39 @@ namespace ImageSharp.Formats
                 return result;
             }
 
-            byte[] sub = ArrayPool<byte>.Shared.Rent(bytesPerScanline + 1);
-            byte[] up = ArrayPool<byte>.Shared.Rent(bytesPerScanline + 1);
-            byte[] average = ArrayPool<byte>.Shared.Rent(bytesPerScanline + 1);
-            byte[] paeth = ArrayPool<byte>.Shared.Rent(bytesPerScanline + 1);
+            SubFilter.Encode(rawScanline, this.sub, this.bytesPerPixel, bytesPerScanline);
+            int currentTotalVariation = this.CalculateTotalVariation(this.sub, bytesPerScanline);
+            int lowestTotalVariation = currentTotalVariation;
 
-            try
-            {
-                SubFilter.Encode(rawScanline, sub, this.bytesPerPixel, bytesPerScanline);
-                int currentTotalVariation = this.CalculateTotalVariation(sub, bytesPerScanline);
-                int lowestTotalVariation = currentTotalVariation;
+            result = this.sub;
 
-                result = sub;
-
-                UpFilter.Encode(rawScanline, previousScanline, up, bytesPerScanline);
-                currentTotalVariation = this.CalculateTotalVariation(up, bytesPerScanline);
-
-                if (currentTotalVariation < lowestTotalVariation)
-                {
-                    lowestTotalVariation = currentTotalVariation;
-                    result = up;
-                }
+            UpFilter.Encode(rawScanline, previousScanline, this.up, bytesPerScanline);
+            currentTotalVariation = this.CalculateTotalVariation(this.up, bytesPerScanline);
 
-                AverageFilter.Encode(rawScanline, previousScanline, average, this.bytesPerPixel, bytesPerScanline);
-                currentTotalVariation = this.CalculateTotalVariation(average, bytesPerScanline);
+            if (currentTotalVariation < lowestTotalVariation)
+            {
+                lowestTotalVariation = currentTotalVariation;
+                result = this.up;
+            }
 
-                if (currentTotalVariation < lowestTotalVariation)
-                {
-                    lowestTotalVariation = currentTotalVariation;
-                    result = average;
-                }
+            AverageFilter.Encode(rawScanline, previousScanline, this.average, this.bytesPerPixel, bytesPerScanline);
+            currentTotalVariation = this.CalculateTotalVariation(this.average, bytesPerScanline);
 
-                PaethFilter.Encode(rawScanline, previousScanline, paeth, this.bytesPerPixel, bytesPerScanline);
-                currentTotalVariation = this.CalculateTotalVariation(paeth, bytesPerScanline);
+            if (currentTotalVariation < lowestTotalVariation)
+            {
+                lowestTotalVariation = currentTotalVariation;
+                result = this.average;
+            }
 
-                if (currentTotalVariation < lowestTotalVariation)
-                {
-                    result = paeth;
-                }
+            PaethFilter.Encode(rawScanline, previousScanline, this.paeth, this.bytesPerPixel, bytesPerScanline);
+            currentTotalVariation = this.CalculateTotalVariation(this.paeth, bytesPerScanline);
 
-                return result;
-            }
-            finally
+            if (currentTotalVariation < lowestTotalVariation)
             {
-                ArrayPool<byte>.Shared.Return(sub);
-                ArrayPool<byte>.Shared.Return(up);
-                ArrayPool<byte>.Shared.Return(average);
-                ArrayPool<byte>.Shared.Return(paeth);
+                result = this.paeth;
             }
+
+            return result;
         }
 
         /// <summary>
@@ -618,6 +623,14 @@ namespace ImageSharp.Formats
             int resultLength = bytesPerScanline + 1;
             byte[] result = ArrayPool<byte>.Shared.Rent(resultLength);
 
+            if (this.PngColorType != PngColorType.Palette)
+            {
+                this.sub = ArrayPool<byte>.Shared.Rent(resultLength);
+                this.up = ArrayPool<byte>.Shared.Rent(resultLength);
+                this.average = ArrayPool<byte>.Shared.Rent(resultLength);
+                this.paeth = ArrayPool<byte>.Shared.Rent(resultLength);
+            }
+
             byte[] buffer;
             int bufferLength;
             MemoryStream memoryStream = null;
@@ -643,6 +656,14 @@ namespace ImageSharp.Formats
                 ArrayPool<byte>.Shared.Return(previousScanline);
                 ArrayPool<byte>.Shared.Return(rawScanline);
                 ArrayPool<byte>.Shared.Return(result);
+
+                if (this.PngColorType != PngColorType.Palette)
+                {
+                    ArrayPool<byte>.Shared.Return(this.sub);
+                    ArrayPool<byte>.Shared.Return(this.up);
+                    ArrayPool<byte>.Shared.Return(this.average);
+                    ArrayPool<byte>.Shared.Return(this.paeth);
+                }
             }
 
             // Store the chunks in repeated 64k blocks.

tests/ImageSharp.Tests/Formats/Png/PngTests.cs

@@ -6,6 +6,7 @@
 namespace ImageSharp.Tests
 {
     using System.IO;
+    using System.Threading.Tasks;
 
     using Formats;
 
@@ -29,5 +30,23 @@ namespace ImageSharp.Tests
                 }
             }
         }
+
+        [Fact]
+        public void ImageCanSavePngInParallel()
+        {
+            string path = this.CreateOutputDirectory("Png");
+
+            Parallel.ForEach(
+                Files,
+                file =>
+                    {
+                        Image image = file.CreateImage();
+
+                        using (FileStream output = File.OpenWrite($"{path}/{file.FileNameWithoutExtension}.png"))
+                        {
+                            image.Save(output, new PngFormat());
+                        }
+                    });
+        }
     }
 }