Fixed concurrency issue on cache used for circles rendering

Without synchronization lock, concurrent rendering of images including
circles could lead to glitches as reported in issue #248

source/net/yacy/visualization/CircleTool.java

@@ -24,17 +24,28 @@ import java.util.ArrayList;
 import java.util.HashSet;
 import java.util.List;
 import java.util.Set;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.locks.ReentrantLock;
 
 
 public class CircleTool {
 
-    private static List<int[]> circles = new ArrayList<>();
+	/** Cache of calculated circles */
+    private static final List<int[]> CIRCLES_CACHE = new ArrayList<>();
+    
+    /** Synchronization lock for cache access */
+    private static final ReentrantLock CIRCLES_CACHE_LOCK = new ReentrantLock();
     
     public static void clearcache() {
-        circles.clear();
+    	CIRCLES_CACHE_LOCK.lock();
+    	try {
+    		CIRCLES_CACHE.clear();
+    	} finally {
+    		CIRCLES_CACHE_LOCK.unlock();
+    	}
     }
 
-    private static int[] getCircleCoords(final short radius) {
+    private static int[] getCircleCoords(final short radius, final List<int[]> circles) {
         if (radius - 1 < circles.size()) return circles.get(radius - 1);
 
         // read some lines from known circles
@@ -90,10 +101,23 @@ public class CircleTool {
     }
 
     public static void circle(final RasterPlotter matrix, final int xc, final int yc, final int radius, final int intensity) {
-        if (radius == 0) {
-            //matrix.plot(xc, yc, 100);
-        } else {
-            final int[] c = getCircleCoords((short) radius);
+        if (radius != 0) {
+			final int[] c;
+			try {
+				if (CIRCLES_CACHE_LOCK.tryLock(1, TimeUnit.SECONDS)) {
+					try {
+						c = getCircleCoords((short) radius, CIRCLES_CACHE);
+					} finally {
+						CIRCLES_CACHE_LOCK.unlock();
+					}
+				} else {
+					/* Cache is too busy : let's calculate without it */
+					c = getCircleCoords((short) radius, new ArrayList<>());
+				}
+			} catch (final InterruptedException e) {
+				Thread.currentThread().interrupt(); // preserve thread interrupted state
+				return;
+			}
             short x, y;
             short limit = (short) c.length;
             int co;
@@ -116,11 +140,23 @@ public class CircleTool {
         while (fromArc < 0  ) fromArc +=360;
         while (  toArc > 360)   toArc -=360;
         while (  toArc < 0  )   toArc +=360;
-        if (radius == 0) {
-            //matrix.plot(xc, yc, 100);
-        } else {
-            int[] c = getCircleCoords((short) radius);
-            if (c == null) c = getCircleCoords((short) radius);
+        if (radius != 0) {
+			final int[] c;
+			try {
+				if (CIRCLES_CACHE_LOCK.tryLock(1, TimeUnit.SECONDS)) {
+					try {
+						c = getCircleCoords((short) radius, CIRCLES_CACHE);
+					} finally {
+						CIRCLES_CACHE_LOCK.unlock();
+					}
+				} else {
+					/* Cache is too busy : let's calculate without it */
+					c = getCircleCoords((short) radius, new ArrayList<>());
+				}
+			} catch (final InterruptedException e) {
+				Thread.currentThread().interrupt(); // preserve thread interrupted state
+				return;
+			}
             final short q = (short) c.length;
             final short q2 = (short) (q * 2);
             final short q3 = (short) (q * 3);

test/java/net/yacy/visualization/CircleToolTest.java

@@ -0,0 +1,119 @@
+// CircleToolTest.java
+// Copyright 2018 by luccioman; https://github.com/luccioman
+//
+// This is a part of YaCy, a peer-to-peer based web search engine
+//
+// LICENSE
+//
+// This program is free software; you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation; either version 2 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program; if not, write to the Free Software
+// Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+
+package net.yacy.visualization;
+
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+import java.util.ArrayList;
+import java.util.concurrent.Callable;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
+
+import org.bouncycastle.util.Arrays;
+import org.junit.Assert;
+import org.junit.Test;
+
+import net.yacy.peers.graphics.EncodedImage;
+
+/**
+ * Unit tests for the {@link CircleTool} class.
+ */
+public class CircleToolTest {
+
+	/**
+	 * Check circle function consistency when run on multiple concurrent threads.
+	 *
+	 * @throws Exception when an unexpected error occurred
+	 */
+	@Test
+	public void testCircleConcurrentConsistency() throws Exception {
+		final int concurrency = 20;
+		final int side = 600;
+		final String ext = "png";
+
+		final Callable<byte[]> drawTask = () -> {
+			final RasterPlotter raster = new RasterPlotter(side, side, RasterPlotter.DrawMode.MODE_SUB, "FFFFFF");
+			for (int radius = side / 4; radius < side / 2; radius++) {
+				raster.setColor(RasterPlotter.GREEN);
+				CircleTool.circle(raster, side / 2, side / 2, radius, 100);
+				raster.setColor(RasterPlotter.RED);
+				CircleTool.circle(raster, side / 2, side / 2, radius, 0, 45);
+			}
+
+			final EncodedImage image = new EncodedImage(raster, ext, true);
+			return image.getImage().getBytes();
+		};
+
+		/* Generate a reference image without concurrency */
+		CircleTool.clearcache();
+		final byte[] refImageBytes = drawTask.call();
+
+		/* Write the reference image to the file system to enable manual visual check */
+		final Path outputPath = Paths.get(System.getProperty("java.io.tmpdir", ""), "CircleToolTest." + ext);
+		try {
+			Files.write(outputPath, refImageBytes);
+			System.out.println("Wrote CircleTool.circle() test image to file " + outputPath.toAbsolutePath());
+		} catch (final IOException e) {
+			/*
+			 * Even if output file writing failed we do not make the test fail as this is
+			 * not the purpose of the test
+			 */
+			e.printStackTrace();
+		}
+
+		/*
+		 * Generate the same image multiple times in concurrent threads without initial
+		 * cache
+		 */
+		CircleTool.clearcache();
+		final ExecutorService executor = Executors.newFixedThreadPool(concurrency);
+		final ArrayList<Future<byte[]>> futures = new ArrayList<>();
+		for (int i = 0; i < concurrency; i++) {
+			futures.add(executor.submit(drawTask));
+		}
+		try {
+			for (final Future<byte[]> future : futures) {
+				/* Check that all concurrently generated images are equal to the reference */
+				final byte[] imageBytes = future.get();
+				if (!Arrays.areEqual(refImageBytes, imageBytes)) {
+					/* Write the image in error to file system to enable manual visual check */
+					final Path errOutputPath = Paths.get(System.getProperty("java.io.tmpdir", ""),
+							"CircleToolTestError." + ext);
+					try {
+						Files.write(errOutputPath, imageBytes);
+						System.out.println(
+								"Wrote CircleTool.circle() error image to file " + errOutputPath.toAbsolutePath());
+					} catch (final IOException e) {
+						e.printStackTrace();
+					}
+					Assert.fail();
+				}
+			}
+		} finally {
+			executor.shutdown();
+		}
+	}
+
+}