Remove workaround for bug in Java's ReferenceQueue

Sun's Java 5, 6, 7 implementation had a bug [1] where a Reference can be
enqueued and dequeued twice on the same reference queue due to a race
condition within ReferenceQueue.enqueue(Reference).

This bug was fixed for Java 8 [2] hence remove the workaround.

[1] http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6837858
[2] http://hg.openjdk.java.net/jdk8/jdk8/jdk/rev/858c75eb83b5

Change-Id: I2deeb607e3d237f9f825a207533acdee305c7e73
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/WindowCache.java

@@ -496,31 +496,16 @@ public class WindowCache {
 	private void gc() {
 		Ref r;
 		while ((r = (Ref) queue.poll()) != null) {
-			// Sun's Java 5 and 6 implementation have a bug where a Reference
+			clear(r);
-			// can be enqueued and dequeued twice on the same reference queue
+
-			// due to a race condition within ReferenceQueue.enqueue(Reference).
+			final int s = slot(r.pack, r.position);
-			//
+			final Entry e1 = table.get(s);
-			// http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6837858
+			for (Entry n = e1; n != null; n = n.next) {
-			//
+				if (n.ref == r) {
-			// We CANNOT permit a Reference to come through us twice, as it will
+					n.dead = true;
-			// skew the resource counters we maintain. Our canClear() check here
-			// provides a way to skip the redundant dequeues, if any.
-			//
-			if (r.canClear()) {
-				clear(r);
-
-				boolean found = false;
-				final int s = slot(r.pack, r.position);
-				final Entry e1 = table.get(s);
-				for (Entry n = e1; n != null; n = n.next) {
-					if (n.ref == r) {
-						n.dead = true;
-						found = true;
-						break;
-					}
-				}
-				if (found)
 					table.compareAndSet(s, e1, clean(e1));
+					break;
+				}
 			}
 		}
 	}
@@ -581,8 +566,6 @@ public class WindowCache {
 
 		long lastAccess;
 
-		private boolean cleared;
-
 		protected Ref(final PackFile pack, final long position,
 				final ByteWindow v, final ReferenceQueue<ByteWindow> queue) {
 			super(v, queue);
@@ -590,13 +573,6 @@ public class WindowCache {
 			this.position = position;
 			this.size = v.size();
 		}
-
-		final synchronized boolean canClear() {
-			if (cleared)
-				return false;
-			cleared = true;
-			return true;
-		}
 	}
 
 	private static final class Lock {