DHT: Fix thread-safety issue in AbstractWriteBuffer

There is a data corruption issue with the 'running' list if a
background thread schedules something onto the buffer while the
application thread is also using it.

Change-Id: I5ba78b98b6632965d677a9c8f209f0cf8320cc3d
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

org.eclipse.jgit.storage.dht/src/org/eclipse/jgit/storage/dht/spi/util/AbstractWriteBuffer.java

@@ -82,6 +82,8 @@ public abstract class AbstractWriteBuffer implements WriteBuffer {
 
 	private final List<Future<?>> running;
 
+	private final Object runningLock;
+
 	private final Semaphore spaceAvailable;
 
 	private int queuedCount;
@@ -102,6 +104,7 @@ public abstract class AbstractWriteBuffer implements WriteBuffer {
 		this.executor = executor;
 		this.bufferSize = bufferSize;
 		this.running = new LinkedList<Future<?>>();
+		this.runningLock = new Object();
 		this.spaceAvailable = new Semaphore(bufferSize);
 	}
 
@@ -189,15 +192,19 @@ public abstract class AbstractWriteBuffer implements WriteBuffer {
 				}
 			}
 
+			synchronized (runningLock) {
 				checkRunningTasks(true);
+			}
 		} finally {
 			flushing = false;
 		}
 	}
 
 	public void abort() throws DhtException {
+		synchronized (runningLock) {
 			checkRunningTasks(true);
 		}
+	}
 
 	private void acquireSpace(int sz) throws DhtException {
 		try {
@@ -259,10 +266,12 @@ public abstract class AbstractWriteBuffer implements WriteBuffer {
 			return;
 		}
 
+		synchronized (runningLock) {
 			if (!flushing)
 				checkRunningTasks(false);
 			running.add(executor.submit(op));
 		}
+	}
 
 	/**
 	 * Wrap a callback to update the buffer.
@@ -284,8 +293,10 @@ public abstract class AbstractWriteBuffer implements WriteBuffer {
 			int size) throws DhtException {
 		int permits = permitsForSize(size);
 		WrappedCallback<T> op = new WrappedCallback<T>(callback, permits);
+		synchronized (runningLock) {
 			checkRunningTasks(false);
 			running.add(op);
+		}
 		return op;
 	}