Fix: output device now has queue of size 1. a newer command always overrides the previous one

voting-booth/src/main/java/meerkat/voting/output/AsyncRunnableOutputDevice.java

@@ -11,7 +11,7 @@ import meerkat.voting.output.outputcommands.*;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
-import java.util.concurrent.LinkedBlockingQueue;
+import java.util.concurrent.ArrayBlockingQueue;
 
 /**
  * This is a base class for simple OutputDevices which run asynchronously (as a separate thread).
@@ -21,13 +21,13 @@ import java.util.concurrent.LinkedBlockingQueue;
 public abstract class AsyncRunnableOutputDevice implements BallotOutputDevice, Runnable  {
 
     private Logger logger;
-    private LinkedBlockingQueue<OutputCommand> queue;
+    private ArrayBlockingQueue<OutputCommand> queue;
     private volatile boolean shutDownHasBeenCalled;
 
     public AsyncRunnableOutputDevice() {
         logger = LoggerFactory.getLogger(AsyncRunnableOutputDevice.class);
         logger.info("AsyncRunnableOutputDevice is constructed");
-        queue = new LinkedBlockingQueue<>();
+        queue = new ArrayBlockingQueue<>(1);
         shutDownHasBeenCalled = false;
     }
 
@@ -87,24 +87,28 @@ public abstract class AsyncRunnableOutputDevice implements BallotOutputDevice, R
                                SignedEncryptedBallot signedEncryptedBallot,
                                FutureCallback<Void> callback) {
         logger.debug("Output interface call to commit to ballot");
+        queue.clear();
         queue.add(new CommitOutputCommand(plaintextBallot, signedEncryptedBallot, (OutputDeviceCommitCallback)callback));
     }
 
     @Override
     public void audit(BallotSecrets ballotSecrets, FutureCallback<Void> callback) {
         logger.debug("an interface call to audit");
+        queue.clear();
         queue.add(new AuditOutputCommand(ballotSecrets, (OutputDeviceFinalizeCallback)callback));
     }
 
     @Override
     public void castBallot(FutureCallback<Void> callback) {
         logger.debug("an interface call to cast ballot");
+        queue.clear();
         queue.add(new CastOutputCommand((OutputDeviceFinalizeCallback)callback));
     }
 
     @Override
     public void cancelBallot(FutureCallback<Void> callback) {
         logger.debug("an interface call to cancel the output");
+        queue.clear();
         queue.add(new CancelOutputCommand((ControllerCallback<Void>)callback));
     }