Serializing batch execution on per-instance basis for intents

Change-Id: Idda3f4a65e78567302d91ba0070e78d435eea8fd

@@ -155,6 +155,9 @@ public class IntentManager
 
     @Override
     public void execute(IntentOperations operations) {
+        if (operations.operations().isEmpty()) {
+            return;
+        }
         batchService.addIntentOperations(operations);
     }
 

@@ -15,7 +15,7 @@
  */
 package org.onlab.onos.store.intent.impl;
 
-import com.google.common.collect.ImmutableSet;
+import com.google.common.collect.Sets;
 import org.apache.felix.scr.annotations.Activate;
 import org.apache.felix.scr.annotations.Component;
 import org.apache.felix.scr.annotations.Deactivate;
@@ -25,7 +25,9 @@ import org.onlab.onos.net.intent.IntentBatchService;
 import org.onlab.onos.net.intent.IntentOperations;
 import org.slf4j.Logger;
 
-import java.util.HashSet;
+import java.util.Collection;
+import java.util.LinkedList;
+import java.util.Queue;
 import java.util.Set;
 
 import static com.google.common.base.Preconditions.checkNotNull;
@@ -37,7 +39,8 @@ import static org.slf4j.LoggerFactory.getLogger;
 public class DistributedIntentBatchQueue implements IntentBatchService {
 
     private final Logger log = getLogger(getClass());
-    private final Set<IntentOperations> pendingBatches = new HashSet<>();
+    private final Queue<IntentOperations> pendingBatches = new LinkedList<>();
+    private final Set<IntentOperations> currentBatches = Sets.newHashSet();
     private IntentBatchDelegate delegate;
 
     @Activate
@@ -53,18 +56,35 @@ public class DistributedIntentBatchQueue implements IntentBatchService {
     @Override
     public void addIntentOperations(IntentOperations operations) {
         checkState(delegate != null, "No delegate set");
-        pendingBatches.add(operations);
-        delegate.execute(operations);
+        synchronized (this) {
+            pendingBatches.add(operations);
+            if (currentBatches.isEmpty()) {
+                IntentOperations work = pendingBatches.poll();
+                currentBatches.add(work);
+                delegate.execute(work);
+            }
+        }
     }
 
     @Override
     public void removeIntentOperations(IntentOperations operations) {
-        pendingBatches.remove(operations);
+        // we allow at most one outstanding batch at a time
+        synchronized (this) {
+            checkState(currentBatches.remove(operations), "Operations not found in current ops.");
+            checkState(currentBatches.isEmpty(), "More than one outstanding batch.");
+            IntentOperations work = pendingBatches.poll();
+            if (work != null) {
+                currentBatches.add(work);
+                delegate.execute(work);
+            }
+        }
     }
 
     @Override
     public Set<IntentOperations> getIntentOperations() {
-        return ImmutableSet.copyOf(pendingBatches);
+        Set<IntentOperations> set = Sets.newHashSet(currentBatches);
+        set.addAll((Collection) pendingBatches);
+        return set;
     }
 
     @Override