avoid synchronization for method that blocks while fetching zero-queue message (#61)

pulsar-client/src/main/java/com/yahoo/pulsar/client/impl/ConsumerImpl.java

@@ -23,8 +23,6 @@ import java.io.IOException;
 import java.util.BitSet;
 import java.util.NavigableMap;
 import java.util.concurrent.CompletableFuture;
-import java.util.concurrent.ConcurrentHashMap;
-import java.util.concurrent.ConcurrentMap;
 import java.util.concurrent.ConcurrentSkipListMap;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.TimeUnit;
@@ -60,7 +58,6 @@ import io.netty.util.concurrent.GenericFutureListener;
 import static com.yahoo.pulsar.common.api.Commands.readChecksum;
 import static com.yahoo.pulsar.checksum.utils.Crc32cChecksum.computeChecksum;
 import static com.yahoo.pulsar.common.api.Commands.hasChecksum;
-import static com.yahoo.pulsar.common.api.Commands.readChecksum;
 
 public class ConsumerImpl extends ConsumerBase {
 
@@ -79,6 +76,8 @@ public class ConsumerImpl extends ConsumerBase {
     private volatile boolean waitingOnReceiveForZeroQueueSize = false;
 
     private final ReadWriteLock lock = new ReentrantReadWriteLock();
+    
+    private final ReadWriteLock zeroQueueLock;
 
     private final ConcurrentSkipListMap<MessageIdImpl, BitSet> batchMessageAckTracker;
 
@@ -104,6 +103,11 @@ public class ConsumerImpl extends ConsumerBase {
         } else {
             stats = ConsumerStats.CONSUMER_STATS_DISABLED;
         }
+        if (conf.getReceiverQueueSize() <= 1) {
+            zeroQueueLock = new ReentrantReadWriteLock();
+        } else {
+            zeroQueueLock = null;
+        }
         grabCnx();
     }
 
@@ -142,7 +146,13 @@ public class ConsumerImpl extends ConsumerBase {
     @Override
     protected Message internalReceive() throws PulsarClientException {
         if (conf.getReceiverQueueSize() == 0) {
-            return fetchSingleMessageFromBroker();
+            checkArgument(zeroQueueLock != null, "Receiver queue size can't be modified");
+            zeroQueueLock.writeLock().lock();
+            try {
+                return fetchSingleMessageFromBroker();
+            } finally {
+                zeroQueueLock.writeLock().unlock();
+            }
         }
         Message message;
         try {
@@ -190,7 +200,7 @@ public class ConsumerImpl extends ConsumerBase {
         return result;
     }
 
-    private synchronized Message fetchSingleMessageFromBroker() throws PulsarClientException {
+    private Message fetchSingleMessageFromBroker() throws PulsarClientException {
         checkArgument(conf.getReceiverQueueSize() == 0);
 
         // Just being cautious
@@ -203,8 +213,10 @@ public class ConsumerImpl extends ConsumerBase {
         try {
             // is cnx is null or if the connection breaks the connectionOpened function will send the flow again
             waitingOnReceiveForZeroQueueSize = true;
-            if (isConnected()) {
-                receiveMessages(cnx(), 1);
+            synchronized (this) {
+                if (isConnected()) {
+                    receiveMessages(cnx(), 1);
+                }
             }
             do {
                 message = incomingMessages.take();