Fix bugs, fix warnings and update disruptor. (#1021)

* Update disruptor and fix warnings.

* Fix bug that holds the memory to all recorded events in disruptor.

(cherry picked from commit 2e9b2f0)

Author: Bogdan Drutu

build.gradle

@@ -148,7 +148,7 @@ subprojects {
                 auto_service: 'com.google.auto.service:auto-service:1.0-rc3',
                 byte_buddy: 'net.bytebuddy:byte-buddy:1.7.10',
                 config: 'com.typesafe:config:1.2.1',
-                disruptor: 'com.lmax:disruptor:3.3.6',
+                disruptor: 'com.lmax:disruptor:3.3.8',
                 errorprone: "com.google.errorprone:error_prone_annotations:${errorProneVersion}",
                 findbugs_annotations: "com.google.code.findbugs:annotations:${findBugsVersion}",
                 google_auth: "com.google.auth:google-auth-library-credentials:${googleAuthVersion}",

impl/src/main/java/io/opencensus/impl/internal/DisruptorEventQueue.java

@@ -24,7 +24,6 @@
 import com.lmax.disruptor.dsl.ProducerType;
 import io.opencensus.implcore.internal.DaemonThreadFactory;
 import io.opencensus.implcore.internal.EventQueue;
-import java.util.concurrent.Executors;
 import javax.annotation.Nullable;
 import javax.annotation.concurrent.ThreadSafe;
 
@@ -101,19 +100,20 @@ public final class DisruptorEventQueue implements EventQueue {
   private final RingBuffer<DisruptorEvent> ringBuffer;
 
   // Creates a new EventQueue. Private to prevent creation of non-singleton instance.
-  // Suppress warnings for disruptor.handleEventsWith and Disruptor constructor
-  @SuppressWarnings({"deprecation", "unchecked", "varargs"})
+  // Suppress warnings for disruptor.handleEventsWith.
+  @SuppressWarnings({"unchecked"})
   private DisruptorEventQueue() {
-    // Create new Disruptor for processing. Note that this uses a single thread for processing; this
-    // ensures that the event handler can take unsynchronized actions whenever possible.
+    // Create new Disruptor for processing. Note that Disruptor creates a single thread per
+    // consumer (see https://github.com/LMAX-Exchange/disruptor/issues/121 for details);
+    // this ensures that the event handler can take unsynchronized actions whenever possible.
     disruptor =
-        new Disruptor<DisruptorEvent>(
-            new DisruptorEventFactory(),
+        new Disruptor<>(
+            DisruptorEventFactory.INSTANCE,
             DISRUPTOR_BUFFER_SIZE,
-            Executors.newSingleThreadExecutor(new DaemonThreadFactory("OpenCensus.Disruptor")),
+            new DaemonThreadFactory("OpenCensus.Disruptor"),
             ProducerType.MULTI,
             new SleepingWaitStrategy());
-    disruptor.handleEventsWith(new DisruptorEventHandler());
+    disruptor.handleEventsWith(DisruptorEventHandler.INSTANCE);
     disruptor.start();
     ringBuffer = disruptor.getRingBuffer();
   }
@@ -145,10 +145,12 @@ public void enqueue(Entry entry) {
 
   // An event in the {@link EventQueue}. Just holds a reference to an EventQueue.Entry.
   private static final class DisruptorEvent {
-    @Nullable private Entry entry;
+    // TODO(bdrutu): Investigate if volatile is needed. This object is shared between threads so
+    // intuitively this variable must be volatile.
+    @Nullable private volatile Entry entry = null;
 
     // Sets the EventQueueEntry associated with this DisruptorEvent.
-    void setEntry(Entry entry) {
+    void setEntry(@Nullable Entry entry) {
       this.entry = entry;
     }
 
@@ -160,7 +162,9 @@ Entry getEntry() {
   }
 
   // Factory for DisruptorEvent.
-  private static final class DisruptorEventFactory implements EventFactory<DisruptorEvent> {
+  private enum DisruptorEventFactory implements EventFactory<DisruptorEvent> {
+    INSTANCE;
+
     @Override
     public DisruptorEvent newInstance() {
       return new DisruptorEvent();
@@ -171,12 +175,17 @@ public DisruptorEvent newInstance() {
    * Every event that gets added to {@link EventQueue} will get processed here. Just calls the
    * underlying process() method.
    */
-  private static final class DisruptorEventHandler implements EventHandler<DisruptorEvent> {
+  private enum DisruptorEventHandler implements EventHandler<DisruptorEvent> {
+    INSTANCE;
+
     @Override
-    // TODO(sebright): Fix the Checker Framework warning.
-    @SuppressWarnings("nullness")
     public void onEvent(DisruptorEvent event, long sequence, boolean endOfBatch) {
-      event.getEntry().process();
+      Entry entry = event.getEntry();
+      if (entry != null) {
+        entry.process();
+      }
+      // Remove the reference to the previous entry to allow the memory to be gc'ed.
+      event.setEntry(null);
     }
   }
 }