Only release event loops when unused

Resolves #926. Transports can still be alive when the Server shuts down,
but they are using the worker event loops. Only release the worker event
loops when all transport's channels are closed.

netty/src/main/java/io/grpc/netty/NettyServer.java

@@ -48,6 +48,8 @@ import io.netty.channel.EventLoopGroup;
 import io.netty.channel.ServerChannel;
 import io.netty.channel.socket.nio.NioServerSocketChannel;
 import io.netty.handler.ssl.SslContext;
+import io.netty.util.AbstractReferenceCounted;
+import io.netty.util.ReferenceCounted;
 
 import java.io.IOException;
 import java.net.SocketAddress;
@@ -74,6 +76,7 @@ public class NettyServer implements Server {
   private Channel channel;
   private final int flowControlWindow;
   private final int maxMessageSize;
+  private final ReferenceCounted eventLoopReferenceCounter = new EventLoopReferenceCounter();
 
   NettyServer(SocketAddress address, Class<? extends ServerChannel> channelType,
               @Nullable EventLoopGroup bossGroup, @Nullable EventLoopGroup workerGroup,
@@ -108,6 +111,12 @@ public class NettyServer implements Server {
     b.childHandler(new ChannelInitializer<Channel>() {
       @Override
       public void initChannel(Channel ch) throws Exception {
+        eventLoopReferenceCounter.retain();
+        ch.closeFuture().addListener(new ChannelFutureListener() {
+          public void operationComplete(ChannelFuture future) {
+            eventLoopReferenceCounter.release();
+          }
+        });
         NettyServerTransport transport
             = new NettyServerTransport(ch, sslContext, maxStreamsPerConnection, flowControlWindow,
                 maxMessageSize);
@@ -142,7 +151,7 @@ public class NettyServer implements Server {
           log.log(Level.WARNING, "Error shutting down server", future.cause());
         }
         listener.serverShutdown();
-        releaseSharedGroups();
+        eventLoopReferenceCounter.release();
       }
     });
   }
@@ -156,20 +165,28 @@ public class NettyServer implements Server {
     }
   }
 
-  private void releaseSharedGroups() {
-    try {
-      if (usingSharedBossGroup && bossGroup != null) {
-        SharedResourceHolder.release(Utils.DEFAULT_BOSS_EVENT_LOOP_GROUP, bossGroup);
-      }
-    } finally {
-      bossGroup = null;
+  class EventLoopReferenceCounter extends AbstractReferenceCounted {
+    @Override
+    protected void deallocate() {
       try {
-        if (usingSharedWorkerGroup && workerGroup != null) {
-          SharedResourceHolder.release(Utils.DEFAULT_WORKER_EVENT_LOOP_GROUP, workerGroup);
+        if (usingSharedBossGroup && bossGroup != null) {
+          SharedResourceHolder.release(Utils.DEFAULT_BOSS_EVENT_LOOP_GROUP, bossGroup);
         }
       } finally {
-        workerGroup = null;
+        bossGroup = null;
+        try {
+          if (usingSharedWorkerGroup && workerGroup != null) {
+            SharedResourceHolder.release(Utils.DEFAULT_WORKER_EVENT_LOOP_GROUP, workerGroup);
+          }
+        } finally {
+          workerGroup = null;
+        }
       }
     }
+
+    @Override
+    public ReferenceCounted touch(Object hint) {
+      return this;
+    }
   }
 }