Implement flush coalescing in OkHttp.

okhttp/src/main/java/io/grpc/okhttp/AsyncFrameWriter.java

@@ -25,6 +25,7 @@
 import java.io.IOException;
 import java.net.Socket;
 import java.util.List;
+import java.util.concurrent.atomic.AtomicLong;
 import java.util.logging.Level;
 import java.util.logging.Logger;
 import okio.Buffer;
@@ -36,10 +37,12 @@ class AsyncFrameWriter implements FrameWriter {
   // Although writes are thread-safe, we serialize them to prevent consuming many Threads that are
   // just waiting on each other.
   private final SerializingExecutor executor;
-  private final OkHttpClientTransport transport;
+  private final TransportExceptionHandler transportExceptionHandler;
+  private final AtomicLong flushCounter = new AtomicLong();
 
-  public AsyncFrameWriter(OkHttpClientTransport transport, SerializingExecutor executor) {
-    this.transport = transport;
+  public AsyncFrameWriter(
+      TransportExceptionHandler transportExceptionHandler, SerializingExecutor executor) {
+    this.transportExceptionHandler = transportExceptionHandler;
     this.executor = executor;
   }
 
@@ -89,10 +92,17 @@ public void doRun() throws IOException {
 
   @Override
   public void flush() {
+    // keep track of version of flushes to skip flush if another flush task is queued.
+    final long flushCount = flushCounter.incrementAndGet();
+
     executor.execute(new WriteRunnable() {
       @Override
       public void doRun() throws IOException {
-        frameWriter.flush();
+        // There can be a flush starvation if there are continuous flood of flush is queued, this
+        // is not an issue with OkHttp since it flushes if the buffer is full.
+        if (flushCounter.get() == flushCount) {
+          frameWriter.flush();
+        }
       }
     });
   }
@@ -219,9 +229,9 @@ public final void run() {
         }
         doRun();
       } catch (RuntimeException e) {
-        transport.onException(e);
+        transportExceptionHandler.onException(e);
       } catch (Exception e) {
-        transport.onException(e);
+        transportExceptionHandler.onException(e);
       }
     }
 
@@ -233,4 +243,11 @@ public int maxDataLength() {
     return frameWriter == null ? 0x4000 /* 16384, the minimum required by the HTTP/2 spec */
         : frameWriter.maxDataLength();
   }
+
+  /** A class that handles transport exception. */
+  interface TransportExceptionHandler {
+
+    /** Handles exception. */
+    void onException(Throwable throwable);
+  }
 }

okhttp/src/main/java/io/grpc/okhttp/OkHttpClientTransport.java

@@ -55,6 +55,7 @@
 import io.grpc.internal.SharedResourceHolder;
 import io.grpc.internal.StatsTraceContext;
 import io.grpc.internal.TransportTracer;
+import io.grpc.okhttp.AsyncFrameWriter.TransportExceptionHandler;
 import io.grpc.okhttp.internal.ConnectionSpec;
 import io.grpc.okhttp.internal.framed.ErrorCode;
 import io.grpc.okhttp.internal.framed.FrameReader;
@@ -98,7 +99,7 @@
 /**
  * A okhttp-based {@link ConnectionClientTransport} implementation.
  */
-class OkHttpClientTransport implements ConnectionClientTransport {
+class OkHttpClientTransport implements ConnectionClientTransport, TransportExceptionHandler {
   private static final Map<ErrorCode, Status> ERROR_CODE_TO_STATUS = buildErrorCodeToStatusMap();
   private static final Logger log = Logger.getLogger(OkHttpClientTransport.class.getName());
   private static final OkHttpClientStream[] EMPTY_STREAM_ARRAY = new OkHttpClientStream[0];
@@ -728,7 +729,8 @@ int getPendingStreamSize() {
   /**
    * Finish all active streams due to an IOException, then close the transport.
    */
-  void onException(Throwable failureCause) {
+  @Override
+  public void onException(Throwable failureCause) {
     Preconditions.checkNotNull(failureCause, "failureCause");
     Status status = Status.UNAVAILABLE.withCause(failureCause);
     startGoAway(0, ErrorCode.INTERNAL_ERROR, status);

okhttp/src/test/java/io/grpc/okhttp/AsyncFrameWriterTest.java

@@ -0,0 +1,124 @@
+/*
+ * Copyright 2018 The gRPC Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.grpc.okhttp;
+
+import static org.mockito.Matchers.anyBoolean;
+import static org.mockito.Matchers.anyInt;
+import static org.mockito.Mockito.inOrder;
+import static org.mockito.Mockito.verify;
+import static org.mockito.internal.verification.VerificationModeFactory.times;
+
+import io.grpc.internal.SerializingExecutor;
+import io.grpc.okhttp.AsyncFrameWriter.TransportExceptionHandler;
+import io.grpc.okhttp.internal.framed.FrameWriter;
+import java.io.IOException;
+import java.net.Socket;
+import java.util.Queue;
+import java.util.concurrent.ConcurrentLinkedQueue;
+import java.util.concurrent.Executor;
+import org.junit.Before;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mockito.InOrder;
+import org.mockito.Mock;
+import org.mockito.runners.MockitoJUnitRunner;
+
+@RunWith(MockitoJUnitRunner.class)
+public class AsyncFrameWriterTest {
+
+  @Mock private Socket socket;
+  @Mock private FrameWriter frameWriter;
+
+  private QueueingExecutor queueingExecutor = new QueueingExecutor();
+  private TransportExceptionHandler transportExceptionHandler =
+      new EscalatingTransportErrorHandler();
+  private AsyncFrameWriter asyncFrameWriter =
+      new AsyncFrameWriter(transportExceptionHandler, new SerializingExecutor(queueingExecutor));
+
+  @Before
+  public void setUp() throws Exception {
+    asyncFrameWriter.becomeConnected(frameWriter, socket);
+  }
+
+  @Test
+  public void noCoalesceRequired() throws IOException {
+    asyncFrameWriter.ping(true, 0, 1);
+    asyncFrameWriter.flush();
+    queueingExecutor.runAll();
+
+    verify(frameWriter, times(1)).ping(anyBoolean(), anyInt(), anyInt());
+    verify(frameWriter, times(1)).flush();
+  }
+
+  @Test
+  public void flushCoalescing_shouldNotMergeTwoDistinctFlushes() throws IOException {
+    asyncFrameWriter.ping(true, 0, 1);
+    asyncFrameWriter.flush();
+    queueingExecutor.runAll();
+
+    asyncFrameWriter.ping(true, 0, 2);
+    asyncFrameWriter.flush();
+    queueingExecutor.runAll();
+
+    verify(frameWriter, times(2)).ping(anyBoolean(), anyInt(), anyInt());
+    verify(frameWriter, times(2)).flush();
+  }
+
+  @Test
+  public void flushCoalescing_shouldMergeTwoQueuedFlushes() throws IOException {
+    asyncFrameWriter.ping(true, 0, 1);
+    asyncFrameWriter.flush();
+    asyncFrameWriter.ping(true, 0, 2);
+    asyncFrameWriter.flush();
+
+    queueingExecutor.runAll();
+
+    InOrder inOrder = inOrder(frameWriter);
+    inOrder.verify(frameWriter, times(2)).ping(anyBoolean(), anyInt(), anyInt());
+    inOrder.verify(frameWriter).flush();
+  }
+
+  /**
+   * Executor queues incoming runnables instead of running it. Runnables can be invoked via {@link
+   * QueueingExecutor#runAll} in serial order.
+   */
+  private static class QueueingExecutor implements Executor {
+
+    private final Queue<Runnable> runnables = new ConcurrentLinkedQueue<Runnable>();
+
+    @Override
+    public void execute(Runnable command) {
+      runnables.add(command);
+    }
+
+    public void runAll() {
+      Runnable r;
+      while ((r = runnables.poll()) != null) {
+        r.run();
+      }
+    }
+  }
+
+  /** Rethrows as Assertion error. */
+  private static class EscalatingTransportErrorHandler implements TransportExceptionHandler {
+
+    @Override
+    public void onException(Throwable throwable) {
+      throw new AssertionError(throwable);
+    }
+  }
+}