factor out write path so that easy to replace with other implementation

servlet/src/main/java/io/grpc/servlet/AsyncServletOutputStreamWriter.java

@@ -0,0 +1,240 @@
+/*
+ * Copyright 2019 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.servlet;
+
+import static io.grpc.servlet.ServletServerStream.toHexString;
+import static java.util.logging.Level.FINE;
+import static java.util.logging.Level.FINEST;
+
+import io.grpc.InternalLogId;
+import io.grpc.Status;
+import io.grpc.servlet.ServletServerStream.ServletTransportState;
+import java.io.IOException;
+import java.time.Duration;
+import java.util.Queue;
+import java.util.concurrent.ConcurrentLinkedQueue;
+import java.util.concurrent.atomic.AtomicReference;
+import java.util.concurrent.locks.LockSupport;
+import java.util.logging.Logger;
+import javax.annotation.CheckReturnValue;
+import javax.servlet.AsyncContext;
+import javax.servlet.ServletOutputStream;
+
+/** Handles write actions from the container thread and the application thread. */
+final class AsyncServletOutputStreamWriter {
+
+  private static final Logger logger =
+      Logger.getLogger(AsyncServletOutputStreamWriter.class.getName());
+
+  /**
+   * Memory boundary for write actions.
+   *
+   * <pre>
+   * WriteState curState = writeState.get();  // mark a boundary
+   * doSomething();  // do something within the boundary
+   * boolean successful = writeState.compareAndSet(curState, newState); // try to mark a boundary
+   * if (successful) {
+   *   // state has not changed since
+   *   return;
+   * } else {
+   *   // state is changed by another thread while doSomething(), need recompute
+   * }
+   * </pre>
+   *
+   * <p>There are two threads, the container thread (calling {@code onWritePossible()}) and the
+   * application thread (calling {@code runOrBufferActionItem()}) that reads and updates the
+   * writeState. Only onWritePossible() may turn readyAndEmpty from false to true, and only
+   * runOrBufferActionItem() may turn it from true to false.
+   */
+  private final AtomicReference<WriteState> writeState = new AtomicReference<>(WriteState.DEFAULT);
+
+  private final ServletOutputStream outputStream;
+  private final ServletTransportState transportState;
+  private final InternalLogId logId;
+  private final ActionItem flushAction;
+  private final ActionItem completeAction;
+
+  /**
+   * New write actions will be buffered into this queue if the servlet output stream is not ready or
+   * the queue is not drained.
+   */
+  // SPSC queue would do
+  private final Queue<ActionItem> writeChain = new ConcurrentLinkedQueue<>();
+  private volatile Thread parkingThread;
+
+  AsyncServletOutputStreamWriter(
+      AsyncContext asyncContext,
+      ServletOutputStream outputStream,
+      ServletTransportState transportState,
+      InternalLogId logId) {
+    this.outputStream = outputStream;
+    this.transportState = transportState;
+    this.logId = logId;
+    this.flushAction = () -> {
+      logger.log(FINEST, "[{0}] flushBuffer", logId);
+      asyncContext.getResponse().flushBuffer();
+    };
+    this.completeAction = () -> {
+      logger.log(FINE, "[{0}] call is completing", logId);
+      transportState.runOnTransportThread(
+          () -> {
+            transportState.complete();
+            asyncContext.complete();
+            logger.log(FINE, "[{0}] call completed", logId);
+          });
+    };
+  }
+
+  /** Called from application thread. */
+  void writeBytes(byte[] bytes, int numBytes) throws IOException {
+    runOrBufferActionItem(
+        // write bytes action
+        () -> {
+          outputStream.write(bytes, 0, numBytes);
+          transportState.runOnTransportThread(() -> transportState.onSentBytes(numBytes));
+          if (logger.isLoggable(FINEST)) {
+            logger.log(
+                FINEST,
+                "[{0}] outbound data: length = {1}, bytes = {2}",
+                new Object[]{logId, numBytes, toHexString(bytes, numBytes)});
+          }
+        });
+  }
+
+  /** Called from application thread. */
+  void flush() throws IOException {
+    runOrBufferActionItem(flushAction);
+  }
+
+  /** Called from application thread. */
+  void complete() {
+    try {
+      runOrBufferActionItem(completeAction);
+    } catch (IOException e) {
+      // actually completeAction does not throw
+      throw Status.fromThrowable(e).asRuntimeException();
+    }
+  }
+
+  /** Called from the container thread {@link javax.servlet.WriteListener#onWritePossible()}. */
+  void onWritePossible() throws IOException {
+    logger.log(
+        FINEST, "[{0}] onWritePossible: ENTRY. The servlet output stream becomes ready", logId);
+    assureReadyAndEmptyFalse();
+    while (outputStream.isReady()) {
+      WriteState curState = writeState.get();
+
+      ActionItem actionItem = writeChain.poll();
+      if (actionItem != null) {
+        actionItem.run();
+        continue;
+      }
+
+      if (writeState.compareAndSet(curState, curState.withReadyAndEmpty(true))) {
+        // state has not changed since.
+        logger.log(
+            FINEST,
+            "[{0}] onWritePossible: EXIT. All data available now is sent out and the servlet output"
+                + " stream is still ready",
+            logId);
+        return;
+      }
+      // else, state changed by another thread (runOrBufferActionItem), need to drain the writeChain
+      // again
+    }
+    logger.log(
+        FINEST, "[{0}] onWritePossible: EXIT. The servlet output stream becomes not ready", logId);
+  }
+
+  private void runOrBufferActionItem(ActionItem actionItem) throws IOException {
+    WriteState curState = writeState.get();
+    if (curState.readyAndEmpty) { // write to the outputStream directly
+      actionItem.run();
+      if (!outputStream.isReady()) {
+        logger.log(FINEST, "[{0}] the servlet output stream becomes not ready", logId);
+        boolean successful = writeState.compareAndSet(curState, curState.withReadyAndEmpty(false));
+        assert successful;
+        LockSupport.unpark(parkingThread);
+      }
+    } else { // buffer to the writeChain
+      writeChain.offer(actionItem);
+      if (!writeState.compareAndSet(curState, curState.newItemBuffered())) {
+        // state changed by another thread (onWritePossible)
+        assert writeState.get().readyAndEmpty;
+        ActionItem lastItem = writeChain.poll();
+        if (lastItem != null) {
+          assert lastItem == actionItem;
+          runOrBufferActionItem(lastItem);
+        }
+      } // state has not changed since
+    }
+  }
+
+  private void assureReadyAndEmptyFalse() {
+    // readyAndEmpty should have been set to false already or right now
+    // It's very very unlikely readyAndEmpty is still true due to a race condition
+    while (writeState.get().readyAndEmpty) {
+      parkingThread = Thread.currentThread();
+      LockSupport.parkNanos(Duration.ofSeconds(1).toNanos());
+    }
+    parkingThread = null;
+  }
+
+  /** Write actions, e.g. writeBytes, flush, complete. */
+  @FunctionalInterface
+  private interface ActionItem {
+    void run() throws IOException;
+  }
+
+  private static final class WriteState {
+
+    static final WriteState DEFAULT = new WriteState(false);
+
+    /**
+     * The servlet output stream is ready and the writeChain is empty.
+     *
+     * <p>readyAndEmpty turns from false to true when:
+     * {@code onWritePossible()} exits while currently there is no more data to write, but the last
+     * check of {@link javax.servlet.ServletOutputStream#isReady()} is true.
+     *
+     * <p>readyAndEmpty turns from false to true when:
+     * {@code runOrBufferActionItem()} exits while either the action item is written directly to the
+     * servlet output stream and the check of {@link javax.servlet.ServletOutputStream#isReady()}
+     * right after that returns false, or the action item is buffered into the writeChain.
+     */
+    final boolean readyAndEmpty;
+
+    WriteState(boolean readyAndEmpty) {
+      this.readyAndEmpty = readyAndEmpty;
+    }
+
+    /**
+     * Only {@code onWritePossible()} can set readyAndEmpty to true, and only {@code
+     * runOrBufferActionItem()} can set it to false.
+     */
+    @CheckReturnValue
+    WriteState withReadyAndEmpty(boolean readyAndEmpty) {
+      return new WriteState(readyAndEmpty);
+    }
+
+    /** Only {@code runOrBufferActionItem()} can call it, and will set readyAndEmpty to false. */
+    @CheckReturnValue
+    WriteState newItemBuffered() {
+      return new WriteState(false);
+    }
+  }
+}