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   /*
    * Copyright 2014 The Netty Project
    *
    * The Netty Project licenses this file to you 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:
    *
    *   https://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.netty.channel;
  
  import io.netty.buffer.AbstractReferenceCountedByteBuf;
  import io.netty.util.ReferenceCountUtil;
  import io.netty.util.concurrent.EventExecutor;
  import io.netty.util.concurrent.PromiseCombiner;
  import io.netty.util.internal.ObjectPool;
  import io.netty.util.internal.ObjectPool.ObjectCreator;
  import io.netty.util.internal.ObjectUtil;
  import io.netty.util.internal.SystemPropertyUtil;
  import io.netty.util.internal.logging.InternalLogger;
  import io.netty.util.internal.logging.InternalLoggerFactory;
  
  /**
   * A queue of write operations which are pending for later execution. It also updates the
   * {@linkplain Channel#isWritable() writability} of the associated {@link Channel}, so that
   * the pending write operations are also considered to determine the writability.
   */
  public final class PendingWriteQueue {
      private static final InternalLogger logger = InternalLoggerFactory.getInstance(PendingWriteQueue.class);
      // Assuming a 64-bit JVM:
      //  - 16 bytes object header
      //  - 4 reference fields
      //  - 1 long fields
      private static final int PENDING_WRITE_OVERHEAD =
              SystemPropertyUtil.getInt("io.netty.transport.pendingWriteSizeOverhead", 64);
  
      private final ChannelOutboundInvoker invoker;
      private final EventExecutor executor;
      private final PendingBytesTracker tracker;
  
      // head and tail pointers for the linked-list structure. If empty head and tail are null.
      private PendingWrite head;
      private PendingWrite tail;
      private int size;
      private long bytes;
  
      public PendingWriteQueue(ChannelHandlerContext ctx) {
          tracker = PendingBytesTracker.newTracker(ctx.channel());
          this.invoker = ctx;
          this.executor = ctx.executor();
      }
  
      public PendingWriteQueue(Channel channel) {
          tracker = PendingBytesTracker.newTracker(channel);
          this.invoker = channel;
          this.executor = channel.eventLoop();
      }
  
      /**
       * Returns {@code true} if there are no pending write operations left in this queue.
       */
      public boolean isEmpty() {
          assert executor.inEventLoop();
          return head == null;
      }
  
      /**
       * Returns the number of pending write operations.
       */
      public int size() {
          assert executor.inEventLoop();
          return size;
      }
  
      /**
       * Returns the total number of bytes that are pending because of pending messages. This is only an estimate so
       * it should only be treated as a hint.
       */
      public long bytes() {
          assert executor.inEventLoop();
          return bytes;
      }
  
      private int size(Object msg) {
          // It is possible for writes to be triggered from removeAndFailAll(). To preserve ordering,
          // we should add them to the queue and let removeAndFailAll() fail them later.
          int messageSize = tracker.size(msg);
          if (messageSize < 0) {
              // Size may be unknown so just use 0
              messageSize = 0;
          }
          return messageSize + PENDING_WRITE_OVERHEAD;
      }
 
     /**
      * Add the given {@code msg} and {@link ChannelPromise}.
      */
     public void add(Object msg, ChannelPromise promise) {
         assert executor.inEventLoop();
         ObjectUtil.checkNotNull(msg, "msg");
         ObjectUtil.checkNotNull(promise, "promise");
         // It is possible for writes to be triggered from removeAndFailAll(). To preserve ordering,
         // we should add them to the queue and let removeAndFailAll() fail them later.
         int messageSize = size(msg);
 
         PendingWrite write = PendingWrite.newInstance(msg, messageSize, promise);
         PendingWrite currentTail = tail;
         if (currentTail == null) {
             tail = head = write;
         } else {
             currentTail.next = write;
             tail = write;
         }
         size ++;
         bytes += messageSize;
         tracker.incrementPendingOutboundBytes(write.size);
         // Touch the message to make it easier to debug buffer leaks.
 
         // this save both checking against the ReferenceCounted interface
         // and makes better use of virtual calls vs interface ones
         if (msg instanceof AbstractReferenceCountedByteBuf) {
             ((AbstractReferenceCountedByteBuf) msg).touch();
         } else {
             ReferenceCountUtil.touch(msg);
         }
     }
 
     /**
      * Remove all pending write operation and performs them via
      * {@link ChannelHandlerContext#write(Object, ChannelPromise)}.
      *
      * @return  {@link ChannelFuture} if something was written and {@code null}
      *          if the {@link PendingWriteQueue} is empty.
      */
     public ChannelFuture removeAndWriteAll() {
         assert executor.inEventLoop();
 
         if (isEmpty()) {
             return null;
         }
 
         ChannelPromise p = invoker.newPromise();
         PromiseCombiner combiner = new PromiseCombiner(executor);
         try {
             // It is possible for some of the written promises to trigger more writes. The new writes
             // will "revive" the queue, so we need to write them up until the queue is empty.
             for (PendingWrite write = head; write != null; write = head) {
                 head = tail = null;
                 size = 0;
                 bytes = 0;
 
                 while (write != null) {
                     PendingWrite next = write.next;
                     Object msg = write.msg;
                     ChannelPromise promise = write.promise;
                     recycle(write, false);
                     if (!(promise instanceof VoidChannelPromise)) {
                         combiner.add(promise);
                     }
                     invoker.write(msg, promise);
                     write = next;
                 }
             }
             combiner.finish(p);
         } catch (Throwable cause) {
             p.setFailure(cause);
         }
         assertEmpty();
         return p;
     }
 
     /**
      * Remove all pending write operation and fail them with the given {@link Throwable}. The message will be released
      * via {@link ReferenceCountUtil#safeRelease(Object)}.
      */
     public void removeAndFailAll(Throwable cause) {
         assert executor.inEventLoop();
         ObjectUtil.checkNotNull(cause, "cause");
         // It is possible for some of the failed promises to trigger more writes. The new writes
         // will "revive" the queue, so we need to clean them up until the queue is empty.
         for (PendingWrite write = head; write != null; write = head) {
             head = tail = null;
             size = 0;
             bytes = 0;
             while (write != null) {
                 PendingWrite next = write.next;
                 ReferenceCountUtil.safeRelease(write.msg);
                 ChannelPromise promise = write.promise;
                 recycle(write, false);
                 safeFail(promise, cause);
                 write = next;
             }
         }
         assertEmpty();
     }
 
     /**
      * Remove a pending write operation and fail it with the given {@link Throwable}. The message will be released via
      * {@link ReferenceCountUtil#safeRelease(Object)}.
      */
     public void removeAndFail(Throwable cause) {
         assert executor.inEventLoop();
         ObjectUtil.checkNotNull(cause, "cause");
 
         PendingWrite write = head;
         if (write == null) {
             return;
         }
         ReferenceCountUtil.safeRelease(write.msg);
         ChannelPromise promise = write.promise;
         safeFail(promise, cause);
         recycle(write, true);
     }
 
     private void assertEmpty() {
         assert tail == null && head == null && size == 0;
     }
 
     /**
      * Removes a pending write operation and performs it via
      * {@link ChannelHandlerContext#write(Object, ChannelPromise)}.
      *
      * @return  {@link ChannelFuture} if something was written and {@code null}
      *          if the {@link PendingWriteQueue} is empty.
      */
     public ChannelFuture removeAndWrite() {
         assert executor.inEventLoop();
         PendingWrite write = head;
         if (write == null) {
             return null;
         }
         Object msg = write.msg;
         ChannelPromise promise = write.promise;
         recycle(write, true);
         return invoker.write(msg, promise);
     }
 
     /**
      * Removes a pending write operation and release it's message via {@link ReferenceCountUtil#safeRelease(Object)}.
      *
      * @return  {@link ChannelPromise} of the pending write or {@code null} if the queue is empty.
      *
      */
     public ChannelPromise remove() {
         assert executor.inEventLoop();
         PendingWrite write = head;
         if (write == null) {
             return null;
         }
         ChannelPromise promise = write.promise;
         ReferenceCountUtil.safeRelease(write.msg);
         recycle(write, true);
         return promise;
     }
 
     /**
      * Return the current message or {@code null} if empty.
      */
     public Object current() {
         assert executor.inEventLoop();
         PendingWrite write = head;
         if (write == null) {
             return null;
         }
         return write.msg;
     }
 
     private void recycle(PendingWrite write, boolean update) {
         final PendingWrite next = write.next;
         final long writeSize = write.size;
 
         if (update) {
             if (next == null) {
                 // Handled last PendingWrite so rest head and tail
                 // Guard against re-entrance by directly reset
                 head = tail = null;
                 size = 0;
                 bytes = 0;
             } else {
                 head = next;
                 size --;
                 bytes -= writeSize;
                 assert size > 0 && bytes >= 0;
             }
         }
 
         write.recycle();
         tracker.decrementPendingOutboundBytes(writeSize);
     }
 
     private static void safeFail(ChannelPromise promise, Throwable cause) {
         if (!(promise instanceof VoidChannelPromise) && !promise.tryFailure(cause)) {
             logger.warn("Failed to mark a promise as failure because it's done already: {}", promise, cause);
         }
     }
 
     /**
      * Holds all meta-data and construct the linked-list structure.
      */
     static final class PendingWrite {
         private static final ObjectPool<PendingWrite> RECYCLER = ObjectPool.newPool(new ObjectCreator<PendingWrite>() {
             @Override
             public PendingWrite newObject(ObjectPool.Handle<PendingWrite> handle) {
                 return new PendingWrite(handle);
             }
         });
 
         private final ObjectPool.Handle<PendingWrite> handle;
         private PendingWrite next;
         private long size;
         private ChannelPromise promise;
         private Object msg;
 
         private PendingWrite(ObjectPool.Handle<PendingWrite> handle) {
             this.handle = handle;
         }
 
         static PendingWrite newInstance(Object msg, int size, ChannelPromise promise) {
             PendingWrite write = RECYCLER.get();
             write.size = size;
             write.msg = msg;
             write.promise = promise;
             return write;
         }
 
         private void recycle() {
             size = 0;
             next = null;
             msg = null;
             promise = null;
             handle.recycle(this);
         }
     }
 }