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Netty关闭连接流程分析

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08/01
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在实际场景中，使用Netty4来实现RPC框架，服务端一般会验证协议，最简单的方法的协议探测，判断魔数是否正确。如果服务端无法识别协议会立即抛出异常，并主动关闭连接，此时客户端会收到read信号，在发现是一个关闭连接请求后会关闭本地连接，这其中用户可控的是InboundHandle收到 channelInactive方法。

今天想搞清楚的是：

假设consumer端 的InboundHandle收到 channelInactive 事件，是否可以立即结束这次请求并返回请求失败，而不必等到timeout才结束。

单纯的这个协议探测失败场景中，答案是可以立即结束，因为服务端不会响应任何数据。其他场景呢，在第1个InboundHandle中可以直接响应操作失败吗？

服务端主动关闭连接时，Netty的处理流程

在netty4中channel都被绑定到特定I/O EventLoop线程中，I/O线程收到信号为SelectionKey.OP_READ的就绪信号，如果allocHandle.lastBytesRead() = -1，则表示这是连接关闭的请求。

单独卡read的处理逻辑在AbstractNioByteChannel.read()中

public final void read() {
final ChannelConfig config = config();
if (shouldBreakReadReady(config)) {
clearReadPending();
return;
}
final ChannelPipeline pipeline = pipeline();
final ByteBufAllocator allocator = config.getAllocator();
final RecvByteBufAllocator.Handle allocHandle = recvBufAllocHandle();
allocHandle.reset(config); ByteBuf byteBuf = null;
boolean close = false;
try {
do {
byteBuf = allocHandle.allocate(allocator);
allocHandle.lastBytesRead(doReadBytes(byteBuf));
if (allocHandle.lastBytesRead() <= 0) {
// nothing was read. release the buffer.
byteBuf.release();
byteBuf = null;
close = allocHandle.lastBytesRead() < 0; // -1表示关闭连接
if (close) {
// There is nothing left to read as we received an EOF.
readPending = false;
}
break;
} allocHandle.incMessagesRead(1);
readPending = false;
pipeline.fireChannelRead(byteBuf);
byteBuf = null;
} while (allocHandle.continueReading()); allocHandle.readComplete();
pipeline.fireChannelReadComplete(); if (close) {
closeOnRead(pipeline); //执行关连接逻辑
}
} catch (Throwable t) {
handleReadException(pipeline, byteBuf, t, close, allocHandle);
} finally {
// Check if there is a readPending which was not processed yet.
// This could be for two reasons:
// * The user called Channel.read() or ChannelHandlerContext.read() in channelRead(...) method
// * The user called Channel.read() or ChannelHandlerContext.read() in channelReadComplete(...) method
//
// See https://github.com/netty/netty/issues/2254
if (!readPending && !config.isAutoRead()) {
removeReadOp();
}
}
}
}

close = allocHandle.lastBytesRead() < 0;

根据最后可读的bytes为-1，即为关闭

接下来是关闭连接的流程

检查此连接之前是否为active状态，并且当前是否为active，进入fireChannelInactiveAndDeregister流程，注销channel

这里不会立即注销channel，而是以一个任务的形式放到eventloop中稍后执行，文档中解释了不立即执行的原因：

private void invokeLater(Runnable task) {
try {
// This method is used by outbound operation implementations to trigger an inbound event later.
// They do not trigger an inbound event immediately because an outbound operation might have been
// triggered by another inbound event handler method. If fired immediately, the call stack
// will look like this for example:
//
// handlerA.inboundBufferUpdated() - (1) an inbound handler method closes a connection.
// -> handlerA.ctx.close()
// -> channel.unsafe.close()
// -> handlerA.channelInactive() - (2) another inbound handler method called while in (1) yet
//
// which means the execution of two inbound handler methods of the same handler overlap undesirably.
eventLoop().execute(task);
} catch (RejectedExecutionException e) {
logger.warn("Can't invoke task later as EventLoop rejected it", e);
}
}

大致的意思是 这个方法被用于出港操作来延迟触发一个到达的事件, 主要针对的场景是InboundHandler中产生了出港操作，比如主动断开连接，可能出现多个InboundHandler都会触发出港操作，这些操作可能是相同的如果立即执行，可能会重叠执行。

如果客户端从未收到任何数据，直接被服务端主动关闭是没有这个顾虑的，因为还没有InboundHandler处理数据。

invokeLater(new Runnable() {
@Override
public void run() {
try {
doDeregister();
} catch (Throwable t) {
logger.warn("Unexpected exception occurred while deregistering a channel.", t);
} finally {
if (fireChannelInactive) {
pipeline.fireChannelInactive();
}
// Some transports like local and AIO does not allow the deregistration of
// an open channel. Their doDeregister() calls close(). Consequently,
// close() calls deregister() again - no need to fire channelUnregistered, so check
// if it was registered.
if (registered) {
registered = false;
pipeline.fireChannelUnregistered();
}
safeSetSuccess(promise);
}
}
});

在从eventloop移除当前channel后（doDeregister()方法），进入finally代码块，如果之前连接是可用的，则fireChannelInactive为true

接下来进入pipeline.fireChannelInactive();流程

DefaultChannelPipreline 中有一个 AbstractChannelHandlerContext 链表，他按用户配置的顺序被写入Pipiline中，表头是Netty自定义的HeadContext，先执行头部的AbstractChannelHandlerContext

@Override
public final ChannelPipeline fireChannelInactive() {
AbstractChannelHandlerContext.invokeChannelInactive(head);
return this;
}

AbstractChannelHandlerContext持有 ChannelHandler对象，此处实际为 ChannelInboundHandler

private void invokeChannelInactive() {
if (invokeHandler()) {
try {
((ChannelInboundHandler) handler()).channelInactive(this);
} catch (Throwable t) {
notifyHandlerException(t);
}
} else {
fireChannelInactive();
}
}

这里会先进入netty预置的DefaultChannelPipeline$HeadContext实现中，然后走到了我们配置的第一个InboundHandle中来

默认的channelInactive实现在ByteToMessageDecoder中

public void channelInactive(ChannelHandlerContext ctx) throws Exception {
channelInputClosed(ctx, true);
}

Netty对关闭连接容忍度非常高，关闭连接前如果连接仍然有可读的数据，会尝试把他读出来

private void channelInputClosed(ChannelHandlerContext ctx, boolean callChannelInactive) throws Exception {
CodecOutputList out = CodecOutputList.newInstance();
try {
channelInputClosed(ctx, out);
} catch (DecoderException e) {
throw e;
} catch (Exception e) {
throw new DecoderException(e);
} finally {
try {
if (cumulation != null) {
cumulation.release();
cumulation = null;
}
int size = out.size();
fireChannelRead(ctx, out, size);
if (size > 0) {
// Something was read, call fireChannelReadComplete()
ctx.fireChannelReadComplete();
}
if (callChannelInactive) {
ctx.fireChannelInactive();
}
} finally {
// Recycle in all cases
out.recycle();
}
}
} /**
* Called when the input of the channel was closed which may be because it changed to inactive or because of
* {@link ChannelInputShutdownEvent}.
*/
void channelInputClosed(ChannelHandlerContext ctx, List<Object> out) throws Exception {
if (cumulation != null) {
callDecode(ctx, cumulation, out);
decodeLast(ctx, cumulation, out);
} else {
decodeLast(ctx, Unpooled.EMPTY_BUFFER, out);
}
}
/**
* Get {@code numElements} out of the {@link CodecOutputList} and forward these through the pipeline.
*/
static void fireChannelRead(ChannelHandlerContext ctx, CodecOutputList msgs, int numElements) {
for (int i = 0; i < numElements; i ++) {
ctx.fireChannelRead(msgs.getUnsafe(i));
}
}

callDecode会尝试去decode已经在channel还为取完的数据，如果取到了，则外部方法中的size就会大于0，会走完fireChannelRead的流程

/**
* A {@link Channel} received a message.
*
* This will result in having the {@link ChannelInboundHandler#channelRead(ChannelHandlerContext, Object)}
* method called of the next {@link ChannelInboundHandler} contained in the {@link ChannelPipeline} of the
* {@link Channel}.
*/
ChannelInboundInvoker fireChannelRead(Object msg);

fireChannelRead方法会调用ChannelPipeline中的下一个ChannelInboundHandler执行channelRead，我们常用的MessageToMessageDecoder执行channelRead时，会调用decode方法，也就是我们实现来处理请求的方法。

然后设置channelReadComplete()

最后调用ctx.fireChannelInactive();将事件传给下一个handler。

Netty4中I/O EventLoop是单线程执行的，对一个channel来说是线程安全的，而channel上的数据必然是先于close信号到达的，那么，当我们收到close新号时，之前已经发送过来的数据，一定已经至少被尝试处理过了吗？如果是这样，还有必要执行一下callDecode逻辑来fireChannelRead吗？

回到最初的代码块中

AbstractNioByteChannel.read()中

public final void read() {
final ChannelConfig config = config();
if (shouldBreakReadReady(config)) {
clearReadPending();
return;
}
final ChannelPipeline pipeline = pipeline();
final ByteBufAllocator allocator = config.getAllocator();
final RecvByteBufAllocator.Handle allocHandle = recvBufAllocHandle();
allocHandle.reset(config); ByteBuf byteBuf = null;
boolean close = false;
try {
do {
byteBuf = allocHandle.allocate(allocator);
allocHandle.lastBytesRead(doReadBytes(byteBuf));
if (allocHandle.lastBytesRead() <= 0) {
// nothing was read. release the buffer.
byteBuf.release();
byteBuf = null;
close = allocHandle.lastBytesRead() < 0;
if (close) {
// There is nothing left to read as we received an EOF.
readPending = false;
}
break;
} allocHandle.incMessagesRead(1);
readPending = false;
pipeline.fireChannelRead(byteBuf); //如果是正常的read，立即 fireChannelRead
byteBuf = null;
} while (allocHandle.continueReading()); allocHandle.readComplete();
pipeline.fireChannelReadComplete(); if (close) {
closeOnRead(pipeline);
}
} catch (Throwable t) {
handleReadException(pipeline, byteBuf, t, close, allocHandle);
} finally {
// Check if there is a readPending which was not processed yet.
// This could be for two reasons:
// * The user called Channel.read() or ChannelHandlerContext.read() in channelRead(...) method
// * The user called Channel.read() or ChannelHandlerContext.read() in channelReadComplete(...) method
//
// See https://github.com/netty/netty/issues/2254
if (!readPending && !config.isAutoRead()) {
removeReadOp();
}
}
}
}

可以发现，如果是普通的读操作，会立即触发fireChannelRead，经过前面的分析，可以知道该方法将会invokeChannelRead，并且当前的executor是在eventLoop中的，那么channelRead会被立即执行，最终触发我们配置的InboundHandle

而close新号必然晚于正常的数据流，因此数据一定是先被InboundHandle处理后才接受到close信号的。

结论：

单从服务端主动断开连接的场景，如果收到close新号，则在第一个inboundHandle的 channelInactive 方法中，直接通知业务任务已失败是安全的，因为数据流如果返回完全，则必然被处理过。

如果客户端inbountHandle过程中主动close掉channel，也是安全的，因为他会稍后执行，并且尝试将最新的Inbound数据再次decode，如果有可处理的数据就走完所有InboundHandle(channel已经关闭，写响应会失败),没有则结束