ZOOKEEPER-4907: Stop client packets processing after server channel closed

Reviewers: kezhuw, tisonkun
Author: jiafu1115
Closes #2236 from jiafu1115/jiafu1115-patch-1

(cherry picked from commit 6e4ec27)
Signed-off-by: Kezhu Wang <[email protected]>

Author: jiafu1115

Diff for: zookeeper-server/src/main/java/org/apache/zookeeper/server/NettyServerCnxn.java

@@ -367,6 +367,11 @@ void processMessage(ByteBuf buf) {
         checkIsInEventLoop("processMessage");
         LOG.debug("0x{} queuedBuffer: {}", Long.toHexString(sessionId), queuedBuffer);
 
+        if (closingChannel) {
+            LOG.debug("Drop incoming message during connection closing for session 0x{}", Long.toHexString(sessionId));
+            return;
+        }
+
         if (LOG.isTraceEnabled()) {
             LOG.trace("0x{} buf {}", Long.toHexString(sessionId), ByteBufUtil.hexDump(buf));
         }