NioEventLoop closes channel even if channel is not registered

Motivation:
If a channel is deregistered from an NioEventLoop the associated SelectionKey is cancelled. If the NioEventLoop has yet to process a pending event as a result of that SelectionKey then the NioEventLoop will see the SelecitonKey is invalid and close the channel. The NioEventLoop should not close a channel if it is not registered with that NioEventLoop.

Modifications:
- NioEventLoop.processSelectedKeys should check that the channel is still registered to itself before closing the channel

Result:
NioEventLoop doesn't close a channel that is no longer registered to it when the SelectionKey is invalid
Fixes https://github.com/netty/netty/issues/5125

transport/src/main/java/io/netty/channel/nio/NioEventLoop.java

@@ -17,6 +17,7 @@ package io.netty.channel.nio;
 
 import io.netty.channel.Channel;
 import io.netty.channel.ChannelException;
+import io.netty.channel.EventLoop;
 import io.netty.channel.EventLoopException;
 import io.netty.channel.SelectStrategy;
 import io.netty.channel.SingleThreadEventLoop;
@@ -512,9 +513,25 @@ public final class NioEventLoop extends SingleThreadEventLoop {
         }
     }
 
-    private static void processSelectedKey(SelectionKey k, AbstractNioChannel ch) {
+    private void processSelectedKey(SelectionKey k, AbstractNioChannel ch) {
         final NioUnsafe unsafe = ch.unsafe();
         if (!k.isValid()) {
+            final EventLoop eventLoop;
+            try {
+                eventLoop = ch.eventLoop();
+            } catch (Throwable ignored) {
+                // If the channel implementation throws an exception because there is no event loop, we ignore this
+                // because we are only trying to determine if ch is registered to this event loop and thus has authority
+                // to close ch.
+                return;
+            }
+            // Only close ch if ch is still registerd to this EventLoop. ch could have deregistered from the event loop
+            // and thus the SelectionKey could be cancelled as part of the deregistration process, but the channel is
+            // still healthy and should not be closed.
+            // See https://github.com/netty/netty/issues/5125
+            if (eventLoop != this || eventLoop == null) {
+                return;
+            }
             // close the channel if the key is not valid anymore
             unsafe.close(unsafe.voidPromise());
             return;