NioEventLoop.register(...) should offload to the EventLoop if not alr… (#8612)

Motivation:

java.nio.channels.spi.AbstractSelectableChannel.register(...) need to obtain multiple locks during execution which may produce a long wait time if we currently select. This lead to multiple CI failures in the past.

Modifications:

Ensure the register call takes place on the EventLoop.

Result:

No more flacky CI test timeouts.

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

@@ -33,8 +33,9 @@ import java.io.IOException;
 import java.lang.reflect.Field;
 import java.nio.channels.CancelledKeyException;
 import java.nio.channels.SelectableChannel;
-import java.nio.channels.SelectionKey;
 import java.nio.channels.Selector;
+import java.nio.channels.SelectionKey;
+
 import java.nio.channels.spi.SelectorProvider;
 import java.security.AccessController;
 import java.security.PrivilegedAction;
@@ -293,6 +294,26 @@ public final class NioEventLoop extends SingleThreadEventLoop {
             throw new IllegalStateException("event loop shut down");
         }
 
+        if (inEventLoop()) {
+            register0(ch, interestOps, task);
+        } else {
+            try {
+                // Offload to the EventLoop as otherwise java.nio.channels.spi.AbstractSelectableChannel.register
+                // may block for a long time while trying to obtain an internal lock that may be hold while selecting.
+                submit(new Runnable() {
+                    @Override
+                    public void run() {
+                        register0(ch, interestOps, task);
+                    }
+                }).sync();
+            } catch (InterruptedException ignore) {
+                // Even if interrupted we did schedule it so just mark the Thread as interrupted.
+                Thread.currentThread().interrupt();
+            }
+        }
+    }
+
+    private void register0(SelectableChannel ch, int interestOps, NioTask<?> task) {
         try {
             ch.register(unwrappedSelector, interestOps, task);
         } catch (Exception e) {