[#3680] Enabled SecurityManager results in ClassNotFoundError during io.netty.util.NetUtil initialization

Motivation:

When a SecurityManager is in place that preven reading the somaxconn file trying to bootstrap a channel later will result in a ClassNotFoundError.

Modifications:

- Reading the file in a privileged block.

Result:

No more ClassNotFoundError when a SecurityManager is in place.

common/src/main/java/io/netty/util/NetUtil.java

@@ -28,6 +28,8 @@ import java.net.InetAddress;
 import java.net.NetworkInterface;
 import java.net.SocketException;
 import java.net.UnknownHostException;
+import java.security.AccessController;
+import java.security.PrivilegedAction;
 import java.util.ArrayList;
 import java.util.Enumeration;
 import java.util.List;
@@ -230,6 +232,12 @@ public final class NetUtil {
         LOOPBACK_IF = loopbackIface;
         LOCALHOST = loopbackAddr;
 
+        // As a SecurityManager may prevent reading the somaxconn file we wrap this in a privileged block.
+        //
+        // See https://github.com/netty/netty/issues/3680
+        SOMAXCONN = AccessController.doPrivileged(new PrivilegedAction<Integer>() {
+            @Override
+            public Integer run() {
                 // Determine the default somaxconn (server socket backlog) value of the platform.
                 // The known defaults:
                 // - Windows NT Server 4.0+: 200
@@ -260,8 +268,9 @@ public final class NetUtil {
                         logger.debug("{}: {} (non-existent)", file, somaxconn);
                     }
                 }
-
+                return somaxconn;
-        SOMAXCONN = somaxconn;
+            }
+        });
     }
 
     /**