NetworkInterface.getByInetAddress() may return null on Android platfo…

…rm (#10056)


Motivation:

NetworkInterface.getByInetAddress() may return null on Android. This is incorrect by the API but still happens. To help our users we should provide a workaround

Modifications:

Just return an empty Enumeration when null is returned.

Result:

Fixes #10045

common/src/main/java/io/netty/util/internal/SocketUtils.java

@@ -32,6 +32,7 @@
 import java.security.PrivilegedAction;
 import java.security.PrivilegedActionException;
 import java.security.PrivilegedExceptionAction;
+import java.util.Collections;
 import java.util.Enumeration;
 
 /**
@@ -42,9 +43,16 @@
  */
 public final class SocketUtils {
 
+    private static final Enumeration<Object> EMPTY = Collections.enumeration(Collections.emptyList());
+
     private SocketUtils() {
     }
 
+    @SuppressWarnings("unchecked")
+    private static <T> Enumeration<T> empty() {
+        return (Enumeration<T>) EMPTY;
+    }
+
     public static void connect(final Socket socket, final SocketAddress remoteAddress, final int timeout)
             throws IOException {
         try {
@@ -176,12 +184,20 @@ public InetSocketAddress run() {
     }
 
     public static Enumeration<InetAddress> addressesFromNetworkInterface(final NetworkInterface intf) {
-        return AccessController.doPrivileged(new PrivilegedAction<Enumeration<InetAddress>>() {
+        Enumeration<InetAddress> addresses =
+                AccessController.doPrivileged(new PrivilegedAction<Enumeration<InetAddress>>() {
             @Override
             public Enumeration<InetAddress> run() {
                 return intf.getInetAddresses();
             }
         });
+        // Android seems to sometimes return null even if this is not a valid return value by the api docs.
+        // Just return an empty Enumeration in this case.
+        // See https://github.com/netty/netty/issues/10045
+        if (addresses == null) {
+            return empty();
+        }
+        return addresses;
     }
 
     @SuppressJava6Requirement(reason = "Usage guarded by java version check")