Respect -Djava.net.preferIPv4Stack when using epoll transport

Motivation:

On a system where ipv4 and ipv6 are supported a user may want to use -Djava.net.preferIPv4Stack=true to restrict it to use ipv4 only.
This is currently ignored with the epoll transport.

Modifications:

Respect java.net.preferIPv4Stack system property.

Result:

-Djava.net.preferIPv4Stack=true will have the effect the user is looking for.

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

@@ -115,6 +115,11 @@ public final class NetUtil {
      */
     private static final int IPV4_SEPARATORS = 3;
 
+    /**
+     * {@code true} if ipv4 should be used on a system that supports ipv4 and ipv6.
+     */
+    private static final boolean IPV4_PREFERRED = Boolean.getBoolean("java.net.preferIPv4Stack");
+
     /**
      * The logger being used by this class
      */
@@ -260,6 +265,13 @@ public final class NetUtil {
         SOMAXCONN = somaxconn;
     }
 
+    /**
+     * Returns {@code true} if ipv4 should be prefered on a system that supports ipv4 and ipv6.
+     */
+    public static boolean isIpV4StackPreferred() {
+        return IPV4_PREFERRED;
+    }
+
     /**
      * Creates an byte[] based on an ipAddressString. No error handling is
      * performed here.

transport-native-epoll/src/main/c/io_netty_channel_epoll_Native.c

@@ -84,6 +84,8 @@ jmethodID closedChannelExceptionMethodId = NULL;
 jclass inetSocketAddressClass = NULL;
 jclass datagramSocketAddressClass = NULL;
 jclass nativeDatagramPacketClass = NULL;
+jclass netUtilClass = NULL;
+jmethodID netUtilClassIpv4PreferredMethodId = NULL;
 
 static int socketType;
 static const char* ip4prefix = "::ffff:";
@@ -305,7 +307,13 @@ static int init_sockaddr(JNIEnv* env, jbyteArray address, jint scopeId, jint jpo
     return 0;
 }
 
-static int socket_type() {
+static int socket_type(JNIEnv* env) {
+    jboolean ipv4Preferred = (*env)->CallStaticBooleanMethod(env, netUtilClass, netUtilClassIpv4PreferredMethodId);
+
+    if (ipv4Preferred) {
+        // User asked to use ipv4 explicitly.
+        return AF_INET;
+    }
     int fd = socket(AF_INET6, SOCK_STREAM | SOCK_NONBLOCK, 0);
     if (fd == -1) {
         if (errno == EAFNOSUPPORT) {
@@ -343,6 +351,24 @@ jint JNI_OnLoad(JavaVM* vm, void* reserved) {
     if ((*vm)->GetEnv(vm, (void**) &env, JNI_VERSION_1_6) != JNI_OK) {
         return JNI_ERR;
     } else {
+        jclass localNetUtilClass = (*env)->FindClass(env, "io/netty/util/NetUtil" );
+        if (localNetUtilClass == NULL) {
+            // pending exception...
+            return JNI_ERR;
+        }
+        netUtilClass = (jclass) (*env)->NewGlobalRef(env, localNetUtilClass);
+        if (netUtilClass == NULL) {
+            // out-of-memory!
+            throwOutOfMemoryError(env);
+            return JNI_ERR;
+        }
+
+        netUtilClassIpv4PreferredMethodId = (*env)->GetStaticMethodID(env, netUtilClass, "isIpV4StackPreferred", "()Z" );
+        if (netUtilClassIpv4PreferredMethodId == NULL) {
+            // position method was not found.. something is wrong so bail out
+            throwRuntimeException(env, "failed to get method ID: NetUild.isIpV4StackPreferred()");
+            return JNI_ERR;
+        }
         // cache classes that are used within other jni methods for performance reasons
         jclass localClosedChannelExceptionClass = (*env)->FindClass(env, "java/nio/channels/ClosedChannelException");
         if (localClosedChannelExceptionClass == NULL) {
@@ -509,7 +535,7 @@ jint JNI_OnLoad(JavaVM* vm, void* reserved) {
             throwRuntimeException(env, "failed to get method ID: InetSocketAddress.<init>(String, int)");
             return JNI_ERR;
         }
-        socketType = socket_type();
+        socketType = socket_type(env);
 
         datagramSocketAddrMethodId = (*env)->GetMethodID(env, datagramSocketAddressClass, "<init>", "(Ljava/lang/String;II)V");
         if (datagramSocketAddrMethodId == NULL) {
@@ -575,6 +601,9 @@ void JNI_OnUnload(JavaVM* vm, void* reserved) {
         if (datagramSocketAddressClass != NULL) {
             (*env)->DeleteGlobalRef(env, datagramSocketAddressClass);
         }
+        if (netUtilClass != NULL) {
+            (*env)->DeleteGlobalRef(env, netUtilClass);
+        }
     }
 }
 
@@ -952,7 +981,6 @@ JNIEXPORT jint JNICALL Java_io_netty_channel_epoll_Native_shutdown0(JNIEnv* env,
 }
 
 static inline jint socket0(JNIEnv* env, jclass clazz, int type) {
-    // TODO: Maybe also respect -Djava.net.preferIPv4Stack=true
     int fd = socket(socketType, type | SOCK_NONBLOCK, 0);
     if (fd == -1) {
         return -errno;