Select right IP address based on Java preferences. (akka#22336)

* Change priority of DNS resolved addresses based on java.net.preferIPv6Addresses property. akka#22330

* Include test specs for IPv6 resolver selection. akka#22330

* Use Java system properties to retrieve the IPv6 preference. akka#22330

(cherry picked from commit 6406c0e)

akka-actor-tests/src/test/scala/akka/io/DnsSpec.scala

@@ -0,0 +1,38 @@
+package akka.io
+
+import java.net.{ Inet4Address, Inet6Address, InetAddress }
+
+import scala.collection.immutable.Seq
+import org.scalatest.{ BeforeAndAfterAll, Matchers, WordSpec }
+
+class DnsSpec extends WordSpec with BeforeAndAfterAll with Matchers {
+
+  val ip4Address = InetAddress.getByAddress("localhost", Array[Byte](127, 0, 0, 1)) match {
+    case address: Inet4Address ⇒ address
+  }
+  val ipv6Address = InetAddress.getByAddress("localhost", Array[Byte](0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1)) match {
+    case address: Inet6Address ⇒ address
+  }
+
+  var temporaryValue: Option[String] = None
+
+  override def beforeAll() = {
+    temporaryValue = sys.props.get("java.net.preferIPv6Addresses")
+  }
+
+  override def afterAll() = temporaryValue match {
+    case Some(value) ⇒ sys.props.put("java.net.preferIPv6Addresses", value)
+    case _           ⇒ sys.props.remove("java.net.preferIPv6Addresses")
+  }
+
+  "Dns" should {
+    "resolve to a IPv6 address if it is the preferred network stack" in {
+      sys.props.put("java.net.preferIPv6Addresses", true.toString)
+      Dns.Resolved("test", Seq(ip4Address), Seq(ipv6Address)).addr should ===(ipv6Address)
+    }
+    "resolve to a IPv4 address if IPv6 is not the preferred network stack" in {
+      sys.props.remove("java.net.preferIPv6Addresses")
+      Dns.Resolved("test", Seq(ip4Address), Seq(ipv6Address)).addr should ===(ip4Address)
+    }
+  }
+}

akka-actor/src/main/scala/akka/io/Dns.scala

@@ -26,7 +26,7 @@ object Dns extends ExtensionId[DnsExt] with ExtensionIdProvider {
   }
 
   case class Resolved(name: String, ipv4: immutable.Seq[Inet4Address], ipv6: immutable.Seq[Inet6Address]) extends Command {
-    val addrOption: Option[InetAddress] = ipv4.headOption orElse ipv6.headOption
+    val addrOption: Option[InetAddress] = IpVersionSelector.getInetAddress(ipv4.headOption, ipv6.headOption)
 
     @throws[UnknownHostException]
     def addr: InetAddress = addrOption match {
@@ -89,3 +89,11 @@ class DnsExt(system: ExtendedActorSystem) extends IO.Extension {
 
   def getResolver: ActorRef = manager
 }
+
+object IpVersionSelector {
+  def getInetAddress(ipv4: Option[Inet4Address], ipv6: Option[Inet6Address]): Option[InetAddress] =
+    System.getProperty("java.net.preferIPv6Addresses") match {
+      case "true" ⇒ ipv6 orElse ipv4
+      case _      ⇒ ipv4 orElse ipv6
+    }
+}