recvfrom should set srcAddress for empty packets

0 is a valid return value for recvfrom. It returns -1 on error, instead
of 0.

Test: libcore.io.OsTest
Bug: 33483694
Change-Id: I66261f7254bc5e4e5155a8a353018ed626259b82

luni/src/main/native/libcore_io_Posix.cpp

@@ -1718,7 +1718,7 @@ static jint Posix_recvfromBytes(JNIEnv* env, jobject, jobject javaFd, jobject ja
     sockaddr* from = (javaInetSocketAddress != NULL) ? reinterpret_cast<sockaddr*>(&ss) : NULL;
     socklen_t* fromLength = (javaInetSocketAddress != NULL) ? &sl : 0;
     jint recvCount = NET_FAILURE_RETRY(env, ssize_t, recvfrom, javaFd, bytes.get() + byteOffset, byteCount, flags, from, fromLength);
-    if (recvCount > 0) {
+    if (recvCount >= 0) {
         fillInetSocketAddress(env, javaInetSocketAddress, ss);
     }
     return recvCount;

luni/src/test/java/libcore/io/OsTest.java

@@ -28,6 +28,8 @@
 import java.io.FileDescriptor;
 import java.io.FileInputStream;
 import java.io.FileOutputStream;
+import java.net.DatagramPacket;
+import java.net.DatagramSocket;
 import java.net.Inet4Address;
 import java.net.Inet6Address;
 import java.net.InetAddress;
@@ -43,6 +45,7 @@
 import java.util.Locale;
 import java.util.concurrent.atomic.AtomicReference;
 import junit.framework.TestCase;
+
 import static android.system.OsConstants.*;
 
 public class OsTest extends TestCase {
@@ -762,4 +765,20 @@ public void test_readlink() throws Exception {
       assertTrue("Could not delete symlink: " + path, new File(path).delete());
     }
   }
+
+  // Address should be correctly set for empty packets. http://b/33481605
+  public void test_recvfrom_EmptyPacket() throws Exception {
+    try (DatagramSocket ds = new DatagramSocket();
+         DatagramSocket srcSock = new DatagramSocket()) {
+      srcSock.send(new DatagramPacket(new byte[0], 0, ds.getLocalSocketAddress()));
+
+      byte[] recvBuf = new byte[16];
+      InetSocketAddress address = new InetSocketAddress();
+      int recvCount =
+          android.system.Os.recvfrom(ds.getFileDescriptor$(), recvBuf, 0, 16, 0, address);
+      assertEquals(0, recvCount);
+      assertTrue(address.getAddress().isLoopbackAddress());
+      assertEquals(srcSock.getLocalPort(), address.getPort());
+    }
+  }
 }