Special return code for "socket closed"

Blocking recv calls return a special return code (-2) for notifying the socket
is closed (this information is useful for MDP socket users).

This implementation is unsatisfactory: return code should instead be consistent
with recvfrom(), that is always returning the length of received data (0 for
socket closed).

mdp_client.c

@@ -251,6 +251,10 @@ int overlay_mdp_recv(int mdp_sockfd, overlay_mdp_frame *mdp, int port, int *ttl)
 
   /* Check if reply available */
   ssize_t len = recvwithttl(mdp_sockfd,(unsigned char *)mdp, sizeof(overlay_mdp_frame),ttl,recvaddr,&recvaddrlen);
+  if (len == 0) {
+    /* Socket is closed. */
+    return -2; /* would be better to always return len */
+  }
 
   recvaddr_un=(struct sockaddr_un *)recvaddr;
   /* Null terminate received address so that the stat() call below can succeed */

mdp_jni.c

@@ -300,6 +300,8 @@ Java_org_servalproject_servald_mdp_MeshSocket__1receive(JNIEnv * env,
   jbyteArray jbuf, jsid;
   jbyte *sid;
   overlay_mdp_frame mdp;
+  int res;
+  char *msg;
 
   /* fd = this.fd; */
   fd = (*env)->GetIntField(env, this, f_meshsocket_fd);
@@ -311,10 +313,15 @@ Java_org_servalproject_servald_mdp_MeshSocket__1receive(JNIEnv * env,
   length = (*env)->GetIntField(env, mdppack, f_meshpacket_length);
 
   /* Receive data. */
-  if (overlay_mdp_recv(fd, &mdp, localport, &ttl)) {
-    (*env)->ThrowNew(env, cl_meshsocketexception,
-                     "Cannot receive data from servald");
-    WHY("Cannot receive data from servald");
+  if (res = (overlay_mdp_recv(fd, &mdp, localport, &ttl))) {
+    if (res == -2) {
+      /* Socket is closed. */
+      msg = "Socket closed";
+    } else {
+      msg = "Cannot receive data from servald";
+    }
+    (*env)->ThrowNew(env, cl_meshsocketexception, msg);
+    WHY(msg);
     return;
   }