ovpndcocli: prepend control packet with peer-id when using TunBuilder

The TunBuilder implementation needs to know which peer to send the
control packet to and for this reason the ovpndcocli component is now
prepending the data with 4 bytes representing the peer-id of the
destination.

Since this approach is incompatible with sending a const Buffer (as it
cannot be modified), send() has to allocate a temporary buffer for the
purpose.

While at it, make transport_send(Buffer) obsolete, as ovpndcocli
is not expected to handle data packets (sent as non-const).

Signed-off-by: Antonio Quartulli <antonio@openvpn.net>

openvpn/dco/ovpndcocli.hpp

@@ -156,25 +156,23 @@ class OvpnDcoClient : public Client,
     if (peer_id == OVPN_PEER_ID_UNDEF)
       return transport->transport_send_const(buf);
 
-    return send(buf);
-  }
-
-  virtual bool transport_send(BufferAllocated &buf) override {
-    if (peer_id == OVPN_PEER_ID_UNDEF)
-      return transport->transport_send(buf);
-
-    return send(buf);
-  }
-
-  bool send(const Buffer &buf) {
-    if (config->builder)
-      pipe->write_some(buf.const_buffer());
-    else
+    if (config->builder) {
+      Buffer tmp(buf);
+      tmp.prepend(&peer_id, sizeof(peer_id));
+      pipe->write_some(tmp.const_buffer());
+    } else {
       genl->send_data(peer_id, buf.c_data(), buf.size());
+    }
 
     return true;
   }
 
+  virtual bool transport_send(BufferAllocated &buf) override {
+    OPENVPN_THROW(dcocli_error,
+                  "Non-const send expected for data channel only, but "
+                  "ovpndcocli is not expected to handle data packets");
+  }
+
   void get_remote_sockaddr(struct sockaddr_storage &sa, socklen_t &salen) {
     memset(&sa, 0, sizeof(sa));