Implement async DNS resolution using c-ares

fix #2533

src/network/dns_socket_handler.cpp

@@ -0,0 +1,45 @@
+#include <config.h>
+#ifdef CARES_FOUND
+
+#include <network/dns_socket_handler.hpp>
+#include <network/dns_handler.hpp>
+#include <network/poller.hpp>
+
+#include <ares.h>
+
+DNSSocketHandler::DNSSocketHandler(std::shared_ptr<Poller> poller,
+                                   const socket_t socket):
+  SocketHandler(poller, socket)
+{
+}
+
+DNSSocketHandler::~DNSSocketHandler()
+{
+}
+
+void DNSSocketHandler::connect()
+{
+}
+
+void DNSSocketHandler::on_recv()
+{
+  // always stop watching send and read events. We will re-watch them if the
+  // next call to ares_fds tell us to
+  this->poller->remove_socket_handler(this->socket);
+  ::ares_process_fd(DNSHandler::instance.get_channel(), this->socket, ARES_SOCKET_BAD);
+}
+
+void DNSSocketHandler::on_send()
+{
+  // always stop watching send and read events. We will re-watch them if the
+  // next call to ares_fds tell us to
+  this->poller->remove_socket_handler(this->socket);
+  ::ares_process_fd(DNSHandler::instance.get_channel(), ARES_SOCKET_BAD, this->socket);
+}
+
+bool DNSSocketHandler::is_connected() const
+{
+  return true;
+}
+
+#endif /* CARES_FOUND */