Implement async DNS resolution using c-ares

fix #2533

src/network/dns_handler.hpp

@@ -0,0 +1,62 @@
+#ifndef DNS_HANDLER_HPP_INCLUDED
+#define DNS_HANDLER_HPP_INCLUDED
+
+#include <config.h>
+#ifdef CARES_FOUND
+
+class TCPSocketHandler;
+class Poller;
+class DNSSocketHandler;
+
+# include <ares.h>
+# include <memory>
+# include <string>
+# include <list>
+
+void on_hostname4_resolved(void* arg, int status, int, struct hostent* hostent);
+void on_hostname6_resolved(void* arg, int status, int, struct hostent* hostent);
+
+/**
+ * Class managing DNS resolution.  It should only be statically instanciated
+ * once in SocketHandler.  It manages ares channel and calls various
+ * functions of that library.
+ */
+
+class DNSHandler
+{
+public:
+  DNSHandler();
+  ~DNSHandler() = default;
+  void gethostbyname(const std::string& name, TCPSocketHandler* socket_handler,
+                     int family);
+  /**
+   * Call ares_fds to know what fd needs to be watched by the poller, create
+   * or destroy DNSSocketHandlers depending on the result.
+   */
+  void watch_dns_sockets(std::shared_ptr<Poller>& poller);
+  /**
+   * Destroy and stop watching all the DNS sockets. Then de-init the channel
+   * and library.
+   */
+  void destroy();
+  ares_channel& get_channel();
+
+  static DNSHandler instance;
+
+private:
+  /**
+   * The list of sockets that needs to be watched, according to the last
+   * call to ares_fds.  DNSSocketHandlers are added to it or removed from it
+   * in the watch_dns_sockets() method
+   */
+  std::list<std::unique_ptr<DNSSocketHandler>> socket_handlers;
+  ares_channel channel;
+
+  DNSHandler(const DNSHandler&) = delete;
+  DNSHandler(DNSHandler&&) = delete;
+  DNSHandler& operator=(const DNSHandler&) = delete;
+  DNSHandler& operator=(DNSHandler&&) = delete;
+};
+
+#endif /* CARES_FOUND */
+#endif /* DNS_HANDLER_HPP_INCLUDED */