Add virtual function set for socket IO

Defines a structure of basic create, close, read/write
functions as virtual function calls, settable for individual
c-ares channels.

ares.h

@@ -356,6 +356,27 @@ CARES_EXTERN void ares_set_socket_configure_callback(ares_channel channel,
 CARES_EXTERN int ares_set_sortlist(ares_channel channel,
                                    const char *sortstr);
 
+/*
+ * Virtual function set to have user-managed socket IO.
+ * Note that all functions need to be defined, and when
+ * set, the library will not do any bind nor set any
+ * socket options, assuming the client handles these
+ * through either socket creation or the
+ * ares_sock_config_callback call.
+ */
+struct iovec;
+struct ares_socket_functions {
+   ares_socket_t(*asocket)(int, int, int, void *);
+   int(*aclose)(ares_socket_t, void *);
+   int(*aconnect)(ares_socket_t, const struct sockaddr *, socklen_t, void *);
+   ssize_t(*arecvfrom)(ares_socket_t, void *, size_t, int, struct sockaddr *, socklen_t *, void *);
+   ssize_t(*asendv)(ares_socket_t, const struct iovec *, int, void *);
+};
+
+CARES_EXTERN void ares_set_socket_functions(ares_channel channel,
+					    const struct ares_socket_functions * funcs,
+					    void *user_data);
+
 CARES_EXTERN void ares_send(ares_channel channel,
                             const unsigned char *qbuf,
                             int qlen,

ares_init.c

@@ -166,6 +166,8 @@ int ares_init_options(ares_channel *channelptr, struct ares_options *options,
   channel->sock_create_cb_data = NULL;
   channel->sock_config_cb = NULL;
   channel->sock_config_cb_data = NULL;
+  channel->sock_funcs = NULL;
+  channel->sock_func_cb_data = NULL;
 
   channel->last_server = 0;
   channel->last_timeout_processed = (time_t)now.tv_sec;
@@ -292,6 +294,8 @@ int ares_dup(ares_channel *dest, ares_channel src)
   (*dest)->sock_create_cb_data = src->sock_create_cb_data;
   (*dest)->sock_config_cb      = src->sock_config_cb;
   (*dest)->sock_config_cb_data = src->sock_config_cb_data;
+  (*dest)->sock_funcs          = src->sock_funcs;
+  (*dest)->sock_func_cb_data   = src->sock_func_cb_data;
 
   strncpy((*dest)->local_dev_name, src->local_dev_name,
           sizeof(src->local_dev_name));
@@ -2103,6 +2107,14 @@ void ares_set_socket_configure_callback(ares_channel channel,
   channel->sock_config_cb_data = data;
 }
 
+void ares_set_socket_functions(ares_channel channel,
+                               const struct ares_socket_functions * funcs,
+                               void *data)
+{
+  channel->sock_funcs = funcs;
+  channel->sock_func_cb_data = data;
+}
+
 int ares_set_sortlist(ares_channel channel, const char *sortstr)
 {
   int nsort = 0;

ares_private.h

@@ -314,6 +314,9 @@ struct ares_channeldata {
 
   ares_sock_config_callback sock_config_cb;
   void *sock_config_cb_data;
+
+  const struct ares_socket_functions * sock_funcs;
+  void *sock_func_cb_data;
 };
 
 /* Memory management functions */