url: Add option CURLOPT_RESOLVER_START_FUNCTION

- Add new option CURLOPT_RESOLVER_START_FUNCTION to set a callback that
  will be called every time before a new resolve request is started
  (ie before a host is resolved) with a pointer to backend-specific
  resolver data. Currently this is only useful for ares.

- Add new option CURLOPT_RESOLVER_START_DATA to set a user pointer to
  pass to the resolver start callback.

Closes https://github.com/curl/curl/pull/2311

lib/hostip.c

@@ -58,6 +58,7 @@
 #include "strerror.h"
 #include "url.h"
 #include "inet_ntop.h"
+#include "multiif.h"
 #include "warnless.h"
 /* The last 3 #include files should be in this order */
 #include "curl_printf.h"
@@ -481,6 +482,17 @@ int Curl_resolv(struct connectdata *conn,
     if(!Curl_ipvalid(conn))
       return CURLRESOLV_ERROR;
 
+    /* notify the resolver start callback */
+    if(data->set.resolver_start) {
+      int st;
+      Curl_set_in_callback(data, true);
+      st = data->set.resolver_start(data->state.resolver, NULL,
+                                    data->set.resolver_start_client);
+      Curl_set_in_callback(data, false);
+      if(st)
+        return CURLRESOLV_ERROR;
+    }
+
     /* If Curl_getaddrinfo() returns NULL, 'respwait' might be set to a
        non-zero value indicating that we need to wait for the response to the
        resolve call */

lib/setopt.c

@@ -2110,6 +2110,21 @@ CURLcode Curl_vsetopt(struct Curl_easy *data, CURLoption option,
     data->set.fclosesocket = va_arg(param, curl_closesocket_callback);
     break;
 
+  case CURLOPT_RESOLVER_START_FUNCTION:
+    /*
+     * resolver start callback function: called before a new resolver request
+     * is started
+     */
+    data->set.resolver_start = va_arg(param, curl_resolver_start_callback);
+    break;
+
+  case CURLOPT_RESOLVER_START_DATA:
+    /*
+     * resolver start callback data pointer. Might be NULL.
+     */
+    data->set.resolver_start_client = va_arg(param, void *);
+    break;
+
   case CURLOPT_CLOSESOCKETDATA:
     /*
      * socket callback data pointer. Might be NULL.

lib/urldata.h

@@ -1681,6 +1681,10 @@ struct UserDefined {
   struct Curl_http2_dep *stream_dependents;
 
   bool abstract_unix_socket;
+
+  curl_resolver_start_callback resolver_start; /* optional callback called
+                                                  before resolver start */
+  void *resolver_start_client; /* pointer to pass to resolver start callback */
 };
 
 struct Names {