CURLOPT_PREREQFUNCTION: add new callback

Triggered before a request is made but after a connection is set up

Changes:

- callback: Update docs and callback for pre-request callback
- Add documentation for CURLOPT_PREREQDATA and CURLOPT_PREREQFUNCTION,
- Add redirect test and callback failure test
- Note that the function may be called multiple times on a redirection
- Disable new 2086 test due to Windows weirdness

Closes #7477

lib/easyoptions.c

@@ -356,6 +356,6 @@ struct curl_easyoption Curl_easyopts[] = {
  */
 int Curl_easyopts_check(void)
 {
-  return ((CURLOPT_LASTENTRY%10000) != (311 + 1));
+  return ((CURLOPT_LASTENTRY%10000) != (313 + 1));
 }
 #endif

lib/multi.c

@@ -2028,6 +2028,28 @@ static CURLMcode multi_runsingle(struct Curl_multi *multi,
       break;
 
     case MSTATE_DO:
+      if(data->set.fprereq) {
+        int prereq_rc;
+
+        /* call the prerequest callback function */
+        Curl_set_in_callback(data, true);
+        prereq_rc = data->set.fprereq(data->set.prereq_userp,
+                                      data->info.conn_primary_ip,
+                                      data->info.conn_local_ip,
+                                      data->info.conn_primary_port,
+                                      data->info.conn_local_port);
+        Curl_set_in_callback(data, false);
+        if(prereq_rc != CURL_PREREQFUNC_OK) {
+          failf(data, "operation aborted by pre-request callback");
+          /* failure in pre-request callback - don't do any other processing */
+          result = CURLE_ABORTED_BY_CALLBACK;
+          Curl_posttransfer(data);
+          multi_done(data, result, FALSE);
+          stream_error = TRUE;
+          break;
+        }
+      }
+
       if(data->set.connect_only) {
         /* keep connection open for application to use the socket */
         connkeep(data->conn, "CONNECT_ONLY");

lib/setopt.c

@@ -3013,6 +3013,12 @@ CURLcode Curl_vsetopt(struct Curl_easy *data, CURLoption option, va_list param)
       return result;
     break;
 #endif
+  case CURLOPT_PREREQFUNCTION:
+    data->set.fprereq = va_arg(param, curl_prereq_callback);
+    break;
+  case CURLOPT_PREREQDATA:
+    data->set.prereq_userp = va_arg(param, void *);
+    break;
   default:
     /* unknown tag and its companion, just ignore: */
     result = CURLE_UNKNOWN_OPTION;

lib/urldata.h

@@ -1652,6 +1652,8 @@ struct UserDefined {
   curl_closesocket_callback fclosesocket; /* function for closing the
                                              socket */
   void *closesocket_client;
+  curl_prereq_callback fprereq; /* pre-initial request callback */
+  void *prereq_userp; /* pre-initial request user data */
 
   void *seek_client;    /* pointer to pass to the seek callback */
   /* the 3 curl_conv_callback functions below are used on non-ASCII hosts */