MXS-1212: use res.length for buffer length instead of computing the s…

…ize of the whole buffer chain

This reduces CPU usage and decreases the response time for result sets
with many rows.

server/modules/filter/maxrows/maxrows.c

@@ -186,6 +186,7 @@ typedef struct maxrows_response_state
     size_t n_rows;        /**< How many rows we have received. */
     size_t offset;        /**< Where we are in the response buffer. */
     size_t rows_offset;   /**< Offset to first row in result set */
+    size_t length;        /**< Buffer size. */
 } MAXROWS_RESPONSE_STATE;
 
 static void maxrows_response_state_reset(MAXROWS_RESPONSE_STATE *state);
@@ -353,6 +354,8 @@ static int routeQuery(MXS_FILTER *instance,
     csdata->state = MAXROWS_IGNORING_RESPONSE;
     csdata->large_packet = false;
     csdata->discard_resultset = false;
+    // Set buffer size to 0
+    csdata->res.length = 0;
 
     switch ((int)MYSQL_GET_COMMAND(data))
     {
@@ -413,23 +416,25 @@ static int clientReply(MXS_FILTER *instance,
     if (csdata->res.data)
     {
         gwbuf_append(csdata->res.data, data);
+        csdata->res.length += gwbuf_length(data);
     }
     else
     {
         csdata->res.data = data;
+        csdata->res.length = gwbuf_length(data);
     }
 
     if (csdata->state != MAXROWS_IGNORING_RESPONSE)
     {
         if (!csdata->discard_resultset)
         {
-            if (gwbuf_length(csdata->res.data) > csdata->instance->config.max_resultset_size)
+            if (csdata->res.length > csdata->instance->config.max_resultset_size)
             {
                 if (csdata->instance->config.debug & MAXROWS_DEBUG_DISCARDING)
                 {
-                    MXS_NOTICE("Current size %uB of resultset, at least as much "
+                    MXS_NOTICE("Current size %luB of resultset, at least as much "
                                "as maximum allowed size %uKiB. Not returning data.",
-                               gwbuf_length(csdata->res.data),
+                               csdata->res.length,
                                csdata->instance->config.max_resultset_size / 1024);
                 }
 
@@ -572,7 +577,7 @@ static int handle_expecting_fields(MAXROWS_SESSION_DATA *csdata)
 
     bool insufficient = false;
 
-    size_t buflen = gwbuf_length(csdata->res.data);
+    size_t buflen = csdata->res.length;
 
     while (!insufficient && (buflen - csdata->res.offset >= MYSQL_HEADER_LEN))
     {
@@ -649,7 +654,7 @@ static int handle_expecting_response(MAXROWS_SESSION_DATA *csdata)
     ss_dassert(csdata->res.data);
 
     int rv = 1;
-    size_t buflen = gwbuf_length(csdata->res.data);
+    size_t buflen = csdata->res.length;
 
     // Reset field counters
     csdata->res.n_fields = 0;
@@ -769,7 +774,7 @@ static int handle_rows(MAXROWS_SESSION_DATA *csdata)
 
     int rv = 1;
     bool insufficient = false;
-    size_t buflen = gwbuf_length(csdata->res.data);
+    size_t buflen = csdata->res.length;
 
     while (!insufficient && (buflen - csdata->res.offset >= MYSQL_HEADER_LEN))
     {