From 0c19d1ea03a60139d5dc29f85f967c039e5553cd Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Markus=20M=C3=A4kel=C3=A4?= <markus.makela@mariadb.com>
Date: Tue, 21 May 2019 12:29:37 +0300
Subject: [PATCH] Fix debug assertion in mysql_backend

The backend read function used a NULL GWBUF with gwbuf_length.
---
 .../protocol/MySQL/mariadbbackend/mysql_backend.cc        | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/server/modules/protocol/MySQL/mariadbbackend/mysql_backend.cc b/server/modules/protocol/MySQL/mariadbbackend/mysql_backend.cc
index ecf4b8d46..b2f50777b 100644
--- a/server/modules/protocol/MySQL/mariadbbackend/mysql_backend.cc
+++ b/server/modules/protocol/MySQL/mariadbbackend/mysql_backend.cc
@@ -788,7 +788,7 @@ static int gw_read_and_write(DCB* dcb)
 {
     GWBUF* read_buffer = NULL;
     MXS_SESSION* session = dcb->session;
-    int nbytes_read;
+    int nbytes_read = 0;
     int return_code = 0;
 
     /* read available backend data */
@@ -800,7 +800,11 @@ static int gw_read_and_write(DCB* dcb)
         return 0;
     }
 
-    nbytes_read = gwbuf_length(read_buffer);
+    if (read_buffer)
+    {
+        nbytes_read = gwbuf_length(read_buffer);
+    }
+
     if (nbytes_read == 0)
     {
         mxb_assert(read_buffer == NULL);