MXS-2562: Fix out-of-order error during COM_CHANGE_USER

If an error is generated while a COM_CHANGE_USER is being done, it would
always use the sequence number 1. To properly handle this case and send
the correct sequence number, the COM_CHANGE_USER progress needs to be
tracked at the session level.

The information needs to be shared between the backend and client
protocols as the final OK to the COM_CHANGE_USER, with the sequence number
3, is the one that the backend server returns. Only after this response
has been received and routed to the client can the COM_CHANGE_USER
processing stop.

include/maxscale/protocol/mysql.h

@@ -142,14 +142,15 @@ typedef enum
  */
 typedef struct mysql_session
 {
-    uint8_t  client_sha1[MYSQL_SCRAMBLE_LEN];       /*< SHA1(password) */
+    uint8_t  client_sha1[MYSQL_SCRAMBLE_LEN];   /*< SHA1(password) */
-    char     user[MYSQL_USER_MAXLEN + 1];           /*< username       */
+    char     user[MYSQL_USER_MAXLEN + 1];       /*< username       */
-    char     db[MYSQL_DATABASE_MAXLEN + 1];         /*< database       */
+    char     db[MYSQL_DATABASE_MAXLEN + 1];     /*< database       */
-    int      auth_token_len;                        /*< token length   */
+    int      auth_token_len;                    /*< token length   */
-    uint8_t* auth_token;                            /*< token          */
+    uint8_t* auth_token;                        /*< token          */
-    bool     correct_authenticator;                 /*< is session using mysql_native_password? */
+    bool     correct_authenticator;             /*< is session using mysql_native_password? */
-    uint8_t  next_sequence;                         /*< Next packet sequence */
+    uint8_t  next_sequence;                     /*< Next packet sequence */
-    bool     auth_switch_sent;                      /*< Expecting a response to AuthSwitchRequest? */
+    bool     auth_switch_sent;                  /*< Expecting a response to AuthSwitchRequest? */
+    bool     changing_user;                     /*< True if a COM_CHANGE_USER is in progress */
 } MYSQL_session;
 
 /** Protocol packing macros. */

server/modules/protocol/MySQL/mariadbbackend/mysql_backend.cc

@@ -916,6 +916,9 @@ static int gw_read_and_write(DCB* dcb)
              */
             GWBUF_DATA(read_buffer)[3] = 0x3;
             proto->changing_user = false;
+
+            auto s = (MYSQL_session*)session->client_dcb->data;
+            s->changing_user = false;
         }
     }
 

server/modules/protocol/MySQL/mariadbclient/mysql_client.cc

@@ -1594,7 +1594,15 @@ static int gw_client_hangup_event(DCB* dcb)
                 errmsg += ": " + extra;
             }
 
-            modutil_send_mysql_err_packet(dcb, 1, 0, 1927, "08S01", errmsg.c_str());
+            int seqno = 1;
+
+            if (dcb->data && ((MYSQL_session*)dcb->data)->changing_user)
+            {
+                // In case a COM_CHANGE_USER is in progress, we need to send the error with the seqno 3
+                seqno = 3;
+            }
+
+            modutil_send_mysql_err_packet(dcb, seqno, 0, 1927, "08S01", errmsg.c_str());
         }
         dcb_close(dcb);
     }
@@ -1797,6 +1805,10 @@ static int route_by_statement(MXS_SESSION* session, uint64_t capabilities, GWBUF
 
             if (!proto->changing_user && proto->current_command == MXS_COM_CHANGE_USER)
             {
+                // Track the COM_CHANGE_USER progress at the session level
+                auto s = (MYSQL_session*)session->client_dcb->data;
+                s->changing_user = true;
+
                 changed_user = true;
                 send_auth_switch_request_packet(session->client_dcb);
 

server/modules/protocol/MySQL/mysql_common.cc

@@ -38,7 +38,7 @@ uint8_t null_client_sha1[MYSQL_SCRAMBLE_LEN] = "";
 MYSQL_session* mysql_session_alloc()
 {
     MYSQL_session* ses = (MYSQL_session*)MXS_CALLOC(1, sizeof(MYSQL_session));
-
+    ses->changing_user = false;
     return ses;
 }