MXS-862: Add create/destroy and remove plugin_name entry points

The create and destroy entry points allow authenticators to store data in
the DCB. This data is not shared by other DCBs related to the same
session.

The plugin_name entry point wasn't really useful as the plugins would
still need to send a AuthSwitchRequest packet if they wanted to change the
authentication mechanism.

server/modules/protocol/MySQLBackend/mysql_backend.c

@@ -848,8 +848,12 @@ mxs_auth_state_t gw_send_backend_auth(DCB *dcb)
     uint32_t capabilities = create_capabilities(conn, (local_session.db && strlen(local_session.db)), false);
     gw_mysql_set_byte4(client_capabilities, capabilities);
 
-    const char* auth_plugin_name = dcb->authfunc.plugin_name ?
-                                   dcb->authfunc.plugin_name : DEFAULT_MYSQL_AUTH_PLUGIN;
+    /**
+     * Use the default authentication plugin name. If the server is using a
+     * different authentication mechanism, it will send an AuthSwitchRequest
+     * packet.
+     */
+    const char* auth_plugin_name =  DEFAULT_MYSQL_AUTH_PLUGIN;
 
     long bytes = response_length(conn, local_session.user, local_session.client_sha1,
                                  local_session.db, auth_plugin_name);
@@ -909,7 +913,7 @@ mxs_auth_state_t gw_send_backend_auth(DCB *dcb)
     }
 
     // if the db is not NULL append it
-    if (local_session.db && strlen(local_session.db))
+    if (local_session.db[0])
     {
         memcpy(payload, local_session.db, strlen(local_session.db));
         payload += strlen(local_session.db);

server/modules/protocol/MySQLClient/mysql_client.c

@@ -311,8 +311,12 @@ int MySQLSendHandshake(DCB* dcb)
 
     memcpy(mysql_plugin_data, server_scramble + 8, 12);
 
-    const char* plugin_name = dcb->authfunc.plugin_name ?
-        dcb->authfunc.plugin_name : DEFAULT_MYSQL_AUTH_PLUGIN;
+    /**
+     * Use the default authentication plugin name in the initial handshake. If the
+     * authenticator needs to change the authentication method, it should send
+     * an AuthSwitchRequest packet to the client.
+     */
+    const char* plugin_name = DEFAULT_MYSQL_AUTH_PLUGIN;
     int plugin_name_len = strlen(plugin_name);
 
     mysql_payload_size =
@@ -562,6 +566,13 @@ int gw_read_client_event(DCB* dcb)
 static int
 gw_read_do_authentication(DCB *dcb, GWBUF *read_buffer, int nbytes_read)
 {
+    /** Allocate the shared session structure */
+    if (dcb->data == NULL && (dcb->data = mysql_session_alloc()) == NULL)
+    {
+        dcb_close(dcb);
+        return 1;
+    }
+
     /**
      * The first step in the authentication process is to extract the
      * relevant information from the buffer supplied and place it
@@ -590,7 +601,17 @@ gw_read_do_authentication(DCB *dcb, GWBUF *read_buffer, int nbytes_read)
      */
     if (MXS_AUTH_SUCCEEDED == auth_val)
     {
-        SESSION *session;
+        if (dcb->user == NULL)
+        {
+            /** User authentication complete, copy the username to the DCB */
+            MYSQL_session *ses = dcb->data;
+            if ((dcb->user = MXS_STRDUP(ses->user)) == NULL)
+            {
+                dcb_close(dcb);
+                gwbuf_free(read_buffer);
+                return 0;
+            }
+        }
 
         protocol->protocol_auth_state = MXS_AUTH_STATE_RESPONSE_SENT;
         /**
@@ -600,7 +621,7 @@ gw_read_do_authentication(DCB *dcb, GWBUF *read_buffer, int nbytes_read)
          * is changed so that future data will go through the
          * normal data handling function instead of this one.
          */
-        session = session_alloc(dcb->service, dcb);
+        SESSION *session = session_alloc(dcb->service, dcb);
 
         if (session != NULL)
         {

server/modules/protocol/mysql_common.c

@@ -56,6 +56,25 @@ uint8_t null_client_sha1[MYSQL_SCRAMBLE_LEN] = "";
 
 static server_command_t* server_command_init(server_command_t* srvcmd, mysql_server_cmd_t cmd);
 
+/**
+ * @brief Allocate a new MySQL_session
+ * @return New MySQL_session or NULL if memory allocation failed
+ */
+MYSQL_session* mysql_session_alloc()
+{
+    MYSQL_session *ses = MXS_CALLOC(1, sizeof(MYSQL_session));
+
+    if (ses)
+    {
+#ifdef SS_DEBUG
+        ses->myses_chk_top = CHK_NUM_MYSQLSES;
+        ses->myses_chk_tail = CHK_NUM_MYSQLSES;
+#endif
+    }
+
+    return ses;
+}
+
 /**
  * Creates MySQL protocol structure
  *