Bug#33912761: Plugin persisted variable restore issue

Description:
-----------
Plugin persisted variables are actually restored from persisted storage after loading the
registered plugin on startup with the call of
Persisted_variables_cache::set_persisted_options(false), although the "false"
parameter means to restore only non-plugin variables.

Analysis:
--------
Initially all plugin persisted variables are loaded in
m_persisted_dynamic_variables, but that's intentional.
After the first call to set_persisted_options(false), all plugin variables
that failed to be assigned to initially loaded plugins will be added to
m_persisted_dynamic_plugin_variables as well, so things will work fine
for subsequent set_persisted_options(true) calls done when installing
plugins with the SQL command.

Solution:
--------
Leave code as is, just document that initial call to
set_persisted_options(false) is required to initialize plugin specific list.

Approved by: Bharathy Satish <bharathy.x.satish@oracle.com>
Approved by: Georgi Kodinov <georgi.kodinov@oracle.com>
RB: 27742

Change-Id: I339005c79d2f270733bba451c5e740e012e3e78e

Author: MRajcic

sql/persisted_variable.cc

@@ -921,7 +921,10 @@ void Persisted_variables_cache::set_parse_early_sources() {
   set_persisted_options() will set the options read from persisted config file
 
   This function does nothing when --no-defaults is set or if
-  persisted_globals_load is set to false
+  persisted_globals_load is set to false.
+  Initial call to set_persisted_options(false) is needed to initialize
+  m_persisted_dynamic_plugin_variables set, so that next subsequent
+  set_persisted_options(true) calls will work with correct state.
 
    @param [in] plugin_options      Flag which tells what options are being set.
                                    If set to false non dynamically-registered