Make connect speed great again

Rather than parsing session_track_system_variables when thread starts, do
it when first trackable event occurs.

Benchmarked on a 2socket/20core/40threads Broadwell system using sysbench
connect brencmark @40 threads (with select 1 disabled):
101379.77 -> 143016.68 CPS, whereas 10.2 is currently at 137766.31 CPS.

Part of MDEV-14984 - regression in connect performance

sql/log.cc

@@ -7903,6 +7903,7 @@ MYSQL_BIN_LOG::trx_group_commit_leader(group_commit_entry *leader)
     */
     for (current= queue; current != NULL; current= current->next)
     {
+      set_current_thd(current->thd);
       binlog_cache_mngr *cache_mngr= current->cache_mngr;
 
       /*
@@ -7938,6 +7939,7 @@ MYSQL_BIN_LOG::trx_group_commit_leader(group_commit_entry *leader)
         cache_mngr->delayed_error= false;
       }
     }
+    set_current_thd(leader->thd);
 
     bool synced= 0;
     if (unlikely(flush_and_sync(&synced)))

sql/session_tracker.cc

@@ -379,16 +379,10 @@ void Session_sysvars_tracker::deinit(THD *thd)
 
 bool Session_sysvars_tracker::enable(THD *thd)
 {
-  LEX_STRING tmp= { thd->variables.session_track_system_variables,
-                    safe_strlen(thd->variables.session_track_system_variables) };
   orig_list.reinit();
-  if (orig_list.parse_var_list(thd, tmp, true, thd->charset()) == true)
-  {
-    orig_list.reinit();
-    m_enabled= false;
-    return true;
-  }
-  m_enabled= true;
+  m_parsed= false;
+  m_enabled= thd->variables.session_track_system_variables &&
+             *thd->variables.session_track_system_variables;
   return false;
 }
 
@@ -433,6 +427,7 @@ bool Session_sysvars_tracker::update(THD *thd, set_var *var)
   my_free(thd->variables.session_track_system_variables);
   thd->variables.session_track_system_variables= static_cast<char*>(copy);
 
+  m_parsed= true;
   orig_list.copy(&tool_list, thd);
   orig_list.construct_var_list(thd->variables.session_track_system_variables,
                                var->save_result.string_value.length + 1);
@@ -540,6 +535,20 @@ void Session_sysvars_tracker::mark_as_changed(THD *thd,
 {
   sysvar_node_st *node;
   sys_var *svar= (sys_var *)var;
+
+  if (!m_parsed)
+  {
+    DBUG_ASSERT(thd->variables.session_track_system_variables);
+    LEX_STRING tmp= { thd->variables.session_track_system_variables,
+                      strlen(thd->variables.session_track_system_variables) };
+    if (orig_list.parse_var_list(thd, tmp, true, thd->charset()))
+    {
+      orig_list.reinit();
+      return;
+    }
+    m_parsed= true;
+  }
+
   /*
     Check if the specified system variable is being tracked, if so
     mark it as changed and also set the class's m_changed flag.

sql/session_tracker.h

@@ -198,6 +198,7 @@ class Session_sysvars_tracker: public State_tracker
     various operations.
   */
   vars_list orig_list;
+  bool m_parsed;
 
 public:
   void init(THD *thd);

sql/sql_class.cc

@@ -7304,6 +7304,7 @@ void THD::set_last_commit_gtid(rpl_gtid &gtid)
 #ifndef EMBEDDED_LIBRARY
   if (changed_gtid && session_tracker.sysvars.is_enabled())
   {
+    DBUG_ASSERT(current_thd == this);
     session_tracker.sysvars.
       mark_as_changed(this, (LEX_CSTRING*)Sys_last_gtid_ptr);
   }