MDEV-3798: [SHOW] EXPLAIN UPDATE/DELETE, Memory leak in binlog.binlog_base64_flag:

- It turns out, there are statements that will call lex_start(thd->lex) 
  after parsing has been finished. lex_start() will set lex->explain=NULL,
  which will lose the pointer to already allocated Explain_plan object.
- To get rid of this, switch to lazy creation of lex->explain.  Now, it is 
  created only when we get a part ot query plan.

sql/log_event_old.cc

@@ -86,6 +86,7 @@ Old_rows_log_event::do_apply_event(Old_rows_log_event *ev, const Relay_log_info
       call might reset the value of current_stmt_binlog_format, so
       we need to do any changes to that value after this function.
     */
+    delete_explain_query(thd->lex);
     lex_start(ev_thd);
     mysql_reset_thd_for_next_command(ev_thd, 0);
 

sql/sp_head.cc

@@ -3246,7 +3246,6 @@ sp_instr_set::execute(THD *thd, uint *nextp)
 int
 sp_instr_set::exec_core(THD *thd, uint *nextp)
 {
-  create_explain_query(thd->lex, thd->mem_root);
   int res= thd->spcont->set_variable(thd, m_offset, &m_value);
 
   if (res)

sql/sql_delete.cc

@@ -228,6 +228,7 @@ bool mysql_delete(THD *thd, TABLE_LIST *table_list, COND *conds,
   query_plan.using_filesort= FALSE;
   DBUG_ENTER("mysql_delete");
 
+  create_explain_query(thd->lex, thd->mem_root);
   if (open_and_lock_tables(thd, table_list, TRUE, 0))
     DBUG_RETURN(TRUE);
 
@@ -457,7 +458,7 @@ bool mysql_delete(THD *thd, TABLE_LIST *table_list, COND *conds,
   */
   if (thd->lex->describe)
     goto exit_without_my_ok;
-
+  
   query_plan.save_explain_data(thd->lex->explain);
 
   DBUG_EXECUTE_IF("show_explain_probe_delete_exec_start", 

sql/sql_explain.cc

@@ -946,3 +946,9 @@ void create_explain_query(LEX *lex, MEM_ROOT *mem_root)
   lex->explain->mem_root= mem_root;
 }
 
+void create_explain_query_if_not_exists(LEX *lex, MEM_ROOT *mem_root)
+{
+  if (!lex->explain)
+    create_explain_query(lex, mem_root);
+}
+

sql/sql_insert.cc

@@ -719,6 +719,7 @@ bool mysql_insert(THD *thd,TABLE_LIST *table_list,
   Item *unused_conds= 0;
   DBUG_ENTER("mysql_insert");
 
+  create_explain_query(thd->lex, thd->mem_root);
   /*
     Upgrade lock type if the requested lock is incompatible with
     the current connection mode or table operation.

sql/sql_lex.cc

@@ -448,7 +448,7 @@ void lex_start(THD *thd)
 
   lex->thd= lex->unit.thd= thd;
   
-  lex->explain= NULL;
+  DBUG_ASSERT(!lex->explain);
 
   lex->context_stack.empty();
   lex->unit.init_query();

sql/sql_lex.h

@@ -622,6 +622,7 @@ class Explain_query;
 
 void delete_explain_query(LEX *lex);
 void create_explain_query(LEX *lex, MEM_ROOT *mem_root);
+void create_explain_query_if_not_exists(LEX *lex, MEM_ROOT *mem_root);
 bool print_explain_query(LEX *lex, THD *thd, String *str);
 
 class st_select_lex_unit: public st_select_lex_node {

sql/sql_parse.cc

@@ -2201,8 +2201,6 @@ mysql_execute_command(THD *thd)
     thd->mdl_context.release_transactional_locks();
   }
   
-  create_explain_query(thd->lex, thd->mem_root);
-
 #ifndef DBUG_OFF
   if (lex->sql_command != SQLCOM_SET_OPTION)
     DEBUG_SYNC(thd,"before_execute_sql_command");

sql/sql_select.cc

@@ -1020,6 +1020,7 @@ int JOIN::optimize()
   */
   if (was_optimized != optimized && !res && have_query_plan != QEP_DELETED)
   {
+    create_explain_query_if_not_exists(thd->lex, thd->mem_root);
     have_query_plan= QEP_AVAILABLE;
     save_explain_data(thd->lex->explain, false /* can overwrite */,
                       need_tmp,

sql/sql_union.cc

@@ -631,8 +631,10 @@ bool st_select_lex_unit::exec()
     item->make_const();
   
   saved_error= optimize();
+  
+  create_explain_query_if_not_exists(thd->lex, thd->mem_root);
 
-  if (!saved_error && !was_executed && thd->lex->explain)
+  if (!saved_error && !was_executed)
     save_union_explain(thd->lex->explain);
 
   if (uncacheable || !item || !item->assigned() || describe)
@@ -782,7 +784,7 @@ bool st_select_lex_unit::exec()
         if (!fake_select_lex->ref_pointer_array)
           fake_select_lex->n_child_sum_items+= global_parameters->n_sum_items;
         
-        if (!was_executed && thd->lex->explain)
+        if (!was_executed)
           save_union_explain_part2(thd->lex->explain);
 
         saved_error= mysql_select(thd, &fake_select_lex->ref_pointer_array,

sql/sql_update.cc

@@ -281,6 +281,7 @@ int mysql_update(THD *thd,
   query_plan.using_filesort= FALSE;
   DBUG_ENTER("mysql_update");
 
+  create_explain_query(thd->lex, thd->mem_root);
   if (open_tables(thd, &table_list, &table_count, 0))
     DBUG_RETURN(1);