Improve comments around FLUSH STATUS

It's not possible to flush the global status variables in 5.0
Update test case so it works by recording the value of handle_rollback
before and compare it to the value after


mysql-test/r/innodb_mysql.result:
  Update result file
mysql-test/t/innodb_mysql.test:
  It's not possible to reset the global status variables in 5.0 so intead its value
  is recorded and compared to the after value.
  It should not have changed.
sql/mysqld.cc:
  Improve comments
sql/set_var.cc:
  Improve comments
sql/sql_class.cc:
  Improve comments
sql/sql_class.h:
  Improve comments

mysql-test/r/innodb_mysql.result

@@ -104,14 +104,12 @@ SELECT `id1` FROM `t1` WHERE `id1` NOT IN (SELECT `id1` FROM `t2` WHERE `id2` =
 id1
 2
 DROP TABLE t1, t2;
-flush status;
 create table t1 (c1 int) engine=innodb;
 handler t1 open;
 handler t1 read first;
 c1
-show /*!50002 GLOBAL */ status like 'Handler_rollback';
-Variable_name	Value
-Handler_rollback	0
+Before and after comparison
+0
 drop table t1;
 End of 4.1 tests
 create table t1m (a int) engine=myisam;

mysql-test/t/innodb_mysql.test

@@ -119,7 +119,8 @@ SELECT `id1` FROM `t1` WHERE `id1` NOT IN (SELECT `id1` FROM `t2` WHERE `id2` =
 DROP TABLE t1, t2;
 # Bug #22728 - Handler_rollback value is growing
 #
-flush status;
+
+let $before= `show /*!50002 GLOBAL */ status like 'Handler_rollback'`;
 create table t1 (c1 int) engine=innodb;
 connect (con1,localhost,root,,);
 connect (con2,localhost,root,,);
@@ -128,7 +129,11 @@ handler t1 open;
 handler t1 read first;
 disconnect con2;
 connection con1;
-show /*!50002 GLOBAL */ status like 'Handler_rollback';
+let $after= `show /*!50002 GLOBAL */ status like 'Handler_rollback'`;
+# Compare the before and after value, it should be equal
+--disable_query_log
+eval select STRCMP("$before", "$after") as "Before and after comparison";
+--enable_query_log
 connection default;
 drop table t1;
 disconnect con1;

sql/mysqld.cc

@@ -5997,6 +5997,10 @@ The minimum value for this variable is 4096.",
 };
 
 
+/*
+  Variables shown by SHOW STATUS in alphabetical order
+*/
+
 struct show_var_st status_vars[]= {
   {"Aborted_clients",          (char*) &aborted_threads,        SHOW_LONG},
   {"Aborted_connects",         (char*) &aborted_connects,       SHOW_LONG},
@@ -7466,15 +7470,19 @@ void refresh_status(THD *thd)
 {
   pthread_mutex_lock(&LOCK_status);
 
-  /* We must update the global status before cleaning up the thread */
+  /* Add thread's status variabes to global status */
   add_to_status(&global_status_var, &thd->status_var);
+
+  /* Reset thread's status variables */
   bzero((char*) &thd->status_var, sizeof(thd->status_var));
 
+  /* Reset some global variables */
   for (struct show_var_st *ptr=status_vars; ptr->name; ptr++)
   {
     if (ptr->type == SHOW_LONG)
       *(ulong*) ptr->value= 0;
   }
+
   /* Reset the counters of all key caches (default and named). */
   process_key_caches(reset_key_cache_counters);
   pthread_mutex_unlock(&LOCK_status);

sql/set_var.cc

@@ -804,7 +804,7 @@ sys_var *sys_variables[]=
 
 
 /*
-  Variables shown by SHOW variables in alphabetical order
+  Variables shown by SHOW VARIABLES in alphabetical order
 */
 
 struct show_var_st init_vars[]= {

sql/sql_class.cc

@@ -464,14 +464,13 @@ THD::~THD()
 
 void add_to_status(STATUS_VAR *to_var, STATUS_VAR *from_var)
 {
-  ulong *end= (ulong*) ((byte*) to_var + offsetof(STATUS_VAR,
-						  last_system_status_var) +
+  ulong *end= (ulong*) ((byte*) to_var +
+                        offsetof(STATUS_VAR, last_system_status_var) +
 			sizeof(ulong));
   ulong *to= (ulong*) to_var, *from= (ulong*) from_var;
 
   while (to != end)
     *(to++)+= *(from++);
-  /* it doesn't make sense to add last_query_cost values */
 }
 
 

sql/sql_class.h

@@ -646,12 +646,17 @@ typedef struct system_status_var
   ulong com_stmt_reset;
   ulong com_stmt_close;
 
+  /*
+    Status variables which it does not make sense to add to
+    global status variable counter
+  */
   double last_query_cost;
 } STATUS_VAR;
 
 /*
-  This is used for 'show status'. It must be updated to the last ulong
-  variable in system_status_var
+  This is used for 'SHOW STATUS'. It must be updated to the last ulong
+  variable in system_status_var which is makes sens to add to the global
+  counter
 */
 
 #define last_system_status_var com_stmt_close