MDEV-27730 Add PLUGIN_VAR_DEPRECATED flag to plugin variables

The sys_var class has the deprecation_substitute member to mark the
deprecated variables. As it's set, the server produces warnings when
these variables are used. However, the plugin has no means to utilize
that functionality.

So, the PLUGIN_VAR_DEPRECATED flag is introduced to set the
deprecation_substitute with the empty string. A non-empty string can
make the warning more informative, but there's no nice way seen to
specify it, and not that needed at the moment.

include/mysql/plugin.h

@@ -229,6 +229,7 @@ typedef int (*mysql_show_var_func)(MYSQL_THD, struct st_mysql_show_var*, void *,
 #define PLUGIN_VAR_NOCMDARG     0x1000 /* No argument for cmd line */
 #define PLUGIN_VAR_RQCMDARG     0x0000 /* Argument required for cmd line */
 #define PLUGIN_VAR_OPCMDARG     0x2000 /* Argument optional for cmd line */
+#define PLUGIN_VAR_DEPRECATED   0x4000 /* Server variable is deprecated */
 #define PLUGIN_VAR_MEMALLOC     0x8000 /* String needs memory allocated */
 
 struct st_mysql_sys_var;
@@ -282,7 +283,8 @@ typedef void (*mysql_var_update_func)(MYSQL_THD thd,
 #define PLUGIN_VAR_MASK \
         (PLUGIN_VAR_READONLY | PLUGIN_VAR_NOSYSVAR | \
          PLUGIN_VAR_NOCMDOPT | PLUGIN_VAR_NOCMDARG | \
-         PLUGIN_VAR_OPCMDARG | PLUGIN_VAR_RQCMDARG | PLUGIN_VAR_MEMALLOC)
+         PLUGIN_VAR_OPCMDARG | PLUGIN_VAR_RQCMDARG | \
+         PLUGIN_VAR_DEPRECATED | PLUGIN_VAR_MEMALLOC)
 
 #define MYSQL_PLUGIN_VAR_HEADER \
   int flags;                    \

mysql-test/main/plugin.result

@@ -40,11 +40,15 @@ a
 set global example_ulong_var=500;
 set global example_enum_var= e1;
 set session example_int_var= -1;
+set global example_deprecated_var=1;
+Warnings:
+Warning	1287	'@@example_deprecated_var' is deprecated and will be removed in a future release
 show status like 'example%';
 Variable_name	Value
 Example_func_example	enum_var is 0, ulong_var is 500, int_var is -1, double_var is 8.500000, really
 show variables like 'example%';
 Variable_name	Value
+example_deprecated_var	0
 example_double_thdvar	8.500000
 example_double_var	8.500000
 example_enum_var	e1

mysql-test/main/plugin.test

@@ -27,6 +27,7 @@ SELECT * FROM t1;
 set global example_ulong_var=500;
 set global example_enum_var= e1;
 set session example_int_var= -1;
+set global example_deprecated_var=1;
 show status like 'example%';
 show variables like 'example%';
 

sql/sql_plugin.cc

@@ -291,7 +291,8 @@ class sys_var_pluginvar: public sys_var, public Sql_alloc
   struct st_mysql_sys_var *plugin_var;
 
   sys_var_pluginvar(sys_var_chain *chain, const char *name_arg,
-                    st_plugin_int *p, st_mysql_sys_var *plugin_var_arg);
+                    st_plugin_int *p, st_mysql_sys_var *plugin_var_arg,
+                    const char *substitute);
   sys_var_pluginvar *cast_pluginvar() { return this; }
   uchar* real_value_ptr(THD *thd, enum_var_type type) const;
   TYPELIB* plugin_var_typelib(void) const;
@@ -3338,11 +3339,11 @@ static int pluginvar_sysvar_flags(const st_mysql_sys_var *p)
 }
 
 sys_var_pluginvar::sys_var_pluginvar(sys_var_chain *chain, const char *name_arg,
-        st_plugin_int *p, st_mysql_sys_var *pv)
+        st_plugin_int *p, st_mysql_sys_var *pv, const char *substitute)
     : sys_var(chain, name_arg, pv->comment, pluginvar_sysvar_flags(pv),
               0, pv->flags & PLUGIN_VAR_NOCMDOPT ? -1 : 0, NO_ARG,
               pluginvar_show_type(pv), 0,
-              NULL, VARIABLE_NOT_IN_BINLOG, NULL, NULL, NULL),
+              NULL, VARIABLE_NOT_IN_BINLOG, NULL, NULL, substitute),
     plugin(p), plugin_var(pv)
 {
   plugin_var->name= name_arg;
@@ -4076,7 +4077,8 @@ static int test_plugin_options(MEM_ROOT *tmp_root, struct st_plugin_int *tmp,
           my_casedn_str(&my_charset_latin1, varname);
           convert_dash_to_underscore(varname, len-1);
         }
-        v= new (mem_root) sys_var_pluginvar(&chain, varname, tmp, o);
+        const char *s= o->flags & PLUGIN_VAR_DEPRECATED ? "" : NULL;
+        v= new (mem_root) sys_var_pluginvar(&chain, varname, tmp, o, s);
         v->test_load= (var ? &var->loaded : &static_unload);
         DBUG_ASSERT(static_unload == FALSE);
 

storage/example/ha_example.cc

@@ -1054,12 +1054,17 @@ static MYSQL_THDVAR_DOUBLE(
   1000.5,
   0);
 
+static MYSQL_THDVAR_INT(
+  deprecated_var, PLUGIN_VAR_RQCMDARG | PLUGIN_VAR_DEPRECATED, "-1..1",
+  NULL, NULL, 0, -1, 1, 0);
+
 static struct st_mysql_sys_var* example_system_variables[]= {
   MYSQL_SYSVAR(enum_var),
   MYSQL_SYSVAR(ulong_var),
   MYSQL_SYSVAR(int_var),
   MYSQL_SYSVAR(double_var),
   MYSQL_SYSVAR(double_thdvar),
+  MYSQL_SYSVAR(deprecated_var),
   MYSQL_SYSVAR(varopt_default),
   NULL
 };