MDEV-22441 implement a generic way to change a value of a variable in a scopebb-10.7-MDEV-22441-scoped-variable

Example:
{
  auto _= make_scope_value(var, tmp_value);
}

make_scope_value(): a function which returns RAII object which temporary
changes a value of a variable

detail::Scope_value: actual implementation of such RAII class.
It shouldn't be used directly! That's why it's inside a namespace detail.

3 files changed, 51 insertions, 4 deletions

diff --git a/include/scope.h b/include/scope.h
index e0e9fc62969..836c4f77b35 100644
--- a/include/scope.h
+++ b/include/scope.h
@@ -72,3 +72,48 @@ make_scope_exit(Callable &&f)
 #define ANONYMOUS_VARIABLE CONCAT(_anonymous_variable, __LINE__)
 
 #define SCOPE_EXIT auto ANONYMOUS_VARIABLE= make_scope_exit
+
+namespace detail
+{
+
+template <typename T> class Scope_value
+{
+public:
+  Scope_value(T &variable, const T &scope_value)
+      : variable_(variable), saved_value_(variable)
+  {
+    variable= scope_value;
+  }
+
+  Scope_value(Scope_value &&rhs)
+      : variable_(rhs.variable_), saved_value_(rhs.saved_value_),
+        engaged_(rhs.engaged_)
+  {
+    rhs.engaged_= false;
+  }
+
+  Scope_value(const Scope_value &)= delete;
+  Scope_value &operator=(const Scope_value &)= delete;
+  Scope_value &operator=(Scope_value &&)= delete;
+
+  ~Scope_value()
+  {
+    if (engaged_)
+      variable_= saved_value_;
+  }
+
+private:
+  T &variable_;
+  T saved_value_;
+  bool engaged_= true;
+};
+
+} // namespace detail
+
+// Use like this:
+// auto _= make_scope_value(var, tmp_value);
+template <typename T>
+detail::Scope_value<T> make_scope_value(T &variable, const T &scope_value)
+{
+  return detail::Scope_value<T>(variable, scope_value);
+}
diff --git a/sql/sql_acl.cc b/sql/sql_acl.cc
index 2d7f62cd725..376dad2373e 100644
--- a/sql/sql_acl.cc
+++ b/sql/sql_acl.cc
@@ -54,6 +54,7 @@
 #include "sql_array.h"
 #include "sql_hset.h"
 #include "password.h"
+#include "scope.h"
 
 #include "sql_plugin_compat.h"
 
@@ -2465,10 +2466,11 @@ static bool acl_load(THD *thd, const Grant_tables& tables)
   READ_RECORD read_record_info;
   bool check_no_resolve= specialflag & SPECIAL_NO_RESOLVE;
   char tmp_name[SAFE_NAME_LEN+1];
-  Sql_mode_save old_mode_save(thd);
   DBUG_ENTER("acl_load");
 
-  thd->variables.sql_mode&= ~MODE_PAD_CHAR_TO_FULL_LENGTH;
+  auto _= make_scope_value(thd->variables.sql_mode,
+                           thd->variables.sql_mode &
+                               ~MODE_PAD_CHAR_TO_FULL_LENGTH);
 
   grant_version++; /* Privileges updated */
 
diff --git a/sql/sql_show.cc b/sql/sql_show.cc
index d01f84fe7d1..2851f867889 100644
--- a/sql/sql_show.cc
+++ b/sql/sql_show.cc
@@ -65,6 +65,7 @@
 #include "transaction.h"
 #include "opt_trace.h"
 #include "my_cpu.h"
+#include "scope.h"
 
 
 #include "lex_symbol.h"
@@ -6409,8 +6410,7 @@ bool store_schema_params(THD *thd, TABLE *table, TABLE *proc_table,
   {
     Field *field;
     LEX_CSTRING tmp_string;
-    Sql_mode_save sql_mode_backup(thd);
-    thd->variables.sql_mode= sql_mode;
+    auto _= make_scope_value(thd->variables.sql_mode, sql_mode);
 
     if (sph->type() == SP_TYPE_FUNCTION)
     {