MySQL Plugin generator now supporting system variables

Currently only simple integer and string variables are supported. Support for ENUM and SET variables and for custom check() and update() functions will probably follow soon though.

A system variable declaration currently looks like this:

  <systemvar scope="global"  type="str" name="baz" default="foobar" />
  <systemvar scope="global"  type="int" name="bar" min="23" max="42" default="38" />
  <systemvar scope="session" type="int" name="foo" min="23" max="42" default="37" />

The scope, type and name attributes are mandatory, setting a default value is recommended and the min and max attributes for integers are optional.

For global system variables a global variable by the same name is generated in the plugins C++ code which can be accessed directly in the code.

Access to session variables from the plugins C++ code is a bit more tricky, but not much. As session variables are tied to the current connection thread the THDVAR() macro is needed to access them for both assigning and retrieving values:

  THDVAR(thd, varname) = 42;
  foo = THDVAR(thd, varname);