MDEV-36047 Package body variables are not allowed as FETCH targets

It was not possible to use a package body variable as a
fetch target:

CREATE PACKAGE BODY pkg AS
  vc INT := 0;
  FUNCTION f1 RETURN INT AS
    CURSOR cur IS SELECT 1 AS c FROM DUAL;
  BEGIN
    OPEN cur;
    FETCH cur INTO vc; -- this returned "Undeclared variable: vc" error.
    CLOSE cur;
    RETURN vc;
  END;
END;

FETCH assumed that all fetch targets reside of the same sp_rcontext
instance with the cursor. This patch fixes the problem.
Now a cursor and its fetch target can reside in different sp_rcontext
instances.

Details:

- Adding a helper class sp_rcontext_addr
  (a combination of Sp_rcontext_handler pointer and an offset in the rcontext)

- Adding a new class sp_fetch_target deriving from sp_rcontext_addr.
  Fetch targets in "FETCH cur INTO target1, target2 ..." are now collected
  into this structure instead of sp_variable.
  sp_variable cannot be used any more to store fetch targets,
  because it does not have a pointer to Sp_rcontext_handler
  (it only has the current rcontext offset).

- Removing members sp_instr_set members m_rcontext_handler and m_offset.
  Deriving sp_instr_set from sp_rcontext_addr instead.

- Renaming sp_instr_cfetch member  "List<sp_variable> m_varlist"
  to "List<sp_fetch_target> m_fetch_target_list".

- Fixing LEX::sp_add_cfetch() to return the pointer to the
  created sp_fetch_target instance (instead of returning bool).
  This helps to make the grammar in sql_yacc.c simpler

- Renaming LEX::sp_add_cfetch() to LEX::sp_add_instr_cfetch(),
  as `if(sp_add_cfetch())` changed its meaning to the opposite,
  to avoid automatic wrong merge from earlier versions.

- Chaning the "List<sp_variable> *vars" parameter to sp_cursor::fetch
  to have the data type "List<sp_fetch_target> *".

- Changing the data type of "List<sp_variable> &vars" in
  sp_cursor::Select_fetch_into_spvars::send_data_to_variable_list()
  to "List<sp_fetch_target> &".

- Adding THD helper methods get_rcontext() and get_variable().

- Moving the code from sql_yacc.yy into a new LEX method
  LEX::make_fetch_target().

- Simplifying the grammar in sql_yacc.yy using the new LEX method.
  Changing the data type of the bison rule sp_fetch_list from "void"
  to "List<sp_fetch_target> *".

mysql-test/main/sp-package-code.result

@@ -258,3 +258,105 @@ Pos	Instruction
 6	set a.a@0["a"] b.a@1["a"] + 1
 DROP PACKAGE pkg1;
 DROP TABLE t1;
+# Start of 11.4 tests
+#
+# MDEV-36047 Package body variables are not allowed as FETCH targets
+#
+CREATE PACKAGE pkg
+FUNCTION f1() RETURNS INT;
+END;
+$$
+CREATE PACKAGE BODY pkg
+DECLARE vc INT DEFAULT 0;
+FUNCTION f1() RETURNS INT
+BEGIN
+DECLARE cur CURSOR FOR SELECT 1 AS c FROM DUAL;
+OPEN cur;
+FETCH cur INTO vc; -- SHOW CODE should display vc with a "PACKAGE_BODY" prefix
+CLOSE cur;
+RETURN vc;
+END;
+BEGIN
+DECLARE cur CURSOR FOR SELECT 1 AS c FROM DUAL;
+OPEN cur;
+FETCH cur INTO vc; -- SHOW CODE should display vc without a prefix
+CLOSE cur;
+END;
+END;
+$$
+SELECT pkg.f1() FROM DUAL;
+pkg.f1()
+1
+SHOW FUNCTION CODE pkg.f1;
+Pos	Instruction
+0	cpush cur@0
+1	copen cur@0
+2	cfetch cur@0 PACKAGE_BODY.vc@0
+3	cclose cur@0
+4	freturn int PACKAGE_BODY.vc@0
+SHOW PACKAGE BODY CODE pkg;
+Pos	Instruction
+0	set vc@0 0
+1	cpush cur@0
+2	copen cur@0
+3	cfetch cur@0 vc@0
+4	cclose cur@0
+5	cpop 1
+DROP PACKAGE pkg;
+CREATE PACKAGE pkg
+FUNCTION f1() RETURNS TEXT;
+END;
+$$
+CREATE PACKAGE BODY pkg
+DECLARE vc1 INT DEFAULT 0;
+FUNCTION f1() RETURNS TEXT
+BEGIN
+DECLARE vc2 INT DEFAULT 0;
+DECLARE cur CURSOR FOR SELECT 1 AS c1, 2 AS c2 FROM DUAL;
+OPEN cur;
+FETCH cur INTO vc1, vc2;
+CLOSE cur;
+RETURN CONCAT(vc1, ' ', vc2);
+END;
+END;
+$$
+SELECT pkg.f1() FROM DUAL;
+pkg.f1()
+1 2
+SHOW FUNCTION CODE pkg.f1;
+Pos	Instruction
+0	set vc2@0 0
+1	cpush cur@0
+2	copen cur@0
+3	cfetch cur@0 PACKAGE_BODY.vc1@0 vc2@0
+4	cclose cur@0
+5	freturn blob concat(PACKAGE_BODY.vc1@0,' ',vc2@0)
+DROP PACKAGE pkg;
+CREATE PACKAGE pkg
+FUNCTION f1() RETURNS TEXT;
+END;
+$$
+CREATE PACKAGE BODY pkg
+DECLARE vc ROW(p1 INT, p2 INT);
+FUNCTION f1() RETURNS TEXT
+BEGIN
+DECLARE cur CURSOR FOR SELECT 1 AS c1, 2 AS c2 FROM DUAL;
+OPEN cur;
+FETCH cur INTO vc;
+CLOSE cur;
+RETURN CONCAT(vc.p1, ' ', vc.p2);
+END;
+END;
+$$
+SELECT pkg.f1() FROM DUAL;
+pkg.f1()
+1 2
+SHOW FUNCTION CODE pkg.f1;
+Pos	Instruction
+0	cpush cur@0
+1	copen cur@0
+2	cfetch cur@0 PACKAGE_BODY.vc@0
+3	cclose cur@0
+4	freturn blob concat(PACKAGE_BODY.vc.p1@0[0],' ',PACKAGE_BODY.vc.p2@0[1])
+DROP PACKAGE pkg;
+# End of 11.4 tests