MDEV-20034 Add support for the pre-defined weak SYS_REFCURSOR

This patch adds support for SYS_REFCURSOR (a weakly typed cursor)
for both sql_mode=ORACLE and sql_mode=DEFAULT.

Works as a regular stored routine variable, parameter and return value:

- can be passed as an IN parameter to stored functions and procedures
- can be passed as an INOUT and OUT parameter to stored procedures
- can be returned from a stored function

Note, strongly typed REF CURSOR will be added separately.

Note, to maintain dependencies easier, some parts of sql_class.h
and item.h were moved to new header files:

- select_results.h:
  class select_result_sink
  class select_result
  class select_result_interceptor

- sp_cursor.h:
  class sp_cursor_statistics
  class sp_cursor

- sp_rcontext_handler.h
  class Sp_rcontext_handler and its descendants

The implementation consists of the following parts:
- A new class sp_cursor_array deriving from Dynamic_array

- A new class Statement_rcontext which contains data shared
  between sub-statements of a compound statement.
  It has a member m_statement_cursors of the sp_cursor_array data type,
  as well as open cursor counter. THD inherits from Statement_rcontext.

- A new data type handler Type_handler_sys_refcursor in plugins/type_cursor/
  It is designed to store uint16 references -
  positions of the cursor in THD::m_statement_cursors.

- Type_handler_sys_refcursor suppresses some derived numeric features.
  When a SYS_REFCURSOR variable is used as an integer an error is raised.

- A new abstract class sp_instr_fetch_cursor. It's needed to share
  the common code between "OPEN cur" (for static cursors) and
  "OPER cur FOR stmt" (for SYS_REFCURSORs).

- New sp_instr classes:
  * sp_instr_copen_by_ref      - OPEN sys_ref_curor FOR stmt;
  * sp_instr_cfetch_by_ref     - FETCH sys_ref_cursor INTO targets;
  * sp_instr_cclose_by_ref     - CLOSE sys_ref_cursor;
  * sp_instr_destruct_variable - to destruct SYS_REFCURSOR variables when
                                 the execution goes out of the BEGIN..END block
                                 where SYS_REFCURSOR variables are declared.
- New methods in LEX:
  * sp_open_cursor_for_stmt   - handles "OPEN sys_ref_cursor FOR stmt".
  * sp_add_instr_fetch_cursor - "FETCH cur INTO targets" for both
                                static cursors and SYS_REFCURSORs.
  * sp_close - handles "CLOSE cur" both for static cursors and SYS_REFCURSORs.

- Changes in cursor functions to handle both static cursors and SYS_REFCURSORs:
  * Item_func_cursor_isopen
  * Item_func_cursor_found
  * Item_func_cursor_notfound
  * Item_func_cursor_rowcount

- A new system variable @@max_open_cursors - to limit the number
  of cursors (static and SYS_REFCURSORs) opened at the same time.
  Its allowed range is [0-65536], with 50 by default.

- A new virtual method Type_handler::can_return_bool() telling
  if calling item->val_bool() is allowed for Items of this data type,
  or if otherwise the "Illegal parameter for operation" error should be raised
  at fix_fields() time.

- New methods in Sp_rcontext_handler:
  * get_cursor()
  * get_cursor_by_ref()

- A new class Sp_rcontext_handler_statement to handle top level statement
  wide cursors which are shared by all substatements.

- A new virtual method expr_event_handler() in classes Item and Field.
  It's needed to close (and make available for a new OPEN)
  unused THD::m_statement_cursors elements which do not have any references
  any more. It can happen in various moments in time, e.g.
  * after evaluation parameters of an SQL routine
  * after assigning a cursor expression into a SYS_REFCURSOR variable
  * when leaving a BEGIN..END block with SYS_REFCURSOR variables
  * after setting OUT/INOUT routine actual parameters from formal
    parameters.

Author: abarkov

libmysqld/CMakeLists.txt

@@ -157,6 +157,7 @@ SET(SQL_EMBEDDED_SOURCES emb_qcache.cc libmysqld.c lib_sql.cc
            ../sql/json_table.cc
            ../sql/opt_histogram_json.cc
            ../sql/sp_instr.cc
+           ../sql/sp_cursor.cc
            ${GEN_SOURCES}
            ${MYSYS_LIBWRAP_SOURCE}
 )

mysql-test/include/bool_to_char.inc

@@ -0,0 +1,18 @@
+SET @sql_mode=@@sql_mode;
+
+SET sql_mode=ORACLE;
+
+DELIMITER /;
+CREATE FUNCTION bool_to_char(b BOOLEAN) RETURN VARCHAR AS
+BEGIN
+  RETURN
+  CASE
+     WHEN b IS NULL THEN 'NULL'
+     WHEN b         THEN 'true'
+     WHEN NOT b     THEN 'false'
+  END;
+END;
+/
+DELIMITER ;/
+
+SET sql_mode=@sql_mode;

mysql-test/include/dbms_output.inc

@@ -0,0 +1,22 @@
+SET @sql_mode=@@sql_mode;
+
+SET sql_mode=ORACLE;
+
+DELIMITER /;
+
+CREATE PACKAGE DBMS_OUTPUT AS
+  PROCEDURE PUT_LINE(s VARCHAR);
+END;
+/
+
+CREATE PACKAGE BODY DBMS_OUTPUT AS
+  PROCEDURE PUT_LINE(s VARCHAR) AS
+  BEGIN
+    SELECT s AS `` FROM DUAL;
+  END;
+END;
+/
+
+DELIMITER ;/
+
+SET sql_mode=@sql_mode;

mysql-test/include/fetch_one_value.inc

@@ -0,0 +1,17 @@
+# Fetch one value from an open SYS_REFCURSOR
+
+SET @sql_mode=@@sql_mode;
+
+SET sql_mode=ORACLE;
+DELIMITER /;
+CREATE FUNCTION fetch_one_value(c SYS_REFCURSOR) RETURN VARCHAR AS
+  v VARCHAR(128) :='none';
+BEGIN
+  IF c%ISOPEN THEN
+    FETCH c INTO v;
+  END IF;
+  RETURN v;
+END;
+/
+DELIMITER ;/
+SET sql_mode=@sql_mode;

mysql-test/main/mysqld--help.result

@@ -658,6 +658,8 @@ The following specify which files/extra groups are read (specified before remain
  max_join_size records return an error
  --max-length-for-sort-data=# 
  Max number of bytes in sorted records
+ --max-open-cursors=# 
+ The maximum number of open cursors allowed per session
  --max-password-errors=# 
  If there is more than this number of failed connect
  attempts due to invalid password, user will be blocked
@@ -1817,6 +1819,7 @@ max-error-count 64
 max-heap-table-size 16777216
 max-join-size 18446744073709551615
 max-length-for-sort-data 1024
+max-open-cursors 50
 max-password-errors 18446744073709551615
 max-prepared-stmt-count 16382
 max-recursive-iterations 1000
@@ -1932,7 +1935,7 @@ performance-schema-max-socket-classes 10
 performance-schema-max-socket-instances -1
 performance-schema-max-sql-text-length 1024
 performance-schema-max-stage-classes 160
-performance-schema-max-statement-classes 223
+performance-schema-max-statement-classes 227
 performance-schema-max-statement-stack 10
 performance-schema-max-table-handles -1
 performance-schema-max-table-instances -1