MDEV-39546 Unclear error message on OPEN strict_cursor FOR 'stmt'

This statement:
  OPEN strict_cursor_variable FOR 'SELECT ...';
caused an unclear error message:

ERROR 4078: Illegal parameter data types row<2> and row<0> for operation 'OPEN..FOR'

Fixing the error to be clearer:
ERROR HY000: Incorrect usage of c0 and OPEN..FOR <dynamic string>

Note, this statement is not supported:
  OPEN strict_cursor_variable FOR 'SELECT ...';

It can be rewritten:
- either to use a SELECT statement instead of the dynamic string:
    OPEN strict_cursor_variable FOR SELECT ...;
- or to make c0 a weak cursor variable (i.e.without the RETURN clause)
    OPEN weak_cursor_variable FOR 'SELECT ...';

Author: abarkov

plugin/type_cursor/mysql-test/type_cursor/sp-ref_cursor.result

@@ -56,6 +56,18 @@ $$
 CALL p1;
 DROP PROCEDURE p1;
 #
+# Strict cursors cannot be opened from dynamic strings
+#
+CREATE PROCEDURE p1 AS
+TYPE rec0_t IS RECORD (a INT, b INT);
+TYPE cur0_t IS REF CURSOR RETURN rec0_t;
+c0 cur0_t;
+BEGIN
+OPEN c0 FOR 'SELECT 1,2';
+END;
+$$
+ERROR HY000: Incorrect usage of c0 and OPEN..FOR <dynamic string>
+#
 # Cover the deep_copy() call in LEX::sp_add_fetch_cursor().
 # Make sure that every FETCH statement uses
 # Column_definition::create_length_to_internal_length_string()

plugin/type_cursor/mysql-test/type_cursor/sp-ref_cursor.test

@@ -71,6 +71,24 @@ DELIMITER ;$$
 CALL p1;
 DROP PROCEDURE p1;
 
+
+--echo #
+--echo # Strict cursors cannot be opened from dynamic strings
+--echo #
+
+DELIMITER $$;
+--error ER_WRONG_USAGE
+CREATE PROCEDURE p1 AS
+  TYPE rec0_t IS RECORD (a INT, b INT);
+  TYPE cur0_t IS REF CURSOR RETURN rec0_t;
+  c0 cur0_t;
+BEGIN
+  OPEN c0 FOR 'SELECT 1,2';
+END;
+$$
+DELIMITER ;$$
+
+
 --echo #
 --echo # Cover the deep_copy() call in LEX::sp_add_fetch_cursor().
 --echo # Make sure that every FETCH statement uses

sql/sql_lex.cc

@@ -7853,15 +7853,30 @@ bool LEX::sp_open_cursor_for_stmt(THD *thd, const LEX_CSTRING *name,
     stmt->sql_command= SQLCOM_EXECUTE;
   }
 
+  /*
+    This statement is not supported:
+        OPEN strict_cursor_variable FOR 'SELECT ...';
+    It can be rewritten:
+    - either to use a SELECT statement instead of the dynamic string:
+        OPEN strict_cursor_variable FOR SELECT ...;
+    - or to make c0 a weak cursor variable (i.e.without the RETURN clause)
+        OPEN weak_cursor_variable FOR 'SELECT ...';
+  */
+  return_type_def= dynamic_cast<const sp_type_def_ref*>(spv[0].field_def.
+                                               get_attr_const_generic_ptr(0));
+  if (return_type_def && stmt->prepared_stmt.code())
+  {
+    my_error(ER_WRONG_USAGE, MYF(0), name->str, "OPEN..FOR <dynamic string>");
+    goto error;
+  }
+
   /*
     If the REF CURSOR declaration has the RETURN clause and
     the query select list does not have asterisks, check
     that the row sizes are equal.
     A more thorough test (field-by-field assignability) is done
     later, after the cursor has been opened.
   */
-  return_type_def= dynamic_cast<const sp_type_def_ref*>(spv[0].field_def.
-                                               get_attr_const_generic_ptr(0));
   row_def_list=
     return_type_def && return_type_def->def().is_row() ?
     return_type_def->def().row_field_definitions() : nullptr;