MDEV-16385 ROW SP variable is allowed in unexpected context

The problem described in the bug report happened because the code
did not test check_cols(1) after fix_fields() in a few places.

Additionally, fix_fields() could be called multiple times for SP variables,
because they are all fixed at a early stage in append_for_log().

Solution:
1. Adding a few helper methods
   - fix_fields_if_needed()
   - fix_fields_if_needed_for_scalar()
   - fix_fields_if_needed_for_bool()
   - fix_fields_if_needed_for_order_by()
  and using it in many cases instead of fix_fields() where
  the "fixed" status is not definitely known to be "false".

2. Adding DBUG_ASSERT(!fixed) into Item_splocal*::fix_fields()
   to catch double execution.

3. Adding tests.

As a good side effect, the patch removes a lot of duplicate code (~60 lines):

   if (!item->fixed &&
       item->fix_fields(..) &&
       item->check_cols(1))
     return true;

1 files changed, 4 insertions, 6 deletions

diff --git a/sql/sp_head.cc b/sql/sp_head.cc
index 3ec234759e3..3a9e83d9d09 100644
--- a/sql/sp_head.cc
+++ b/sql/sp_head.cc
@@ -99,7 +99,7 @@ sp_map_item_type(const Type_handler *handler)
 
 bool Item_splocal::append_for_log(THD *thd, String *str)
 {
-  if (fix_fields(thd, NULL))
+  if (fix_fields_if_needed(thd, NULL))
     return true;
 
   if (limit_clause_param)
@@ -137,7 +137,7 @@ bool Item_splocal::append_value_for_log(THD *thd, String *str)
 
 bool Item_splocal_row_field::append_for_log(THD *thd, String *str)
 {
-  if (fix_fields(thd, NULL))
+  if (fix_fields_if_needed(thd, NULL))
     return true;
 
   if (limit_clause_param)
@@ -373,16 +373,14 @@ Item *THD::sp_prepare_func_item(Item **it_addr, uint cols)
 Item *THD::sp_fix_func_item(Item **it_addr)
 {
   DBUG_ENTER("THD::sp_fix_func_item");
-  if (!(*it_addr)->fixed &&
-      (*it_addr)->fix_fields(this, it_addr))
+  if ((*it_addr)->fix_fields_if_needed(this, it_addr))
   {
     DBUG_PRINT("info", ("fix_fields() failed"));
     DBUG_RETURN(NULL);
   }
   it_addr= (*it_addr)->this_item_addr(this, it_addr);
 
-  if (!(*it_addr)->fixed &&
-      (*it_addr)->fix_fields(this, it_addr))
+  if ((*it_addr)->fix_fields_if_needed(this, it_addr))
   {
     DBUG_PRINT("info", ("fix_fields() failed"));
     DBUG_RETURN(NULL);