MDEV-23054 Assertion `!item->null_value' failed in Type_handler_inet6::make_sort_key_part (#2)

IFNULL(inet6_not_null_expr, 'foo') erroneously set its nullability to NOT NULL.

Fix:
- Moving the line "maybe_null= args[1]->maybe_null" before the call
  of fix_length_and_dec2(), so the call of Type_handler method
  Item_hybrid_func_fix_attributes() can reset it when desired.

- Fixing Type_handler_inet6::Item_hybrid_func_fix_attributes()
  to ignore args[0] when detecting nullability of IFNULL().

1 files changed, 11 insertions, 1 deletions

diff --git a/sql/item_cmpfunc.h b/sql/item_cmpfunc.h
index 40728f244f3..fe6cba607a7 100644
--- a/sql/item_cmpfunc.h
+++ b/sql/item_cmpfunc.h
@@ -1117,9 +1117,19 @@ public:
   bool native_op(THD *thd, Native *to);
   bool fix_length_and_dec()
   {
+    /*
+      Set nullability from args[1] by default.
+      Note, some type handlers may reset maybe_null
+      in Item_hybrid_func_fix_attributes() if args[1]
+      is NOT NULL but cannot always be converted to
+      the data type of "this" safely.
+      E.g. Type_handler_inet6 does:
+        IFNULL(inet6_not_null_expr, 'foo') -> INET6 NULL
+        IFNULL(inet6_not_null_expr, '::1') -> INET6 NOT NULL
+    */
+    maybe_null= args[1]->maybe_null;
     if (Item_func_case_abbreviation2::fix_length_and_dec2(args))
       return TRUE;
-    maybe_null= args[1]->maybe_null;
     return FALSE;
   }
   const char *func_name() const { return "ifnull"; }