Fixed mdev-15162 Query with CTE hangs if assignment operator (:=) is used

If setting user variable was used in the specification of a recursive CTE
then Item_func_set_user_var::fix_fields() went into an infinite loop.

mysql-test/r/cte_recursive.result

@@ -3050,3 +3050,16 @@ Warnings:
 Note	1003	with recursive destinations as (select `test`.`a`.`arrival` AS `city`,1 AS `legs` from `test`.`flights` `a` where `test`.`a`.`departure` = 'Cairo' union select `test`.`b`.`arrival` AS `arrival`,`r`.`legs` + 1 AS `r.legs + 1` from `destinations` `r` join `test`.`flights` `b` where `r`.`city` = `test`.`b`.`departure` and !<in_optimizer>(`test`.`b`.`arrival`,<exists>(select `destinations`.`city` from `destinations` where trigcond(`test`.`b`.`arrival` = `destinations`.`city` or `destinations`.`city` is null) having trigcond(`destinations`.`city` is null))))select `destinations`.`city` AS `city`,`destinations`.`legs` AS `legs` from `destinations`
 set standard_compliant_cte=default;
 drop table flights;
+#
+# MDEV-15162: Setting user variable in recursive CTE
+#
+SET @c=1;
+WITH RECURSIVE cte AS
+(SELECT 5
+UNION
+SELECT @c:=@c+1 FROM cte WHERE @c<3)
+SELECT * FROM cte;
+5
+5
+2
+3

mysql-test/t/cte_recursive.test

@@ -2085,3 +2085,15 @@ eval explain extended $q;
 set standard_compliant_cte=default;
 
 drop table flights;
+
+--echo #
+--echo # MDEV-15162: Setting user variable in recursive CTE
+--echo #
+
+SET @c=1;
+
+WITH RECURSIVE cte AS
+	(SELECT 5
+	UNION
+	SELECT @c:=@c+1 FROM cte WHERE @c<3)
+SELECT * FROM cte;

sql/item_func.cc

@@ -53,6 +53,7 @@
 #include "sp.h"
 #include "set_var.h"
 #include "debug_sync.h"
+#include "sql_cte.h"
 
 #ifdef NO_EMBEDDED_ACCESS_CHECKS
 #define sp_restore_security_context(A,B) while (0) {}
@@ -4703,10 +4704,13 @@ bool Item_func_set_user_var::fix_fields(THD *thd, Item **ref)
     TABLE_LIST *derived;
     for (derived= unit->derived;
          derived;
-         derived= derived->select_lex->master_unit()->derived)
+         derived= unit->derived)
     {
       derived->set_materialized_derived();
       derived->prohibit_cond_pushdown= true;
+      if (unit->with_element && unit->with_element->is_recursive)
+        break;
+      unit= derived->select_lex->master_unit();
     }
   }