MDEV-17635 Server hangs after the query with recursive CTE

This bug in the code of the function With_element::check_unrestricted_recursive()
could force a recursive CTE to be executed in a non-standard compliant mode
in which recursive UNION ALL could lead to an infinite execution. This
problem could occur only in the case when this CTE was used by another
recursive CTE at least twice.

mysql-test/t/cte_recursive.test

@@ -2440,3 +2440,58 @@ WITH RECURSIVE expired_map AS (
 SELECT  * FROM expired_map;
 
 DROP TABLE purchases, expired;
+
+--echo #
+--echo # MDEV-17635: Two recursive CTEs, the second using the first
+--echo #
+
+WITH RECURSIVE
+x AS (SELECT 0 as k UNION ALL SELECT k + 1 FROM x WHERE k < 1),
+z AS
+  ( SELECT  k1 AS cx, k2 AS cy, k1, k2
+      FROM (SELECT k  AS k1 FROM x) x1 JOIN (SELECT k  AS k2 FROM x) y1
+    UNION
+    SELECT 1,1,1,1 FROM z)
+SELECT * FROM z;
+
+--echo # https://wiki.postgresql.org/wiki/Mandelbrot_set:
+
+WITH RECURSIVE x(i) AS (
+    SELECT CAST(0 AS DECIMAL(13, 10))
+     UNION ALL
+    SELECT i + 1
+      FROM x
+     WHERE i < 101
+),
+Z(Ix, Iy, Cx, Cy, X, Y, I) AS (
+    SELECT Ix, Iy, X, Y, X, Y, 0
+      FROM (SELECT CAST(-2.2 + 0.031 * i AS DECIMAL(13, 10)) AS X,
+                  i AS Ix FROM x) AS xgen
+           CROSS JOIN (
+               SELECT CAST(-1.5 + 0.031 * i AS DECIMAL(13, 10)) AS Y,
+                      i AS iY FROM x
+           ) AS ygen
+    UNION ALL
+    SELECT Ix, Iy, Cx, Cy,
+           CAST(X * X - Y * Y + Cx AS DECIMAL(13, 10)) AS X,
+           CAST(Y * X * 2 + Cy AS DECIMAL(13, 10)), I + 1
+      FROM Z
+     WHERE X * X + Y * Y < 16.0
+           AND I < 27
+),
+Zt (Ix, Iy, I) AS (
+    SELECT Ix, Iy, MAX(I) AS I
+      FROM Z
+     GROUP BY Iy, Ix
+     ORDER BY Iy, Ix
+)
+SELECT GROUP_CONCAT(
+           SUBSTRING(
+               ' .,,,-----++++%%%%@@@@#### ',
+               GREATEST(I, 1),
+               1
+           ) ORDER BY Ix SEPARATOR ''
+       ) AS 'Mandelbrot Set'
+  FROM Zt
+GROUP BY Iy
+ORDER BY Iy;

sql/sql_cte.cc

@@ -1263,7 +1263,7 @@ bool With_element::check_unrestricted_recursive(st_select_lex *sel,
       With_element *with_elem= unit->with_element;
       if (encountered & with_elem->get_elem_map())
         unrestricted|= with_elem->mutually_recursive;
-      else
+      else if (with_elem ==this)
         encountered|= with_elem->get_elem_map();
     }
   }