Bug#35654240: A Recursive CTE Causing MySQL Server Crash

Problem: Right-nested UNIONs are not supported for recursive query
blocks in CTEs. A check for this is missing. In WL#11350, we started
allowing right-nested set operations. The missing check resulted in
a crash.

Solution: If there are right-nested UNIONs of recursive query blocks
that cannot be safely flattened, we throw an error indicating that
the operation is not yet supported.

Change-Id: I8ac01f91bf63e81fe7393179a3d01f9ba1eaa4e2

Author: mithuncy

sql/sql_derived.cc

@@ -365,6 +365,17 @@ bool Table_ref::resolve_derived(THD *thd, bool apply_semijoin) {
         if (sl->parent()->term_type() != QT_UNION) {
           my_error(ER_CTE_RECURSIVE_NOT_UNION, MYF(0));
           return true;
+        } else if (sl->parent()->parent() != nullptr) {
+          /*
+            Right-nested UNIONs with recursive query blocks are not allowed. It
+            is expected that all possible flattening of UNION blocks is done
+            beforehand. Any nested UNION indicates a mixing of UNION DISTINCT
+            and UNION ALL, which cannot be flattened further.
+          */
+          my_error(ER_NOT_SUPPORTED_YET, MYF(0),
+                   "right nested recursive query blocks, in "
+                   "Common Table Expression");
+          return true;
         }
         if (sl->is_ordered() || sl->has_limit() || sl->is_distinct()) {
           /*