MDEV-23619 MariaDB crash on WITH RECURSIVE UNION ALL (CTE) query

Due to a premature cleanup of the unit that specified a recursive CTE
used in the second operand of union the server fell into an infinite
loop in the reported test case. In other cases this premature cleanup
could cause other problems.
The bug is the result of a not quite correct fix for MDEV-17024. The
unit that specifies a recursive CTE has to be cleaned only after the
cleanup of the last external reference to this CTE. It means that
cleanups of the unit triggered not by the cleanup of a external
reference to the CTE must be blocked.
Usage of local table chains in selects to get external references to
recursive CTEs was not correct either because of possible merges of
some selects.

Also fixed a minor bug in st_select_lex::set_explain_type() that caused
typing 'RECURSIVE UNION' instead of 'UNION' in EXPLAIN output for external
references to a recursive CTE.

mysql-test/r/cte_recursive.result

@@ -1301,7 +1301,7 @@ select ancestors.name, ancestors.dob from ancestors;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	<derived4>	ALL	NULL	NULL	NULL	NULL	24	
 4	DERIVED	folks	ALL	NULL	NULL	NULL	NULL	12	Using where
-6	RECURSIVE UNION	<derived3>	ALL	NULL	NULL	NULL	NULL	12	
+6	UNION	<derived3>	ALL	NULL	NULL	NULL	NULL	12	
 5	RECURSIVE UNION	<derived4>	ALL	NULL	NULL	NULL	NULL	24	
 NULL	UNION RESULT	<union4,6,5>	ALL	NULL	NULL	NULL	NULL	NULL	
 3	DERIVED	folks	ALL	NULL	NULL	NULL	NULL	12	Using where
@@ -3964,5 +3964,232 @@ YEAR	d1	d2
 DROP PROCEDURE p;
 DROP TABLE t1,t2,t3,t4;
 #
+# MDEV-23619: recursive CTE used only in the second operand of UNION
+#
+create table t1 (
+a bigint(10) not null auto_increment,
+b int(5) not null,
+c bigint(10) default null,
+primary key (a)
+) engine myisam;
+insert into t1 values
+(1,3,12), (2,7,15), (3,1,3), (4,3,1);
+explain with recursive r_cte as
+( select * from t1 as s
+union
+select t1.* from t1, r_cte as r where t1.c = r.a )
+select 0 as b FROM dual union all select b FROM r_cte as t;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+2	DERIVED	s	ALL	NULL	NULL	NULL	NULL	4	
+3	RECURSIVE UNION	t1	ALL	NULL	NULL	NULL	NULL	4	Using where
+3	RECURSIVE UNION	<derived2>	ref	key0	key0	8	test.t1.c	2	
+NULL	UNION RESULT	<union2,3>	ALL	NULL	NULL	NULL	NULL	NULL	
+4	UNION	<derived2>	ALL	NULL	NULL	NULL	NULL	4	
+with recursive r_cte as
+( select * from t1 as s
+union
+select t1.* from t1, r_cte as r where t1.c = r.a )
+select 0 as b FROM dual union all select b FROM r_cte as t;
+b
+0
+3
+7
+1
+3
+analyze format=json with recursive r_cte as
+( select * from t1 as s
+union
+select t1.* from t1, r_cte as r where t1.c = r.a )
+select 0 as b FROM dual union all select b FROM r_cte as t;
+ANALYZE
+{
+  "query_block": {
+    "union_result": {
+      "table_name": "<union1,4>",
+      "access_type": "ALL",
+      "r_loops": 0,
+      "r_rows": null,
+      "query_specifications": [
+        {
+          "query_block": {
+            "select_id": 1,
+            "table": {
+              "message": "No tables used"
+            }
+          }
+        },
+        {
+          "query_block": {
+            "select_id": 4,
+            "r_loops": 1,
+            "r_total_time_ms": "REPLACED",
+            "table": {
+              "table_name": "<derived2>",
+              "access_type": "ALL",
+              "r_loops": 1,
+              "rows": 4,
+              "r_rows": 4,
+              "r_total_time_ms": "REPLACED",
+              "filtered": 100,
+              "r_filtered": 100,
+              "materialized": {
+                "query_block": {
+                  "recursive_union": {
+                    "table_name": "<union2,3>",
+                    "access_type": "ALL",
+                    "r_loops": 0,
+                    "r_rows": null,
+                    "query_specifications": [
+                      {
+                        "query_block": {
+                          "select_id": 2,
+                          "r_loops": 1,
+                          "r_total_time_ms": "REPLACED",
+                          "table": {
+                            "table_name": "s",
+                            "access_type": "ALL",
+                            "r_loops": 1,
+                            "rows": 4,
+                            "r_rows": 4,
+                            "r_total_time_ms": "REPLACED",
+                            "filtered": 100,
+                            "r_filtered": 100
+                          }
+                        }
+                      },
+                      {
+                        "query_block": {
+                          "select_id": 3,
+                          "r_loops": 1,
+                          "r_total_time_ms": "REPLACED",
+                          "table": {
+                            "table_name": "t1",
+                            "access_type": "ALL",
+                            "r_loops": 1,
+                            "rows": 4,
+                            "r_rows": 4,
+                            "r_total_time_ms": "REPLACED",
+                            "filtered": 100,
+                            "r_filtered": 100,
+                            "attached_condition": "t1.c is not null"
+                          },
+                          "table": {
+                            "table_name": "<derived2>",
+                            "access_type": "ref",
+                            "possible_keys": ["key0"],
+                            "key": "key0",
+                            "key_length": "8",
+                            "used_key_parts": ["a"],
+                            "ref": ["test.t1.c"],
+                            "r_loops": 4,
+                            "rows": 2,
+                            "r_rows": 0.5,
+                            "r_total_time_ms": "REPLACED",
+                            "filtered": 100,
+                            "r_filtered": 100
+                          }
+                        }
+                      }
+                    ]
+                  }
+                }
+              }
+            }
+          }
+        }
+      ]
+    }
+  }
+}
+prepare stmt from "with recursive r_cte as
+( select * from t1 as s
+union
+select t1.* from t1, r_cte as r where t1.c = r.a )
+select 0 as b FROM dual union all select b FROM r_cte as t";
+execute stmt;
+b
+0
+3
+7
+1
+3
+execute stmt;
+b
+0
+3
+7
+1
+3
+deallocate prepare stmt;
+#checking hanging cte that uses a recursive cte
+explain with h_cte as
+( with recursive r_cte as
+( select * from t1 as s
+union
+select t1.* from t1, r_cte as r where t1.c = r.a )
+select 0 as b FROM dual union all select b FROM r_cte as t)
+select * from t1 as tt;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	tt	ALL	NULL	NULL	NULL	NULL	4	
+with h_cte as
+( with recursive r_cte as
+( select * from t1 as s
+union
+select t1.* from t1, r_cte as r where t1.c = r.a )
+select 0 as b FROM dual union all select b FROM r_cte as t)
+select * from t1 as tt;
+a	b	c
+1	3	12
+2	7	15
+3	1	3
+4	3	1
+analyze format=json with h_cte as
+( with recursive r_cte as
+( select * from t1 as s
+union
+select t1.* from t1, r_cte as r where t1.c = r.a )
+select 0 as b FROM dual union all select b FROM r_cte as t)
+select * from t1 as tt;
+ANALYZE
+{
+  "query_block": {
+    "select_id": 1,
+    "r_loops": 1,
+    "r_total_time_ms": "REPLACED",
+    "table": {
+      "table_name": "tt",
+      "access_type": "ALL",
+      "r_loops": 1,
+      "rows": 4,
+      "r_rows": 4,
+      "r_total_time_ms": "REPLACED",
+      "filtered": 100,
+      "r_filtered": 100
+    }
+  }
+}
+prepare stmt from "with h_cte as
+( with recursive r_cte as
+( select * from t1 as s
+union
+select t1.* from t1, r_cte as r where t1.c = r.a )
+select 0 as b FROM dual union all select b FROM r_cte as t)
+select * from t1 as tt";
+execute stmt;
+a	b	c
+1	3	12
+2	7	15
+3	1	3
+4	3	1
+execute stmt;
+a	b	c
+1	3	12
+2	7	15
+3	1	3
+4	3	1
+deallocate prepare stmt;
+drop table t1;
+#
 # End of 10.2 tests
 #

mysql-test/t/cte_recursive.test

@@ -2640,6 +2640,56 @@ SELECT * FROM t4;
 DROP PROCEDURE p;
 DROP TABLE t1,t2,t3,t4;
 
+--echo #
+--echo # MDEV-23619: recursive CTE used only in the second operand of UNION
+--echo #
+
+create table t1 (
+  a bigint(10) not null auto_increment,
+  b int(5) not null,
+  c bigint(10) default null,
+  primary key (a)
+) engine myisam;
+insert into t1 values
+  (1,3,12), (2,7,15), (3,1,3), (4,3,1);
+
+let $q=
+with recursive r_cte as
+( select * from t1 as s
+  union
+  select t1.* from t1, r_cte as r where t1.c = r.a )
+select 0 as b FROM dual union all select b FROM r_cte as t;
+
+eval explain $q;
+eval $q;
+--source include/analyze-format.inc
+eval analyze format=json $q;
+eval prepare stmt from "$q";
+execute stmt;
+execute stmt;
+deallocate prepare stmt;
+
+--echo #checking hanging cte that uses a recursive cte
+let $q1=
+with h_cte as
+( with recursive r_cte as
+  ( select * from t1 as s
+    union
+    select t1.* from t1, r_cte as r where t1.c = r.a )
+  select 0 as b FROM dual union all select b FROM r_cte as t)
+select * from t1 as tt;
+
+eval explain $q1;
+eval $q1;
+--source include/analyze-format.inc
+eval analyze format=json $q1;
+eval prepare stmt from "$q1";
+execute stmt;
+execute stmt;
+deallocate prepare stmt;
+
+drop table t1;
+
 --echo #
 --echo # End of 10.2 tests
 --echo #

sql/sql_lex.cc

@@ -4450,9 +4450,11 @@ void st_select_lex::set_explain_type(bool on_the_fly)
             /*
               pos_in_table_list=NULL for e.g. post-join aggregation JOIN_TABs.
             */
-            if (tab->table && tab->table->pos_in_table_list &&
-                tab->table->pos_in_table_list->with &&
-                tab->table->pos_in_table_list->with->is_recursive)
+            if (!(tab->table && tab->table->pos_in_table_list))
+	      continue;
+            TABLE_LIST *tbl= tab->table->pos_in_table_list;
+            if (tbl->with && tbl->with->is_recursive &&
+                tbl->is_with_table_recursive_reference())
             {
               uses_cte= true;
               break;

sql/sql_select.cc

@@ -12275,6 +12275,9 @@ void JOIN::join_free()
   for (tmp_unit= select_lex->first_inner_unit();
        tmp_unit;
        tmp_unit= tmp_unit->next_unit())
+  {
+    if (tmp_unit->with_element && tmp_unit->with_element->is_recursive)
+      continue;
     for (sl= tmp_unit->first_select(); sl; sl= sl->next_select())
     {
       Item_subselect *subselect= sl->master_unit()->item;
@@ -12292,7 +12295,7 @@ void JOIN::join_free()
       /* Can't unlock if at least one JOIN is still needed */
       can_unlock= can_unlock && full_local;
     }
-
+  }
   /*
     We are not using tables anymore
     Unlock all tables. We may be in an INSERT .... SELECT statement.

sql/sql_union.cc

@@ -1370,13 +1370,7 @@ bool st_select_lex_unit::cleanup()
   {
     DBUG_RETURN(FALSE);
   }
-  /*
-    When processing a PS/SP or an EXPLAIN command cleanup of a unit can
-    be performed immediately when the unit is reached in the cleanup
-    traversal initiated by the cleanup of the main unit.
-  */
-  if (!thd->stmt_arena->is_stmt_prepare() && !thd->lex->describe &&
-      with_element && with_element->is_recursive && union_result)
+  if (with_element && with_element->is_recursive && union_result)
   {
     select_union_recursive *result= with_element->rec_result;
     if (++result->cleanup_count == with_element->rec_outer_references)
@@ -1554,12 +1548,9 @@ bool st_select_lex::cleanup()
 
   if (join)
   {
-    DBUG_ASSERT((st_select_lex*)join->select_lex == this);
-    error= join->destroy();
-    delete join;
-    join= 0;
-  }
-  for (TABLE_LIST *tbl= get_table_list(); tbl; tbl= tbl->next_local)
+    List_iterator<TABLE_LIST> ti(leaf_tables);
+    TABLE_LIST *tbl;
+    while ((tbl= ti++))
     {
       if (tbl->is_recursive_with_table() &&
           !tbl->is_with_table_recursive_reference())
@@ -1572,9 +1563,16 @@ bool st_select_lex::cleanup()
         error|= (bool) error | (uint) unit->cleanup();
       }
     }
+    DBUG_ASSERT((st_select_lex*)join->select_lex == this);
+    error= join->destroy();
+    delete join;
+    join= 0;
+  }
   for (SELECT_LEX_UNIT *lex_unit= first_inner_unit(); lex_unit ;
        lex_unit= lex_unit->next_unit())
   {
+    if (lex_unit->with_element && lex_unit->with_element->is_recursive)
+      continue;
     error= (bool) ((uint) error | (uint) lex_unit->cleanup());
   }
   inner_refs_list.empty();
@@ -1594,8 +1592,12 @@ void st_select_lex::cleanup_all_joins(bool full)
     join->cleanup(full);
 
   for (unit= first_inner_unit(); unit; unit= unit->next_unit())
+  {
+    if (unit->with_element && unit->with_element->is_recursive)
+      continue;
     for (sl= unit->first_select(); sl; sl= sl->next_select())
       sl->cleanup_all_joins(full);
+  }
   DBUG_VOID_RETURN;
 }