MDEV-28615 Crash caused by multi-table UPDATE over derived with hanging CTE

This bug affected only multi-table update statements and in very rare
cases: one of the tables used at the top level of the statement must be
a derived table containg a row construct with a subquery including hanging
CTE.

Before this patch was applied the function prepare_unreferenced() of the
class With_element when invoked for the the hangin CTE did not properly
restored the value of thd->lex->context_analysis_only. As a result it
became 0 after the call of this function.
For a query affected by the bug this function is called when
JOIN::prepare() is called for the subquery with a hanging CTE. This happens
when Item_row::fix_fields() calls fix_fields() for the subquery. Setting
the value of thd->lex->context_analysis_only forces the caller function
Item_row::fix_fields() to invoke the virtual method is_null() for the
subquery that leads to execution of it. It causes an assertion failure
because the call of Item_row::fix_fields() happens during the invocation
of Multiupdate_prelocking_strategy::handle_end() that calls the function
mysql_derived_prepare() for the derived table used by the UPDATE at the
time when proper locks for the statement tables has not been acquired yet.

With this patch the value of thd->lex->context_analysis_only is restored
to CONTEXT_ANALYSIS_ONLY_DERIVED that is set in the function
mysql_multi_update_prepare().

Approved by Oleksandr Byelkin <sanja@mariadb.com>

mysql-test/main/cte_nonrecursive.result

@@ -2599,4 +2599,29 @@ src	path	dest	distance	population
 E	FD	D	251	80000
 drop procedure sp;
 drop table distances, city_population;
+#
+# MDEV-28615: Multi-table UPDATE over derived table containing
+#             row that uses subquery with hanging CTE
+#
+CREATE TABLE t1 (a int) ENGINE=MYISAM;
+INSERT INTO t1 VALUES (3), (7), (1);
+UPDATE
+(SELECT (5, (WITH cte AS (SELECT 1) SELECT a FROM t1))) dt
+JOIN t1 t
+ON t.a=dt.a
+SET t.a = 1;
+ERROR 21000: Operand should contain 1 column(s)
+UPDATE
+(SELECT a FROM t1
+WHERE (5, (WITH cte AS (SELECT 1) SELECT a FROM t1 WHERE a > 4)) <=
+(5,a)) dt
+JOIN t1 t
+ON t.a=dt.a
+SET t.a = 1;
+SELECT * FROM t1;
+a
+3
+1
+1
+DROP TABLE t1;
 # End of 10.4 tests

mysql-test/main/cte_nonrecursive.test

@@ -1784,8 +1784,6 @@ with data as (select 1 as id)
 select id into @myid from data;
 set sql_mode= @save_sql_mode;
 
-
-
 --echo #
 --echo # MDEV-31995 CTE column name specification inconsistency
 --echo #
@@ -1942,4 +1940,31 @@ drop procedure sp;
 
 drop table distances, city_population;
 
+--echo #
+--echo # MDEV-28615: Multi-table UPDATE over derived table containing
+--echo #             row that uses subquery with hanging CTE
+--echo #
+
+CREATE TABLE t1 (a int) ENGINE=MYISAM;
+INSERT INTO t1 VALUES (3), (7), (1);
+
+--error ER_OPERAND_COLUMNS
+UPDATE
+  (SELECT (5, (WITH cte AS (SELECT 1) SELECT a FROM t1))) dt
+  JOIN t1 t
+  ON t.a=dt.a
+SET t.a = 1;
+
+UPDATE
+  (SELECT a FROM t1
+     WHERE (5, (WITH cte AS (SELECT 1) SELECT a FROM t1 WHERE a > 4)) <=
+           (5,a)) dt
+  JOIN t1 t
+  ON t.a=dt.a
+SET t.a = 1;
+
+SELECT * FROM t1;
+
+DROP TABLE t1;
+
 --echo # End of 10.4 tests

sql/sql_cte.cc

@@ -1241,14 +1241,14 @@ bool With_element::prepare_unreferenced(THD *thd)
        sl= sl->next_select())
     sl->context.outer_context= 0;
 
+  uint8 save_context_analysys_only= thd->lex->context_analysis_only;
   thd->lex->context_analysis_only|= CONTEXT_ANALYSIS_ONLY_DERIVED;
   if (!spec->prepared &&
       (spec->prepare(spec->derived, 0, 0) ||
        rename_columns_of_derived_unit(thd, spec) ||
        check_duplicate_names(thd, first_sl->item_list, 1)))
     rc= true;
-
-  thd->lex->context_analysis_only&= ~CONTEXT_ANALYSIS_ONLY_DERIVED;
+  thd->lex->context_analysis_only= save_context_analysys_only;
   return rc;
 }