MDEV-31632 Unresolvable outer reference causes null pointer exception#4513
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mariadb-RexJohnston merged 1 commit into10.11from Jan 22, 2026
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MDEV-31632 Unresolvable outer reference causes null pointer exception#4513mariadb-RexJohnston merged 1 commit into10.11from
mariadb-RexJohnston merged 1 commit into10.11from
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SELECT 1 union select 2 UNION SELECT 1 from a JOIN a b ON
(SELECT 1 FROM dual WHERE AAA)
Crashes during fix_outer_field while resolving field item AAA
In our resolver, once we have determined that a field item isn't
local to our select, we call Item::fix_outer_field(), which
iterates outwards towards the top level select, looking for where
our Item_field might be resolvable.
In our example here, the item isn't resolvable and we expose
fragility in the loop, which i will detail here.
After we initialize the variable 'outer_context' (to a context
containing /* select#3 */ select 1 AS `1` from (a join a b on
((subquery#4))) ) we enter a loop
│ 5927 for (;
│ 5928 outer_context;
│ 5929 outer_context= outer_context->outer_context)
│ 5930 {
│ 5931 select= outer_context->select_lex;
│ 5932 Item_subselect *prev_subselect_item=
│ 5933 last_checked_context->select_lex->master_unit()->item;
│ 5934 last_checked_context= outer_context;
here 'last_checked_context' is the context inner to the current
'outer_context', and we initialize prev_subselect_item to the
Item enclosing the unit containing this inner select.
So for the first iteration of the loop,
select: select #3
last_checked_context: from select #4 to select #3.
prev_subselect_item: item enclosing select #4 (where
field item AAA is defined)
The rest of the loop calls find_field_in_tables() /
resolve_ref_in_select_and_group() in an attempt to
resolve this item with this 'outer_context'.
After the item fails resolution, we move to an outer context
select: select #4294967295 (fake_select_lex)
last_checked_context: from select #3 to the fake select lex
containing the union (i.e. outermost)
prev_subselect_item: null, there is no Item that contains this,
it is the outermost select.
We still need to execute the rest of the loop to determine whether
AAA is resolvable here, but executing
│ 5937 place= prev_subselect_item->parsing_place;
We are now following a null pointer. We introduce a test for this
null pointer, indicating that we are now evaluating the outermost
select and we are not to try accessing the enclosing subselect item.
Approved by: Oleksandr "Sanja" Byelkin (sanja@mariadb.com)
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Pull request created in: https://jira.mariadb.org/browse/MDEV-31632