Hello, hackers!

The last query in the following script crashes Postgres:

create table t (id serial, amount int);
insert into t (amount) select random() * 1000 from generate_series(1, 100);
create extension btree_gist;
create index t_gist_idx on t using gist(id, amount);

select p.id, p.amount, s.nearest
from t as p left join lateral
(
  select p.id, array_agg(l.id) as nearest from (
    select id from t order by amount <-> p.amount limit 10
  ) l
) s using(id);

In gistrescan() IndexScanDesc.xs_hitup is not reset after MemoryContextReset() 
of
so->queueCxt in which xs_hitup was allocated, then getNextNearest() tries to 
pfree()
dangling xs_hitup, which results in the reuse of this pointer and the 
subsequent crash.

Attached patches fix this bug introduced in commit
d04c8ed9044eccebce043143a930617e3998c005 "Add support for index-only scans in 
GiST".
The bug is present in v9.5, v9.6, v10.0.

--
Nikita Glukhov
Postgres Professional:http://www.postgrespro.com
The Russian Postgres Company

diff --git a/src/backend/access/gist/gistscan.c b/src/backend/access/gist/gistscan.c
index 2526a39..580b6ac 100644
--- a/src/backend/access/gist/gistscan.c
+++ b/src/backend/access/gist/gistscan.c
@@ -148,6 +148,11 @@ gistrescan(IndexScanDesc scan, ScanKey key, int nkeys,
 		/* third or later time through */
 		MemoryContextReset(so->queueCxt);
 		first_time = false;
+		/*
+		 * scan->xs_itup is allocated in so->queueCxt and now it is invalid,
+		 * so we need to reset it to prevent it from freeing in getNextNearest().
+		 */
+		scan->xs_itup = NULL;
 	}
 
 	/*
diff --git a/src/backend/access/gist/gistscan.c b/src/backend/access/gist/gistscan.c
index 81ff8fc..5e10ada 100644
--- a/src/backend/access/gist/gistscan.c
+++ b/src/backend/access/gist/gistscan.c
@@ -148,6 +148,11 @@ gistrescan(IndexScanDesc scan, ScanKey key, int nkeys,
 		/* third or later time through */
 		MemoryContextReset(so->queueCxt);
 		first_time = false;
+		/*
+		 * scan->xs_hitup is allocated in so->queueCxt and now it is invalid,
+		 * so we need to reset it to prevent it from freeing in getNextNearest().
+		 */
+		scan->xs_hitup = NULL;
 	}
 
 	/*
-- 
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