PG> I found this while analyzing a report from IRC that initially PG> looked like a PostGIS bug, but which I now think is breakage in PG> spgist:
PG> spgrescan starts out by doing PG> MemoryContextReset(so->traversalCxt);
PG> then later it calls resetSpGistScanOpaque(so); PG> which calls freeScanStack(so) PG> which calls freeScanStackEntry(so) PG> which does:
PG> if (stackEntry->traversalValue) PG> pfree(stackEntry->traversalValue);
PG> But stackEntry->traversalValue, if not NULL, is supposed to have PG> been allocated in so->traversalCxt, and so it's already gone. [...] PG> Unfortunately I don't think this can be demonstrated with the PG> built-in spgist opclasses, which don't allocate traversalValues.
Turns out I was looking in the wrong place, and in fact we can reproduce this easily (at least on an assert build):
create table boxes (b box); insert into boxes select box(point(i,j),point(i+s,j+s)) from generate_series(1,100,5) i, generate_series(1,100,5) j, generate_series(1,10) s; create index on boxes using spgist (b); select * from (values (box(point(5,5),point(8,8))),(box(point(2,2),point(12,12)))) v(b) cross join lateral (select * from boxes where boxes.b && v.b limit 1) pt;
So this logic was added in ccd6eb49a and was wrong from the start. Testing suggests that removing the offending pfree does indeed fix the issue; any objections?
No objections from me. What about moving MemoryContextReset(so->traversalCxt) into resetSpGistScanOpaque()? For me it seems that resetting of traversal memory context is part of opaque reset.
------ Alexander Korotkov Postgres Professional: http://www.postgrespro.com The Russian Postgres Company
