Re: [HACKERS] Memory leaks in relcache

> I have been looking into why a reference to a nonexistent table, eg
> INSERT INTO nosuchtable VALUES(1);
> leaks a small amount of memory per occurrence. What I find is a
> memory leak in the indexscan support. Specifically,
> RelationGetIndexScan in backend/access/index/genam.c palloc's both
> an IndexScanDesc and some keydata storage. The IndexScanDesc
> block is eventually pfree'd, at the bottom of CatalogIndexFetchTuple
> in backend/catalog/indexing.c. But the keydata block is not.
>
> This wouldn't matter so much if the palloc were coming from a
> transaction-local context. But what we're doing is a lookup in pg_class
> on behalf of RelationBuildDesc in backend/utils/cache/relcache.c, and
> it's done a MemoryContextSwitchTo into the global CacheCxt before
> starting the lookup. Therefore, the un-pfreed block represents a
> permanent memory leak.
>
> In fact, *every* reference to a relation that is not already present in
> the relcache causes a similar leak. The error case is just the one that
> is easiest to repeat. The missing pfree of the keydata block is
> probably causing a bunch of other short-term and long-term leaks too.
>
> It seems to me there are two things to fix here: indexscan ought to
> pfree everything it pallocs, and RelationBuildDesc ought to be warier
> about how much work gets done with CacheCxt as the active palloc
> context. (Even if indexscan didn't leak anything ordinarily, there's
> still the risk of elog(ERROR) causing an abort before the indexscan code
> gets to clean up.)
>
> Comments? In particular, where is the cleanest place to add the pfree
> of the keydata block? I don't especially like the fact that callers
> of index_endscan have to clean up the toplevel scan block; I think that
> ought to happen inside index_endscan.

You are certainly on to something. Every call to index_endscan() either
calls pfree() just after the call to free the descriptor, or should. I
recommend doing the pfree in the index_endscan, and removing the
individual pfree's after the index_endscan call. I also recommend doing
pfree'ing the keys inside index_endscan() and see what happens. The
regression test should show any problems. I can easily do this is you
wish.

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