Re: [HACKERS] Memory leaks in relcache

Bruce Momjian <maillist(at)candle(dot)pha(dot)pa(dot)us> writes:
> Tom, where are we on this. As I remember, it is still an open issue,
> right? I can add it to the TODO list.

I have not done anything about it yet; it ought to be in TODO.

I'm also aware of two or three other sources of small but permanent
memory leaks, btw; have them in my todo list.

regards, tom lane

>> 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.
>>
>> regards, tom lane
>>
>>

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