Asymmetry in opening and closing indices for partition routing

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Asymmetry in opening and closing indices for partition routing

Ashutosh Bapat-3
Hi All,
ExecInitPartitionInfo() has code
 536     /*
 537      * Open partition indices.  The user may have asked to check for conflicts
 538      * within this leaf partition and do "nothing" instead of throwing an
 539      * error.  Be prepared in that case by initializing the index information
 540      * needed by ExecInsert() to perform speculative insertions.
 541      */
 542     if (partrel->rd_rel->relhasindex &&
 543         leaf_part_rri->ri_IndexRelationDescs == NULL)
 544         ExecOpenIndices(leaf_part_rri,
 545                         (node != NULL &&
 546                          node->onConflictAction != ONCONFLICT_NONE));

This calls ExecOpenIndices only when ri_indexRelationDescs has not been initialized which is fine. I think this is done so that we don't open indices again and again when multiple tuples are routed to the same partition. But as part of opening indices, we also open corresponding index relations using index_open. 

The indices opened here are closed in ExecCleanupTupleRouting(), but it does this unconditionally. This means that for any reason we had called ExecOpenIndices on a partition before ExecInitPartitionInfo() following things will happen
1. ExecOpenIndices will overwrite the old arrays for index descriptors leaking memory.
2. ExecCleanupTupleRouting will close index relations that were opened second time cleaning up memory. But the relcache references corresponding to the first call to ExecOpenIndices will leak.

Similar situation can happen if ExecOpenIndices is called on a partition after it has been called in ExecInitPartitionInfo.

I couldn't find code where this can happen but I don't see any code which prevents this. This looks like a recipe for memory and reference leaks.

We could fix this by
1. Make ExecOpenIndices and ExecCloseIndices so that they can be called multiple times on the same relation similar to heap_open. The second time onwards ExecOpenIndices doesn't allocate memory and open indexes but increases a refcount. ExecCloseIndices releases memory and closes the index relations when the refcount drops to 0. Then we don't need to check leaf_part_rri->ri_IndexRelationDescs == NULL in ExecInitPartitionInfo(). 

2. Throw an error in ExecOpenIndices if all the arrays are present. We will need to check leaf_part_rri->ri_IndexRelationDescs == NULL in ExecInitPartitionInfo().

--
Best Wishes,
Ashutosh
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Re: Asymmetry in opening and closing indices for partition routing

Alvaro Herrera-9
Hi Ashutosh,

On 2020-Jun-22, Ashutosh Bapat wrote:

> I couldn't find code where this can happen but I don't see any code which
> prevents this. This looks like a recipe for memory and reference leaks.
>
> We could fix this by
> 1. Make ExecOpenIndices and ExecCloseIndices so that they can be called
> multiple times on the same relation similar to heap_open. The second time
> onwards ExecOpenIndices doesn't allocate memory and open indexes but
> increases a refcount. ExecCloseIndices releases memory and closes the index
> relations when the refcount drops to 0. Then we don't need to
> check leaf_part_rri->ri_IndexRelationDescs == NULL in
> ExecInitPartitionInfo().

I think there are a couple of places in executor related to partition
tuple routing where code is a bit weirdly structured.  It might be nice
to improve on that if you either find inefficiencies that can be fixed,
or clear code structure improvements, as long as they don't make
performance worse.  Feel free to have a look around and see if you can
propose some concrete proposals.

I'm not sure that expecting the relcache entry's refcount drops to zero
at the right time is a good approach; that may cause leaks some other
place might have refcounts you're not expecting (say, an open cursor
that's not fully read).

(I'm not terribly worried about refcount leakage as a theoretical
concern, since the ResourceOwner mechanism will warn us about that if it
happens.)

> 2. Throw an error in ExecOpenIndices if all the arrays are present. We will
> need to check leaf_part_rri->ri_IndexRelationDescs == NULL in
> ExecInitPartitionInfo().

This sounds like a job for an assert rather than an error.

--
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services


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Re: Asymmetry in opening and closing indices for partition routing

Ashutosh Bapat-3


On Mon, 22 Jun 2020 at 23:22, Alvaro Herrera <[hidden email]> wrote:

I'm not sure that expecting the relcache entry's refcount drops to zero
at the right time is a good approach; that may cause leaks some other
place might have refcounts you're not expecting (say, an open cursor
that's not fully read).

My proposal was to maintain a refcount counting the number of times an index is opened in ResultRelInfo itself, not to rely on the relcache ref count. But I think that would be an overkill. Please read ahead
 

(I'm not terribly worried about refcount leakage as a theoretical
concern, since the ResourceOwner mechanism will warn us about that if it
happens.)

> 2. Throw an error in ExecOpenIndices if all the arrays are present. We will
> need to check leaf_part_rri->ri_IndexRelationDescs == NULL in
> ExecInitPartitionInfo().

This sounds like a job for an assert rather than an error.

I agree. Here's a patch to fix to add Assert'ion in ExecOpenIndices(). I ran make check with this patch and the assertion didn't trip. I think this will be a good step forward.

--
Best Wishes,
Ashutosh

0001-Assert-if-ExecOpenIndices-is-called-twice-on-same-re.patch (1K) Download Attachment