Обсуждение: GetRelationPublicationActions. - Remove unreachable code

Hi hackers.

There appears to be some unreachable code in the relcache function
GetRelationPublicationActions.

When the 'relation->rd_pubactions' is not NULL then the function
unconditionally returns early (near the top).

Therefore, the following code (near the bottom) seems to have no
purpose because IIUC the 'rd_pubactions' can never be not NULL here.

if (relation->rd_pubactions)
{
pfree(relation->rd_pubactions);
relation->rd_pubactions = NULL;
}

The latest Code Coverage report [1] also shows that this code is never executed.

    5601        3556 :     if (relation->rd_pubactions)
    5602             :     {
    5603           0 :         pfree(relation->rd_pubactions);
    5604           0 :         relation->rd_pubactions = NULL;
    5605             :     }

~~

PSA a patch to remove this unreachable code.

------
[1] https://coverage.postgresql.org/src/backend/utils/cache/relcache.c.gcov.html

Kind Regards,
Peter Smith.
Fujitsu Australia

Peter Smith <smithpb2250@gmail.com> writes:
> There appears to be some unreachable code in the relcache function
> GetRelationPublicationActions.
> When the 'relation->rd_pubactions' is not NULL then the function
> unconditionally returns early (near the top).
> Therefore, the following code (near the bottom) seems to have no
> purpose because IIUC the 'rd_pubactions' can never be not NULL here.

I'm not sure it's as unreachable as all that.  What you're not
accounting for is the possibility of recursive cache loading,
ie something down inside the catalog fetches we have to do here
could already have made the field valid.

Admittedly, that's probably not very likely given expected usage
patterns (to wit, that system catalogs shouldn't really have
publication actions).  But there are other places in relcache.c where
a coding pattern similar to this is demonstrably necessary to avoid
memory leakage.  I'd rather leave the code as-is, maybe add a comment
along the lines of "rd_pubactions is probably still NULL, but just in
case something already made it valid, avoid leaking memory".

            regards, tom lane

On Thu, Feb 10, 2022 at 11:34 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> Peter Smith <smithpb2250@gmail.com> writes:
> > There appears to be some unreachable code in the relcache function
> > GetRelationPublicationActions.
> > When the 'relation->rd_pubactions' is not NULL then the function
> > unconditionally returns early (near the top).
> > Therefore, the following code (near the bottom) seems to have no
> > purpose because IIUC the 'rd_pubactions' can never be not NULL here.
>
> I'm not sure it's as unreachable as all that.  What you're not
> accounting for is the possibility of recursive cache loading,
> ie something down inside the catalog fetches we have to do here
> could already have made the field valid.
>
> Admittedly, that's probably not very likely given expected usage
> patterns (to wit, that system catalogs shouldn't really have
> publication actions).  But there are other places in relcache.c where
> a coding pattern similar to this is demonstrably necessary to avoid
> memory leakage.  I'd rather leave the code as-is, maybe add a comment
> along the lines of "rd_pubactions is probably still NULL, but just in
> case something already made it valid, avoid leaking memory".

OK. Thanks for your explanation and advice.

PSA another patch to just a comment as suggested.

------
Kind Regards,
Peter Smith.
Fujitsu Australia