Обсуждение: GIN tries to form a tuple with a partial compressedList during insertion

Hi,

In the functions addItemPointersToLeafTuple and buildFreshLeafTuple
(in gininsert.c), the result of ginCompressPostingList()
is passed to GinFormTuple without checking whether
ginCompressPostingList() successfully packed all items.
These GinFormTuple calls consistently fail because the resulting
tuples always exceed the maximum size.
While this doesn’t result in data corruption, it might still be worth
addressing.
Additionally, the condition if (compressedList) is unnecessary, since
ginCompressPostingList() never returns NULL.

Please find the attached patch fixing it.

Best regards,
Arseniy Mukhin

Hi!

Here is a new version. I added a commit message. I will add it to PG19-2.


Best regards,
Arseniy Mukhin

On Wed, Jul 2, 2025 at 12:41 PM Arseniy Mukhin
<arseniy.mukhin.dev@gmail.com> wrote:
>
> Hi!
>
> Here is a new version. I added a commit message. I will add it to PG19-2.

Thank you for the patch.

I think the proposed change is reasonable; if we fail to compress all
ItemPointers, it doesn't make sense to try to form a tuple from it.

Here are some review comments:

---
-       compressedList = ginCompressPostingList(newItems, newNPosting,
GinMaxItemSize,
-
                 NULL);
+       compressedList = ginCompressPostingList(newItems, newNPosting,
GinMaxItemSize - GinGetPostingOffset(old),
+
                 &nwritten);

Why does it need to subtract GinGetPostingOffset(old) from the maxsize?

---
        pfree(newItems);
-       if (compressedList)
+       if (nwritten == newNPosting)
        {
                res = GinFormTuple(ginstate, attnum, key, category,
                                                   (char *) compressedList,

SizeOfGinPostingList(compressedList),
                                                   newNPosting,
                                                   false);
-               pfree(compressedList);
        }
+       pfree(compressedList);

I think it would be cleaner if we move 'pfree(newItems)' to before
'pfree(compressedList)'.

Regards,

--
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com

Hi,

On Fri, Sep 26, 2025 at 1:21 AM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>
> On Wed, Jul 2, 2025 at 12:41 PM Arseniy Mukhin
> <arseniy.mukhin.dev@gmail.com> wrote:
> >
> > Hi!
> >
> > Here is a new version. I added a commit message. I will add it to PG19-2.
>
> Thank you for the patch.
>

Thank you for looking into this!

> I think the proposed change is reasonable; if we fail to compress all
> ItemPointers, it doesn't make sense to try to form a tuple from it.
>
> Here are some review comments:
>
> ---
> -       compressedList = ginCompressPostingList(newItems, newNPosting,
> GinMaxItemSize,
> -
>                  NULL);
> +       compressedList = ginCompressPostingList(newItems, newNPosting,
> GinMaxItemSize - GinGetPostingOffset(old),
> +
>                  &nwritten);
>
> Why does it need to subtract GinGetPostingOffset(old) from the maxsize?
>

While writing the patch I realized that there is a room for small
optimization. The idea here is that as we know the index tuple size
limit and we know how much space we need for everything except the
posting list (we can get it with GinGetPostingOffset(old)), then we
can calculate the size limit for the posting list more precisely. But
now I think it was a bad idea to add this optimization to the fix.
Even if it relates to the same line of the code, it's a different
thing and it's confusing. Sorry about that. I removed it from the
patch.

> ---
>         pfree(newItems);
> -       if (compressedList)
> +       if (nwritten == newNPosting)
>         {
>                 res = GinFormTuple(ginstate, attnum, key, category,
>                                                    (char *) compressedList,
>
> SizeOfGinPostingList(compressedList),
>                                                    newNPosting,
>                                                    false);
> -               pfree(compressedList);
>         }
> +       pfree(compressedList);
>
> I think it would be cleaner if we move 'pfree(newItems)' to before
> 'pfree(compressedList)'.
>

Done.

Please find the attached new version.

Thank you!


Best regards,
Arseniy Mukhin