Re: Bug in ginRedoRecompress that causes opaque data on page to be overrun

On Wed, Sep 5, 2018 at 4:59 AM, R, Siva <sivasubr@amazon.com> wrote:
> Hi,
>
> We recently encountered an issue where the opaque data flags on a gin data
> leaf page was corrupted while replaying a gin insert WAL record. Upon
> further examination of the redo code, we found a bug in ginRedoRecompress
> code, which extracts the WAL information and updates the page.
>
> Specifically, when a new segment is inserted in the middle of a page, a
> memmove operation is performed [1] at the current point in the page to make
> room for the new segment. If this segment insertion is followed by delete
> segment actions that are yet to be processed and the total data size is very
> close to GinDataPageMaxDataSize, then we may move the data portion beyond
> the boundary causing the opaque data to be corrupted.
>
> One way of solving this problem is to perform the replay work on a scratch
> space, perform sanity check on the total size of the data portion before
> copying it back to the actual page. While it involves additional memory
> allocation and memcpy operations, it is safer and similar to the 'do' code
> path where we ensure to make a copy of all segment past the first modified
> segment before placing them back on the page [2].
>

Hmm, could you share the sequence of what kind of WAL has applied to
the broken page? I suspect the segment list contains
GIN_SEGMENT_REPLACE before GIN_SEGMENT_INSERT.

Regards,

--
Masahiko Sawada
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center