On Tue, Jul 19, 2022 at 6:34 AM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>
> On Mon, Jul 18, 2022 at 1:12 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > On Fri, Jul 15, 2022 at 8:09 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> > >
> > > This patch should have the fix for the issue that Shi yu reported. Shi
> > > yu, could you please test it again with this patch?
> > >
> >
> > Can you explain the cause of the failure and your fix for the same?
>
> @@ -1694,6 +1788,8 @@ out:
> /* be tidy */
> if (ondisk)
> pfree(ondisk);
> + if (catchange_xip)
> + pfree(catchange_xip);
>
> Regarding the above code in the previous version patch, looking at the
> generated assembler code shared by Shi yu offlist, I realized that the
> “if (catchange_xip)” is removed (folded) by gcc optimization. This is
> because we dereference catchange_xip before null-pointer check as
> follow:
>
> + /* copy catalog modifying xacts */
> + sz = sizeof(TransactionId) * catchange_xcnt;
> + memcpy(ondisk_c, catchange_xip, sz);
> + COMP_CRC32C(ondisk->checksum, ondisk_c, sz);
> + ondisk_c += sz;
>
> Since sz is 0 in this case, memcpy doesn’t do anything actually.
>
> By checking the assembler code, I’ve confirmed that gcc does the
> optimization for these code and setting
> -fno-delete-null-pointer-checks flag prevents the if statement from
> being folded. Also, I’ve confirmed that adding the check if
> "catchange.xcnt > 0” before the null-pointer check also can prevent
> that. Adding a check if "catchange.xcnt > 0” looks more robust. I’ve
> added a similar check for builder->committed.xcnt as well for
> consistency. builder->committed.xip could have no transactions.
>
Good work. I wonder without comments this may create a problem in the
future. OTOH, I don't see adding a check "catchange.xcnt > 0" before
freeing the memory any less robust. Also, for consistency, we can use
a similar check based on xcnt in the SnapBuildRestore to free the
memory in the below code:
+ /* set catalog modifying transactions */
+ if (builder->catchange.xip)
+ pfree(builder->catchange.xip);
--
With Regards,
Amit Kapila.