Re: Skipping logical replication transactions on subscriber side

On Mon, Apr 04, 2022 at 08:20:08AM +0530, Amit Kapila wrote:
> On Mon, Apr 4, 2022 at 8:01 AM Noah Misch <noah(at)leadboat(dot)com> wrote:
> > On Mon, Apr 04, 2022 at 10:28:30AM +0900, Masahiko Sawada wrote:
> > > On Sun, Apr 3, 2022 at 9:45 AM Noah Misch <noah(at)leadboat(dot)com> wrote:
> > > > On Sat, Apr 02, 2022 at 08:44:45PM +0900, Masahiko Sawada wrote:
> > > > > On Sat, Apr 2, 2022 at 7:04 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > > > > > On Sat, Apr 2, 2022 at 1:43 PM Noah Misch <noah(at)leadboat(dot)com> wrote:
> > > > > > > Some options:
> > > > > > > - Move subskiplsn after subdbid, so it's always aligned anyway. I've
> > > > > > > confirmed that this lets the test pass, in 44s.
> >
> > > --- a/src/include/catalog/pg_subscription.h
> > > +++ b/src/include/catalog/pg_subscription.h
> > > @@ -54,6 +54,17 @@ CATALOG(pg_subscription,6100,SubscriptionRelationId) BKI_SHARED_RELATION BKI_ROW
> > >
> > > Oid subdbid BKI_LOOKUP(pg_database); /* Database the
> > > * subscription is in. */
> > > +
> > > + /*
> > > + * All changes finished at this LSN are skipped.
> > > + *
> > > + * Note that XLogRecPtr, pg_lsn in the catalog, is 8-byte alignment
> > > + * (TYPALIGN_DOUBLE) and it does not match the alignment on some platforms
> > > + * such as AIX. Therefore subskiplsn needs to be placed here so it is
> > > + * always aligned.
> >
> > I'm reading this comment as saying that TYPALIGN_DOUBLE is always 8 bytes, but
> > the problem arises precisely because TYPALIGN_DOUBLE==4 on AIX.
>
> How about a comment like: "It has to be kept at 8-byte alignment
> boundary so as to be accessed directly via C struct as it uses
> TYPALIGN_DOUBLE for storage which has 4-byte alignment on platforms
> like AIX."? Can you please suggest a better comment if you don't like
> this one?

I'd write it like this, though I'm not sure it's an improvement on your words:

When ALIGNOF_DOUBLE==4 (e.g. AIX), the C ABI may impose 8-byte alignment on
some of the C types that correspond to TYPALIGN_DOUBLE SQL types. To ensure
catalog C struct layout matches catalog tuple layout, arrange for the tuple
offset of each fixed-width, attalign='d' catalog column to be divisible by 8
unconditionally. Keep such columns before the first NameData column of the
catalog, since packagers can override NAMEDATALEN to an odd number.

The best place for such a comment would be in one of
src/test/regress/sql/*sanity*.sql, next to a test written to detect new
violations. If adding such a test would materially delay getting the
buildfarm green, putting the comment in pg_subscription.h works for me.