Re: Skipping logical replication transactions on subscriber side

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:
> >
> > On Sat, Apr 02, 2022 at 04:33:44PM +0900, Masahiko Sawada wrote:
> > > It seems that 0/B0706F72 is not a random value. Two subscriber logs
> > > show the same value. Since 0x70 = 'p', 0x6F = 'o', and 0x72 = 'r', it
> > > might show the next field in the pg_subscription catalog, i.e.,
> > > subconninfo. The subscription is created by "CREATE SUBSCRIPTION sub
> > > CONNECTION 'port=57851 host=/tmp/6u2vRwQYik dbname=postgres'
> > > PUBLICATION pub WITH (disable_on_error = true, streaming = on,
> > > two_phase = on)".
> > >
> > > Given subscription.sql passes, something is wrong when we read the
> > > subskiplsn value by like "sub->skiplsn = subform->subskiplsn;".
> >
> > That's a good clue. We've never made pg_type.typalign able to represent
> > alignment as it works on AIX. A uint64 like pg_lsn has 8-byte alignment, so
> > the C struct follows from that. At the typalign level, we have only these:
> >
> > #define TYPALIGN_CHAR 'c' /* char alignment (i.e. unaligned) */
> > #define TYPALIGN_SHORT 's' /* short alignment (typically 2 bytes) */
> > #define TYPALIGN_INT 'i' /* int alignment (typically 4 bytes) */
> > #define TYPALIGN_DOUBLE 'd' /* double alignment (often 8 bytes) */
> >
> > On AIX, they are:
> >
> > #define ALIGNOF_DOUBLE 4
> > #define ALIGNOF_INT 4
> > #define ALIGNOF_LONG 8
> > /* #undef ALIGNOF_LONG_LONG_INT */
> > /* #undef ALIGNOF_PG_INT128_TYPE */
> > #define ALIGNOF_SHORT 2
> >
> > uint64 and pg_lsn use TYPALIGN_DOUBLE. For AIX, they really need a typalign
> > corresponding to ALIGNOF_LONG. Hence, the C struct layout doesn't match the
> > tuple layout. Columns potentially affected:
> >
> > [local] test=*# select attrelid::regclass, attname from pg_attribute a join pg_class c on c.oid = attrelid where attalign = 'd' and relkind = 'r' and attnotnull and attlen <> -1;
> > attrelid │ attname
> > ─────────────────┼──────────────
> > pg_sequence │ seqstart
> > pg_sequence │ seqincrement
> > pg_sequence │ seqmax
> > pg_sequence │ seqmin
> > pg_sequence │ seqcache
> > pg_subscription │ subskiplsn
> > (6 rows)
> >
> > The pg_sequence fields evade trouble, because there's exactly eight bytes (two
> > oids) before them.

Thanks for helping with the investigation!

> >
> >
> > Some options:
> > - Move subskiplsn after subdbid, so it's always aligned anyway. I've
> > confirmed that this lets the test pass, in 44s.
> > - Move subskiplsn to the CATALOG_VARLEN section, despite its fixed length.
> >
>
> +1 to any one of the above. I mildly prefer the first option as that
> will allow us to access the value directly instead of going via
> SysCacheGetAttr but I am fine either way.

+1. I also prefer the first option.

Regards,

--
Masahiko Sawada
EDB: https://www.enterprisedb.com/