Re: Skipping logical replication transactions on subscriber side

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.
>
>
> 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.

> - Introduce a new typalign value suitable for uint64. This is more intrusive,
> but it's more future-proof. Looking beyond catalog columns, it might
> improve performance by avoiding unaligned reads.
>
> > Is it possible to run the test again with the attached patch?
>
> Logs attached. The test "passed", though it printed "poll_query_until timed
> out" three times and took awhile.

Thanks for helping in figuring out the problem.

--
With Regards,
Amit Kapila.