Show effective xmin in pg_replication_slots when xmin is not set

Hi,

While working on XID age based slot invalidation [1] and reviewing the
pg_xmin_horizon patch [2], I realized that a logical replication slot
can hold back VACUUM on user tables while its xmin is shown as NULL in
pg_replication_slots. The documentation describes xmin as the oldest
transaction that the slot needs the database to retain, so the view
doesn't match the documentation in this case.

There are two cases where this happens.

1/ When creating a logical slot with an exported or used snapshot, the
slot holds back the data xmin through its in-memory effective xmin
alone, never the one saved to disk and shown in the view. The slot
creation waits for the transactions with an assigned XID that are
running when it starts, so a long running or prepared transaction can
keep a slot in this state for a long time [3]. Table synchronization
workers in logical replication create their slots this way too, though
they hold the state only for the slot creation.

2/ In PG19 and later, REPACK (CONCURRENTLY) creates its temporary slot
in the same way and keeps it in this state for the whole command.

I would like to fix this by reporting the slot's effective xmin in
pg_replication_slots when the saved xmin is not set. I thought of
adding new columns for the effective values, but they would mostly be
the same as the existing ones. Outside the above cases, they differ
only for a moment while a new value is being saved, and catalog_xmin
is never unset while its effective value is set.

I checked that the first case reproduces on all supported branches, so
I think we can backpatch the fix. Please find the attached v1 patch.

Thoughts?

[1] https://postgr.es/m/CALj2ACVRpA1UnZx%3Ds0Ljsn01FcsEHEfj1mQXvQn69eO3Nq2YOA%40mail.gmail.com
[2] https://postgr.es/m/CALj2ACV9FYmK9nHZ+DtNkOvN_z8uMvWF+MwsMgJu2V686vuYHg@mail.gmail.com
[3]
-- session 1
CREATE TABLE t AS SELECT generate_series(1, 1000) AS a;
BEGIN; DELETE FROM t;

-- session 2
BEGIN; SELECT pg_current_xact_id();

-- session 3, waits for sessions 1 and 2
psql "dbname=postgres replication=database" -c
"CREATE_REPLICATION_SLOT s LOGICAL pgoutput"

-- session 1
COMMIT;
-- xmin is NULL
SELECT slot_name, xmin, catalog_xmin FROM pg_replication_slots;
-- only session 2, newer than session 1
SELECT pid, backend_xid FROM pg_stat_activity WHERE backend_xid IS NOT NULL;
-- 1000 dead rows not removable, cutoff is session 1's xid
VACUUM (VERBOSE) t;

-- session 2
-- slot creation finishes
COMMIT;

-- session 1
-- 1000 dead rows removed
VACUUM (VERBOSE) t;

--
Bharath Rupireddy
Amazon Web Services: https://aws.amazon.com