Re: [PoC] pg_upgrade: allow to upgrade publisher node

Here are some minor review comments for patch v28-0002

======
src/sgml/ref/pgupgrade.sgml

1.
-       with the primary.)  Replication slots are not copied and must
-       be recreated.
+       with the primary.)  Replication slots on old standby are not copied.
+       Only logical slots on the primary are migrated to the new standby,
+       and other slots must be recreated.
       </para>

/on old standby/on the old standby/

======
src/bin/pg_upgrade/info.c

2. get_old_cluster_logical_slot_infos

+void
+get_old_cluster_logical_slot_infos(void)
+{
+ int dbnum;
+
+ /* Logical slots can be migrated since PG17. */
+ if (GET_MAJOR_VERSION(old_cluster.major_version) <= 1600)
+ return;
+
+ pg_log(PG_VERBOSE, "\nsource databases:");
+
+ for (dbnum = 0; dbnum < old_cluster.dbarr.ndbs; dbnum++)
+ {
+ DbInfo    *pDbInfo = &old_cluster.dbarr.dbs[dbnum];
+
+ get_old_cluster_logical_slot_infos_per_db(pDbInfo);
+
+ if (log_opts.verbose)
+ {
+ pg_log(PG_VERBOSE, "Database: \"%s\"", pDbInfo->db_name);
+ print_slot_infos(&pDbInfo->slot_arr);
+ }
+ }
+}

It might be worth putting an Assert before calling the
get_old_cluster_logical_slot_infos_per_db(...) just as a sanity check:
Assert(pDbInfo->slot_arr.nslots == 0);

This also helps to better document the "Note" of the
count_old_cluster_logical_slots() function comment.

~~~

3. count_old_cluster_logical_slots

+/*
+ * count_old_cluster_logical_slots()
+ *
+ * Sum up and return the number of logical replication slots for all databases.
+ *
+ * Note: this function always returns 0 if the old_cluster is PG16 and prior
+ * because old_cluster.dbarr.dbs[dbnum].slot_arr is set only for PG17 and
+ * later.
+ */
+int
+count_old_cluster_logical_slots(void)

Maybe that "Note" should be expanded a bit to say who does this:

SUGGESTION

Note: This function always returns 0 if the old_cluster is PG16 and
prior because old_cluster.dbarr.dbs[dbnum].slot_arr is set only for
PG17 and later. See where get_old_cluster_logical_slot_infos_per_db()
is called.

======
src/bin/pg_upgrade/pg_upgrade.c

4.
+ /*
+ * Logical replication slot upgrade only supported for old_cluster >=
+ * PG17.
+ *
+ * Note: This must be done after doing the pg_resetwal command because
+ * pg_resetwal would remove required WALs.
+ */
+ if (count_old_cluster_logical_slots())
+ {
+ start_postmaster(&new_cluster, true);
+ create_logical_replication_slots();
+ stop_postmaster(false);
+ }
+

4a.
I felt this comment needs a bit more detail otherwise you can't tell
how the >= PG17 version check works.

4b.
/slot upgrade only supported/slot upgrade is only supported/

~

SUGGESTION

Logical replication slot upgrade is only supported for old_cluster >=
PG17. An explicit version check is not necessary here because function
count_old_cluster_logical_slots() will always return 0 for old_cluster
<= PG16.

------
Kind Regards,
Peter Smith.
Fujitsu Australia