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

Dear Hou-san,

> Based on this, it’s possible that the slots we get each time when checking
> wal_status are different, because they may get changed in between these checks.
> This may not cause serious problems for now, because we will either copy all
> the slots including ones invalidated when upgrading or we report ERROR. But I
> feel it's better to get consistent result each time we check the slots to close
> the possibility for problems in the future. So, I feel we could centralize the
> check for wal_status and slots fetch, so that even if some slots status changed
> after that, it won't have a risk to affect our check. What do you think ?

Thank you for giving the suggestion! I agreed that to centralize checks, and I
had already started to modify. Here is the updated patch.

In this patch all slot infos are extracted in the get_old_cluster_logical_slot_infos(),
upcoming functions uses them. Based on the change, two attributes confirmed_flush
and wal_status were added in LogicalSlotInfo.

IIUC we cannot use strcut List in the client codes, so structures and related
functions are added in the function.c. These are used for extracting unique
plugins, but it may be overkill because check_loadable_libraries() handle
duplicated entries. If we can ignore duplicated entries, these functions can be
removed.

Also, for simplifying codes, only a first-met invalidated slot is output in the
check_old_cluster_for_valid_slots(). Warning messages int the function were
removed. I think it may be enough because check_new_cluster_is_empty() do
similar thing. Please tell me if it should be reverted...

Best Regards,
Hayato Kuroda
FUJITSU LIMITED