A review of the code has showed up a couple of issues fixed by this
commit:
- Physical slots have been using the confirmed LSN position as a start
comparison point which is always 0/0, instead use the restart LSN
position (logical slots need to use the confirmed LSN position, which
was correct).
- The actual slot update was incorrect for both physical and logical
slots.  Physical slots need to use their restart_lsn as base comparison
point (confirmed_flush was used because of previous point), and logical
slots need to begin reading WAL from restart_lsn (confirmed_flush was
used as well), while confirmed_flush is compiled depending on the
decoding context and record read, and is the LSN position returned back
to the caller.
- Never return 0/0 if a slot cannot be advanced.  This way, if a slot is
advanced while the activity is idle, then the same position is returned
to the caller over and over without raising an error.  Instead return
the LSN the slot has been advanced to.  With repetitive calls, the same
position is returned hence caller can directly monitor the difference in
progress in bytes by doing simply LSN difference calculations, which
should be monotonic.

Note that as the slot is owned by the backend advancing it, then the
read of those fields is fine lock-less, while updates need to happen
while the slot mutex is held, so fix that on the way as well.  Other
locks for in-memory data of replication slots have been already fixed
previously.

Some of those issues have been pointed out by Petr and Simon during the
patch, while I noticed some of them after looking at the code.  This
also visibly takes of a recently-discovered bug causing assertion
failures which can be triggered by a two-step slot forwarding which
first advanced the slot to a WAL page boundary and secondly advanced it
to the latest position, say 'FF/FFFFFFF' to make sure that the newest
LSN is used as forward point.  It would have been nice to drop a test
for that, but the set of operators working on pg_lsn limits it, so this
is left for a future exercise.

Author: Michael Paquier
Reviewed-by: Petr Jelinek, Simon Riggs
Discussion: https://postgr.es/m/CANP8+jLyS=X-CAk59BJnsxKQfjwrmKicHQykyn52Qj-Q=9GLCw@mail.gmail.com
Discussion: https://www.postgresql.org/message-id/2840048a-1184-417a-9da8-3299d207a1d7%40postgrespro.ru