Re: Slot's restart_lsn may point to removed WAL segment after hard restart unexpectedly

So, my proposal is to commit the attached patchset to the HEAD, and
commit [1] to the back branches.  Any objections?

As the buildfarm animal prion shows [1], the 046_checkpoint_logical_slot
test fails with "-DRELCACHE_FORCE_RELEASE -DCATCACHE_FORCE_RELEASE":
# poll_query_until timed out executing this query:
#
#         SELECT count(*) > 0 FROM pg_stat_activity
#         WHERE backend_type = 'client backend' AND wait_event = 'logical-replication-slot-advance-segment'
#     
# expecting this output:
# t
# last actual query output:
# f
# with stderr:
[04:16:27] t/046_checkpoint_logical_slot.pl ......
Dubious, test returned 29 (wstat 7424, 0x1d00)
No subtests run
[04:20:58] t/047_checkpoint_physical_slot.pl ..... ok   271294 ms ( 0.00 usr  0.00 sys +  0.37 cusr  0.26 csys =  0.63 CPU)

I'm able to reproduce this locally as well. Though the test passes for me
with the increased timeout, that is it's not stuck:
PG_TEST_TIMEOUT_DEFAULT=360 PROVE_TESTS="t/046*" make -s check -C src/test/recovery/
# +++ tap check in src/test/recovery +++
t/046_checkpoint_logical_slot.pl .. ok
All tests successful.
Files=1, Tests=1, 533 wallclock secs ( 0.01 usr  0.00 sys +  4.70 cusr  9.61 csys = 14.32 CPU)
Result: PASS

Could you have a look?

[1] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=prion&dt=2025-06-14%2001%3A58%3A06

Best regards,
Alexander