replication slot stats memory bug

Hi,

in the course of https://postgr.es/m/3471359.1615937770%40sss.pgh.pa.us
I saw a leak in pgstat_read_statsfiles(), more precisely:
/* Allocate the space for replication slot statistics */
replSlotStats = palloc0(max_replication_slots * sizeof(PgStat_ReplSlotStats));

the issue is that the current memory context is not set by
pgstat_read_statsfiles().

In some cases CurrentMemoryContext is going to be a long-lived context,
accumulating those allocations over time. In other contexts it will be a
too short lived context, e.g. an ExprContext from the pg_stat_*
invocation in the query. A reproducer for the latter:

postgres[2252294][1]=# SELECT pg_create_logical_replication_slot('test', 'test_decoding');
┌────────────────────────────────────┐
│ pg_create_logical_replication_slot │
├────────────────────────────────────┤
│ (test,0/456C1878) │
└────────────────────────────────────┘
(1 row)

postgres[2252294][1]=# BEGIN ;
BEGIN

postgres[2252294][1]*=# SELECT * FROM pg_stat_replication_slots ;
┌───────────┬────────────┬─────────────┬─────────────┬─────────────┬──────────────┬──────────────┬─────────────┐
│ slot_name │ spill_txns │ spill_count │ spill_bytes │ stream_txns │ stream_count │ stream_bytes │ stats_reset │
├───────────┼────────────┼─────────────┼─────────────┼─────────────┼──────────────┼──────────────┼─────────────┤
│ test │ 0 │ 0 │ 0 │ 0 │ 0 │ 0 │ (null) │
└───────────┴────────────┴─────────────┴─────────────┴─────────────┴──────────────┴──────────────┴─────────────┘
(1 row)

postgres[2252294][1]*=# SELECT * FROM pg_stat_replication_slots ;
┌─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────>
│ >
├─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────>
│ \x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F>
└─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────>
(1 row)

I'll push the minimal fix of forcing the allocation to happen in
pgStatLocalContext and setting it to NULL in pgstat_clear_snapshot().

But it seems like we just shouldn't allocate it dynamically at all?
max_replication_slots doesn't change during postmaster lifetime, so it
seems like it should just be allocated once?

Greetings,

Andres Freund