>   0002: allocate the static buffer once per process via
>         pq_beginmessage_reuse / pq_endmessage_reuse, to avoid the
>         per-call allocation (master only)

I am less convinced this will have any benefits for the additional complexity.
The callers of pgstat_progress_parallel_incr_param() are not frequent enough
to make a measurable difference here. cost delay reporting for parallel workers
is throttled by PARALLEL_VACUUM_DELAY_REPORT_INTERVAL_NS and
index progress reporting does not happen very frequently either.

Thanks for the review, Sami.

Agreed, at those call frequencies the per-call palloc is not worth