max_parallel_workers can't actually be set?

Try this:
alter system set max_parallel_workers = 20;
and restart the system.

max_parallel_workers is still 8, according to both SHOW and
pg_controldata, nor can you launch more than 8 workers.

Even odder, if you just do

regression=# set max_parallel_workers = 200;
SET
regression=# show max_parallel_workers;
max_parallel_workers
----------------------
200
(1 row)

which should certainly not happen for a PGC_POSTMASTER parameter.

We seem to have an awful lot of mechanism that's concerned with
adjustments of max_parallel_workers, for something that apparently
might as well be a compile-time #define ... so I assume it's supposed
to be changeable at restart and somebody broke it. But it's not
working as I'd expect in any branch from 10 onwards.

regards, tom lane