pg_basebackup throttling doesn't throttle as promised

The "-r" option to pg_basebackup is supposed to throttle the rate of the
backup. But it only works properly if the server is mostly idle.

Every non-trivial call to XLogFlush or XLogBackgroundFlush will wake up the
wal sender (the one which is not really sending wal, but base files), and
the throttling routine doesn't go back to sleep after being awoke early.
Rather, it releases another 32kb of data.

Should the basebackup.c throttle sleep in a loop until its time has
expired?

Or should walsender.c WalSndWakeup not wake a wal sender whose status
is WALSNDSTATE_BACKUP?

Or both?

Cheers,

Jeff