scache_save_endp_service: unable to receive file descriptor

I woke up this morning to see this message flying across my maillogs.

Feb 1 05:00:41 gir postfix/scache[26913]: warning:
scache_save_endp_service: unable to receive file descriptor
Feb 1 05:00:41 gir postfix/smtp[31210]: warning: problem talking to
service private/scache socket: Invalid argument

It started out as every 2 seconds (but still attempting to deliver
other mail), until it degenerated to just printing that message and
not delivering mail. It looks like it was mailman doing its monthly
password reminder batch. As far as I can tell, only
scache_save_endp_service was reported as having that problem.

OpenBSD/macppc -current, Postfix snapshot 2.2.20050119, with
CANT_WRITE_BEFORE_SENDING_FD defined.