The symptom is that syslog() fails to log anything but the "ident"
string if LOG_PERROR is specified to openlog(3) and the extensible
printf is in action.
For unclear, likely quaint historical reasons, syslog uses fwopen()
on a stack buffer, rather than using the more straightforward
and faster snprintf().
Along the way, fflush(3) is called, and since the callback writer
function returns zero instead of the length "written", __SERR
naturally gets set on the filedescriptor.
The extensible printf, in difference from the normal printf refuses
to output anything to an __SERR marked filedescriptor, and thus
the actual syslog message is supressed.
MFC: after 2 weeks
5b1deb3ce8