un-dedicating a line to a dialer

[William L. Sebok]

> The solution is to have the dialler program open the dial
> line with O_EXCL, which will return an error if a getty or login
> still has the line open.
> 
> If you have signalled init, it is about to hangup the line anyway,
> so uucico just needs to wait until its exclusive open returns.

There is a severe problem with Exclusive Use mode on lines used for both
dialing in and dialing out.  If someone dials in and releases a long-running
background job the special file will be held open until the background job
finishes (which happens quite often on our system).  This will prevent locking
mechanisms like the above from working. 
  The 4.1 BSD uucp expected that closing the port would clear the Exclusive
use bit.  Under the above circumstances this didn't work.  The cure was to
make sure uucp cleared Exclusive use before closing the port.  Another
possible place to handle this problem is in the program that enables/disables
dialins on the port.
-- 
Bill Sebok			Princeton University, Astrophysics
{allegra,akgua,burl,cbosgd,decvax,ihnp4,kpno,princeton,vax135}!astrovax!wls