uucp seqno wraparound

[utzoo!henry]

The code in uux (and elsewhere?) that picks filenames for spool files
makes no attempt to cope with wraparound of the 4-digit sequence number.
A busy site like decvax can wrap the sequence number in less than a week.
This is bad news if you are a site that polls a busy site infrequently.
The odds are excellent that you will lose X-files (containing uux commands
to be executed at your site) since their spool files have names which are
protected against collisions only by the sequence number.  The C-files
(which actually control transmission) and D-files (which contain the
data the X-files operate on) are less vulnerable because their spool names
contain the name of your site as well, but it is still possible for them
to get stomped (the chances increase as the quantity of traffic spooled
for you increases).

The symptoms of X-file loss are files with names like D.mysiteB2345
piling up in /usr/spool/uucp.  (If you use uuclean or the equivalent
to clean out the directory, you may have to look sharp to notice them
before they vanish!)  The symptoms of C-file or D-file stomping are
missing traffic and mismatches between data and commands (e.g. something
that looks like mail being fed to news instead).

Clearly uux (et al?) should iterate until it finds an unused filename.
Due to race conditions etc., this may not be as easy as it sounds.
I have not investigated in detail;  at the moment the problem can be
controlled by frequent polling.  This obviously won't work forever if
the network traffic keeps increasing.