We have some software which sits behind our version control repository
and which notifies roundup via the email gateway when certain kinds of
commits have been made.  It is frequently the case that several of these
emails are sent out in quick succession (as this process is automated)

Recently, we've been getting error messages of this sort:
An unexpected error occurred during the processing                     
                                                                     
of your message. The tracker administrator is being                    
                                                                     
notified.                                                              
                                                                     
                                                                       
                                                                     
----------------                                                       
                                                                     
Traceback (most recent call last):                                     
                                                                     
  File "/usr/lib/pymodules/python2.5/roundup/mailgw.py", line 738, in
handle_Message                                                         
    return self.handle_message(message)                                
                                                                     
  File "/usr/lib/pymodules/python2.5/roundup/mailgw.py", line 794, in
handle_message                                                         
    return self._handle_message (message)                              
                                                                     
  File "/usr/lib/pymodules/python2.5/roundup/mailgw.py", line 1369, in
_handle_message                                                       
    cl.set(nodeid, **props)                                            
                                                                     
  File "/usr/lib/pymodules/python2.5/roundup/backends/rdbms_common.py",
line 1606, in set                                                    
    propvalues = self.set_inner(nodeid, **propvalues)                  
                                                                     
  File "/usr/lib/pymodules/python2.5/roundup/backends/rdbms_common.py",
line 1816, in set_inner                                              
    self.db.setnode(self.classname, nodeid, propvalues,
multilink_changes)                                                     
             
  File "/usr/lib/pymodules/python2.5/roundup/backends/rdbms_common.py",
line 933, in setnode                                                 
    self.sql(sql, vals)                                                
                                                                     
  File "/usr/lib/pymodules/python2.5/roundup/backends/rdbms_common.py",
line 159, in sql                                                     
    self.cursor.execute(sql, args)                                     
                                                                     
ProgrammingError: ERROR:  could not serialize access due to concurrent
update                                                                
                                                                       
                                                                     
update _issue set _activity='2010-10-15
13:14:44.653',_actor=1837,_resolvedin=2,_status=8 where id=1942 

Strangely it seems that the change we want to make takes effect anyway.

See also http://bugs.darcs.net and the attached mail that our
auto-updater tries to send

Also,$ roundup-admin -v
1.4.13 (python 2.5.2)

This is running on Debian 5.0.3 (Debian stable at the time of this
writing, with roundup installed via an unstable package)

Which database backend are you using?
Does the error message occurr each time? (if not, what is the 
frequency?) Does it depend on the message?

It's Postgres.  The conditions we'd need to trigger the bug don't happen
very often (so I can't really provide a useful answer about frequency).
I think it's happened in two batches, with each batch giving me 3 or 4
of these at a time.

I can keep an eye out for this in the future.

The script that generates these updates lives here:
http://darcs.net/contrib/update_roundup.pl

What I find troubling in this report is that I'd expect each mailgw
session to open up a new session, such that multiple sessions are
independent transactions, which can't interfere with each other.

The tracelog however seems to suggest that the internal cursor variable
gets corrupted due to multiple simultaneous sessions.

I'm also seeing this problem in a production tracker. The reason is that
the cookies are manipulated with the same database cursor as the
production data. I'll have to dig up another issue where this has been
discussed in depth (including a patch as far as I remember).