I got an error similar to the reports on
http://sourceforge.net/p/roundup/mailman/message/32025543/
https://mail.python.org/pipermail/python-dev/2012-April/118498.html
with Roundup devel (changeset e56047711df2 from April 2014).

Traceback (most recent call last):
  File "/home/roundup/current/lib/python/roundup/cgi/client.py", line
469, in inner_main
    html = self.handle_action()
  File "/home/roundup/current/lib/python/roundup/cgi/client.py", line
1243, in handle_action
    return action_klass(self).execute()
  File "/home/roundup/current/lib/python/roundup/cgi/actions.py", line
39, in execute
    return self.handle()
  File "/home/roundup/current/lib/python/roundup/cgi/actions.py", line
620, in handle
    message = self._editnodes(props, links)
  File "/home/roundup/current/lib/python/roundup/cgi/actions.py", line
463, in _editnodes
    newid = self._createnode(cn, props)
  File "/home/roundup/current/lib/python/roundup/cgi/actions.py", line
518, in _createnode
    return cl.create(**props)
  File
"/home/roundup/current/lib/python/roundup/backends/rdbms_common.py",
line 2981, in create
    content, mime_type)
  File
"/home/roundup/current/lib/python/roundup/backends/indexer_xapian.py",
line 58, in add_text
    database = self._get_database()
  File
"/home/roundup/current/lib/python/roundup/backends/indexer_xapian.py",
line 20, in _get_database
    return xapian.WritableDatabase(index, xapian.DB_CREATE_OR_OPEN)
  File "/usr/lib/python2.7/dist-packages/xapian/__init__.py", line 4303,
in __init__
   
_xapian.WritableDatabase_swiginit(self,_xapian.new_WritableDatabase(*args))
DatabaseLockError: Unable to get write lock on /path/to/db/text-index:
already locked

I used the tracker via the web interface to enter a single-line comment and
change the assignedto attribute. I did not get an error message in the web
interface and mails to other people in the nosy got sent without a problem,
too.

Adding a message to the same shortly before and a little bit later worked
fine.

Searching for a single word that appears in this issue only in this one
message using the "All test" field works.

The system is Debian wheezy x86, relevant packages are:
python-xapian 1.2.12-2
libxapian22 1.2.12-2
python2.7 2.7.3-6+deb7u2
postgresql-9.1 9.1.12-0wheezy1