issue2551142 - Import of retired node ... unique constraint failure. Title: Import of retired node with username after active node fails with unique constraint failure. More fixes needed for mysql and postgresql. mysql: add unique constraint for (keyvalue, __retired__) when creating class in the database. On schema change if class is changed, remove the unique constraint too. upgrade version of rdbms database from 5 to 6 to add constraint to all version 5 databases that were created as version 5 and didn't get the unique constraint. Make no changes on version 5 databases upgraded from version 4, the upgrade process to 5 added the constraint. Make no changes to other databases (sqlite, postgres) during upgrade from version 5 to 6. postgres: Handle the exception raised on unique constraint violation. The exception invalidates the database connection so it can't be used to recover from the exception. Added two new database methods: checkpoint_data - performs a db.commit under postgres does nothing on other backends restore_connection_on_error - does a db.rollback on postgres, does nothing on other backends with the rollback() done on the connection I can use the database connection to fixup the import that failed on the unique constraint. This makes postgres slower but without the commit after every imported object, the rollback will delete all the entries done up to this point. Trying to figure out how to make the caller do_import batch and recover from this failure is beyond me. Also dismissed having to process the export csv file before importing. Pushing that onto a user just seems wrong. Also since import/export isn't frequently done the lack of surprise on having a failing import and reduced load/frustration for the user seems worth it. Also the import can be run in verbose mode where it prints out a row as it is processed, so it may take a while, ut the user can get feedback. db_test-base.py: add test for upgrade from 5 to 6.

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Implementation notes
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[see also the roundup package docstring]

There have been some modifications to the spec. I've marked these in the
source with 'XXX' comments when I remember to.

In short:
 Class.find() - may match multiple properties, uses keyword args.

 Class.filter() - isn't in the spec and it's very useful to have at the
    Class level.

 CGI interface index view specifier layout part - lose the '+' from the
    sorting arguments (it's a reserved URL character ;). Just made no
    prefix mean ascending and '-' prefix descending.

 ItemClass - renamed to IssueClass to better match it only having one
    hypderdb class "issue". Allowing > 1 hyperdb class breaks the
    "superseder" multilink (since it can only link to one thing, and
    we'd want bugs to link to support and vice-versa).

 template - the call="link()" is handled by special-case mechanisms in
    my top-level CGI handler. In a nutshell, the handler looks for a
    method on itself called 'index%s' or 'item%s' where %s is a class.
    Most items pass on to the templating mechanism, but the file class
    _always_ does downloading. It'll probably stay this way too...

 template - call="link(property)" may be used to link "the current item"
    (from an index) - the link text is the property specified.

 template - added functions that I found very useful: List, History and
    Submit.

 template - items must specify the message lists, history, etc. Having
    them by default was sometimes not wanted.

 template - index view determines its default columns from the
    template's ``tal:condition="request/show/<property>"`` directives.

 template - menu() and field() look awfully similar now .... ;)

 roundup_admin.py - the command-line tool has a lot more commands at its
    disposal

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