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Mads Kiilerich

branches: fix performance of branch selectors with many branches - only show the first 200 results

The way we use select2, it will cause browser performance problems when a
select list contains thousands of entries. The primary bottleneck is the DOM
creation, secondarily for the query to filter through the entries and decide
what to show. We thus primarily have to limit how many entries we put in the
drop-down, secondarily limit the iteration over data.

One tricky case is where the user specifies a short but full branch name (like
'trunk') but many other branches contains the same string (not necessarily at
the beginning, like 'for-trunk-next-week') which come before the perfect match
in the branch list. It is thus not a solution to just stop searching when a
fixed amount of matches have been found.

Instead, we limit the amount of ordinary query matches, but always show all
prefix matches. We thus always have to iterate through all entries, but we
start using the (presumably) cheaper prefix search when the limit has been
reached.

There is no filtering initially when there is no query term, so that case has
to be handled specially.

Upstream select2 is now at 4.x. Upgrading is not trivial, and getting this
fixed properly upstream is not a short term solution. Instead, we customize our
copy. The benefit from this patch is bigger than the overhead of "maintaining"
it locally.

=======================
Database schema changes
=======================

Kallithea uses Alembic for :ref:`database migrations <upgrade_db>`
(upgrades and downgrades).

If you are developing a Kallithea feature that requires database schema
changes, you should make a matching Alembic database migration script:

1. :ref:`Create a Kallithea configuration and database <setup>` for testing
   the migration script, or use existing ``development.ini`` setup.

   Ensure that this database is up to date with the latest database
   schema *before* the changes you're currently developing. (Do not
   create the database while your new schema changes are applied.)

2. Create a separate throwaway configuration for iterating on the actual
   database changes::

    paster make-config Kallithea temp.ini

   Edit the file to change database settings. SQLite is typically fine,
   but make sure to change the path to e.g. ``temp.db``, to avoid
   clobbering any existing database file.

3. Make your code changes (including database schema changes in ``db.py``).

4. After every database schema change, recreate the throwaway database
   to test the changes::

    rm temp.db
    paster setup-db temp.ini --repos=/var/repos --user=doe --email doe@example.com --password=123456 --no-public-access --force-yes
    paster repo-scan temp.ini

5. Once satisfied with the schema changes, auto-generate a draft Alembic
   script using the development database that has *not* been upgraded.
   (The generated script will upgrade the database to match the code.)

   ::

    alembic -c development.ini revision -m "area: add cool feature" --autogenerate

6. Edit the script to clean it up and fix any problems.

   Note that for changes that simply add columns, it may be appropriate
   to not remove them in the downgrade script (and instead do nothing),
   to avoid the loss of data. Unknown columns will simply be ignored by
   Kallithea versions predating your changes.

7. Run ``alembic -c development.ini upgrade head`` to apply changes to
   the (non-throwaway) database, and test the upgrade script. Also test
   downgrades.

   The included ``development.ini`` has full SQL logging enabled. If
   you're using another configuration file, you may want to enable it
   by setting ``level = DEBUG`` in section ``[handler_console_sql]``.

The Alembic migration script should be committed in the same revision as
the database schema (``db.py``) changes.

See the `Alembic documentation`__ for more information, in particular
the tutorial and the section about auto-generating migration scripts.

.. __: http://alembic.zzzcomputing.com/en/latest/


Troubleshooting
---------------

* If ``alembic --autogenerate`` responds "Target database is not up to
  date", you need to either first use Alembic to upgrade the database
  to the most recent version (before your changes), or recreate the
  database from scratch (without your schema changes applied).