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Thomas De Schampheleire

issues: support generic regex replacements in issue_url and issue_prefix

Issue reference linking is pretty limited:
- the issue_url is a literal with only three special tokens {id},
{repo} and {repo_name}. There is no way to let the URL be dependent on
other elements of the input issue reference.
- The value for {id} is somewhat oddly determined by the concatenation of
all parenthesized groups in the issue_pat regular expression
- the link text of the resulting link is limited to the contents of the
literal issue_prefix with the determined {id}. It is not possible to
retain the input issue reference verbatim, nor to let the link text be
dependent on other elements of the input issue reference.

This commit makes the issue reference linking more flexible:

- issue_prefix is replaced by the more generic issue_sub(stitution), which
is a string that may contain backreferences to regex groups specified in
issue_pat. This string, with backreferences resolved, is used as the
link text of urlified issue references.
- if issue_sub is empty, the entire text matched by issue_pat is used as
the link text.
- like issue_sub, also issue_url can contain backreferences to regex groups.
- {id} is no longer treated as a special token, as it can be solved by
generic backreferences ('\g<id>' assuming issue pattern contains something
like '(P<id>\d+)'. {repo} and {repo_name} are still supported, because
their value is provided externally and not normally part of the
issue pattern.

Documentation and ini file template is updated as well.

=======================
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::

    gearbox make-config 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
    gearbox setup-db -c temp.ini --repos=/var/repos --user=doe --email doe@example.com --password=123456 --no-public-access --force-yes
    gearbox repo-scan -c 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).