Introduce buildout:zcml-additional-fragments buildout variable

[lukasgraf]

This PR introduces a buildout:zcml-additional-fragments buildout variable that can be used to effectively extend the resulting zcml-additional with += assignments.

This is motivated by ZCML needed for ftw.tika:
tika-server.cfg currently includes

[instance0]
zcml-additional += ${tika:zcml}

While it's a nice idea that tika-server.cfg is completely self-configuring, I don't think this can work reliably:

• instance0:zcml-additional often gets overridden in buildouts that extend from the ftw-buildouts.
• Because everything in zcml-additional needs to be wrapped in a single <configure /> element, appending to it from multiple places using += doesn't work.

Therefore I suggest to remove that zcml-additional += ${tika:zcml} and instead document in README.rst how to add tika's ZCML to instance0:zcml-additional in the buildout that extends from tika-server.cfg. That way its left to the developer to make sure the ZCML is included properly in whatever other additional ZCML he has in his buildout.

This issue really should be fixed in plone.recipe.zope2instance I guess - always wrap the contents of zcml-additional in an outer <configure /> element if it contains more than one element.

As a work-around, we do this in some of our buildouts:

• Define buildout:zcml-additional-fragments
• Append to buildout:zcml-additional-fragments from multiple places using +=
• Wrap its contents in a <configure /> node and set zcml-additional to that:

    [instance0]
    zcml-additional =
        <configure xmlns="http://namespaces.zope.org/zope">
            ${buildout:zcml-additional-fragments}
        </configure>

Maybe this pattern could also be useful in ftw-buildouts.
@jone any thoughts on this?

[lukasgraf]

PullRequest created for plone.recipe.zope2instance: plone/plone.recipe.zope2instance#13

[jone]

@lukasgraf I see your points.

The most important thing for me is that I don't have to think / do things when I use it in a "default" setup, meaning when I don't override zcml-additional. Currently this is the only place in ftw-buildouts where zcml-additional is used, so the buildouts here don't yet have this problem.

The zcml-additional-fragments looks very promising, I think we should define this as standard for all ftw-buildouts and wrap the zcml-additional by default as you suggested. But I'd like to have this included in this pull-request, so that existing buildouts using the tika configuration from here do not break.

[jone]

@lukasgraf I've pushed my proposal to your branch, implementing the zcml-additional-fragments buildout variable. Please take a look 😉

[lukasgraf]

@jone excellent! Going to drop my commit, change the title of the PR to something more generic and merge it.