[BuildStream] Migration of plugins away from BuildStream core

[Phil Dawson <phil dawson codethink com>]

Hi All

As per various discussions from the last couple of gatherings, I'm
about to start work on migrating a number of plugins away from
BuildStream core. I've summarised below what I think this will look
like.

# What we want in core

From various discussions on the mailing list and during the last two
gatherings, this part seems to be fairly uncontroversial. However,
please shout out if you disagree.

The end goal is to have the following plugins in Core:
    import
    junction
    compose
    script
    junction
    filter
    manual
    local
    remote


# The migration of plugins from core

The above list of plugins which we want in BuildStream core leaves the
following plugins to migrate:

| elements    | sources |
|-------------+---------|
| make        | git     |
| cmake       | tar     |
| autotools   | bzip    |
| qmake       | bzr     |
| distutils   | ostree  |
| makemaker   | patch   |
| modulebuild | deb     |
| meson       | pip     |
| pip         |         |


The plugins which are moved out of BuildStream core will be moved into
use-case specific repositories. While their seems to be general
agreement on this point, the specifics have not had much discussion
and so I'm particularly interested to hear other peoples thoughts here.

There are a couple of obvious groupings:

 * pip element, pip source, distutils element => bst-plugins-python
 * makemaker element, modulebuild element => bst-plugins-perl

The remainder seem to me to be fairly stand-alone, but may belong to
categories which are likely to be expanded on in the future.

Having said that. Grouping for example git, bzr, ostree, and patch
into a bst-plugins-vcs repository would help keep the number of
repositories easier to manage, but perhaps doesn't make so much sense
from the perspective of grouping by use-case. I'd be interested to
hear peoples thoughts here.

In order to maintain our current levels of test coverage, as each
plugin is migrated I plan to hook its tests into our test suite using
the machinery in [1]

[1] https://gitlab.com/BuildStream/buildstream/merge_requests/1158

Cheers,

Phil


--
Phil Dawson, Software Engineer                          Codethink Ltd
https://www.codethink.co.uk/
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