Pants plugin contributions

Pants plugins that are expected to be widely useful, but not shipped with core pants belong here.

Having plugin code live in the main pants repo allows plugin authors to have their plugin tested and refactored alongside pants code while allowing pants core contributors a better grasp of how plugin APIs are used.

Most new plugins should get their own top-level contrib/ subdirectory although it may make sense to house the source code for related plugins under one top-level contrib/ subdirectory. The contrib/scrooge directory is an example of this and houses two plugin tasks that both use the same underlying Scrooge tool.

Contrib plugins should generally follow 3 basic setup steps:

1. Create a contrib source tree for the new plugin and register source roots A typical pure python layout would be:

   contrib/example/
     src/python/pants/contrib/example/...
     tests/python/pants_test/contrib/example/...
   

   The only hard requirement though is the register.py entry point file in the main src dir - in this example: contrib/example/src/python/pants/contrib/example/register.py.

   Source roots for this layout should match your source root patterns. The default patterns are typically sufficient.

   NB: python code should be in the pants/contrib and pants_test/contrib namespaces and the __init__.py files for these shared root namespaces should contain a namespace declaration like so:

   __import__('pkg_resources').declare_namespace(__name__)

2. Make the local pants aware of your plugin This involves 2 edits to pants.ini. You'll need to add one entry in each of the pythonpath and backend_packages lists:

   [DEFAULT]
   # Enable our own custom loose-source plugins as well as contribs.
   pythonpath: [
       "%(buildroot)s/pants-plugins/src/python",
       ...
       "%(buildroot)s/contrib/example/src/python",  # 1
       ...
     ]
   
   backend_packages: [
       "internal_backend.optional",
       "internal_backend.repositories",
       "internal_backend.sitegen",
       "internal_backend.utilities",
       ...
       "pants.contrib.example",  # 2
       ...
     ]

3. When you're ready for your plugin to be distributed, convert your main python_library plugin target to a contrib_plugin target and register the plugin with the release script.

   The contrib_plugin target assumes 1 source of register.py; so, the sources argument should be removed. It still accepts dependencies and other python target arguments with some special additions to help define the plugin distribution. You'll need to supply a distribution_name and a description of the plugin suitable for pypi as well as parameters indicating which plugin entry points your plugin implements:

   contrib_plugin(
     name='plugin',
     distribution_name='pantsbuild.pants.contrib.example',
     description='An example pants contrib plugin.'
     build_file_aliases=True,
     register_goals=True,
   )

   In this example, the plugin implements the build_file_aliases and register_goals entry point methods, but a plugin may additionally implement the global_subsystems entry point method, in which case it's contrib_plugin target would have a global_subsystems=True, entry as well.

   To register with the release script, add an entry to contrib/release_packages.sh:

   PKG_EXAMPLE=(
     "pantsbuild.pants.contrib.example"
     "//contrib/example/src/python/pants/contrib/example:plugin"
     "pkg_example_install_test"
   )
   function pkg_example_install_test() {
     execute_packaged_pants_with_internal_backends \
       --plugins="['pantsbuild.pants.contrib.example==$(local_version)']" \
       goals | grep "example-goal" &> /dev/null
   }
   
   # Once an individual (new) package is declared above, insert it into the array below)
   CONTRIB_PACKAGES=(
     PKG_SCROOGE
     PKG_EXAMPLE
   )

   NB: The act of releasing your contrib distribution is part of of the normal pantsbuild.pants release process. You may need to request a release from the owners if you have a change that should be fast-tracked before the next pantsbuild.pants release. You can always test that your contrib distribution works though by doing a release dry run:

   ./build-support/bin/release.sh -n