tox 4: enable plugins to discover config sections & testenvs

[dcrosta]

My plugin tox-docker could benefit from being able to discover all configuration sections and/or to get a list of all testenvs from the Config.

tox-docker starts docker containers as configured by each testenv; the testenv references another section (eg [docker:nginx] by way of saying docker=nginx in the testenv). In order to discover all configuration, I need to enumerate all testenvs or all sections (and filter the list to docker:* sections).

I'm currently doing the former with:

class EnvDockerConfigSet(ConfigSet):
    def __init__(self, conf: Config) -> None:
        super().__init__(conf)

        self.add_config(
            keys=["docker"],
            of_type=EnvList,
            default=EnvList([]),
            desc="docker image configs to load",
            post_process=list,
        )

def discover_container_configs(config: Config) -> Sequence[str]:
    docker_configs = set()
    for env_name in config:
        env_config = config.get_section_config(f"testenv:{env_name}", EnvDockerConfigSet)
        docker_configs.update(env_config.load("docker"))
    return list(docker_configs)

It would be more explicit to have eg. config.get_test_envs() that returns either all testenvs, or better yet, the specific testenvs that will be run (eg due to setting -e on the commandline, or the envlist, etc).

[gaborbernat]

1. For getting a list of all tox environments you can use https://github.com/tox-dev/tox/blob/rewrite/src/tox/session/cmd/show_config.py#L61. We need to expose this somehow though on the config argument or something like that.

2. For the new configuration section: So there's some complexity we need to tackle here: different configu2ration sources. Today this is mostly INI file only. However, going ahead we'll have other files, such as the pyproject.toml. For that file, it's not allowed to reuse the docker namespace directly, but instead, https://www.python.org/dev/peps/pep-0518/#tool-table governs that. To follow that ad litera you'll likely want to add the docker sections under tool.tox-docker.docker-nginx, and tool.tox-docker.docker-db. The system should hideaway this from you somehow.