✨ Add get_installation_config helper

It allows users to grab the config file from current repo easily.

As in:

    from octomachinery.app.runtime.installation_utils import (
        get_installation_config
    )
    await get_installation_config()

octomachinery/app/runtime/installation_utils.py

@@ -0,0 +1,80 @@
+"""Utility helpers for App/Action installations."""
+
+from base64 import b64decode
+from io import StringIO
+from pathlib import Path
+import typing
+
+import yaml
+
+from .context import RUNTIME_CONTEXT
+
+
+def _get_file_contents_from_fs(file_name: str) -> typing.Optional[str]:
+    """Read file contents from file system checkout.
+
+    This code path is synchronous.
+
+    It doesn't matter much in GitHub Actions
+    but can be refactored later.
+    """
+    config_path = (Path('.') / file_name)
+
+    try:
+        return config_path.read_text()
+    except FileNotFoundError:
+        return None
+
+
+async def _get_file_contents_from_api(
+        file_name: str,
+        ref: typing.Optional[str],
+) -> typing.Optional[str]:
+    """Read file contents using GitHub API."""
+    github_api = RUNTIME_CONTEXT.app_installation_client
+    repo_slug = RUNTIME_CONTEXT.github_event.data['repository']['full_name']
+
+    api_query_params = f'?ref={ref}' if ref else ''
+    config_response = await github_api.getitem(
+        f'/repos/{repo_slug}/contents'
+        f'/{file_name}{api_query_params}',
+    )
+
+    config_file_found = (
+        config_response.get('encoding') == 'base64' and
+        'content' in config_response
+    )
+    if not config_file_found:
+        return None
+
+    return b64decode(config_response['content']).decode()
+
+
+async def get_installation_config(
+        *,
+        config_name: str = 'config.yml',
+        ref: typing.Optional[str] = None,
+) -> typing.Mapping[str, typing.Any]:
+    """Get a config object from the current installation.
+
+    Read from file system checkout in case of GitHub Action env.
+    Grab it via GitHub API otherwise.
+
+    Usage::
+
+        >>> from octomachinery.app.runtime.installation_utils import (
+        ...     get_installation_config
+        ... )
+        >>> await get_installation_config()
+    """
+    config_path = f'.github/{config_name}'
+
+    if RUNTIME_CONTEXT.IS_GITHUB_ACTION and ref is None:
+        config_content = _get_file_contents_from_fs(config_path)
+    else:
+        config_content = await _get_file_contents_from_api(config_path, ref)
+
+    if config_content is None:
+        return {}
+
+    return yaml.load(StringIO(config_content))

setup.cfg

@@ -94,6 +94,7 @@ install_requires =
     envparse
     gidgethub
     pyjwt
+    pyyaml
     setuptools_scm
 
 [options.packages.find]