Add plugin mechanism. (#35)

The motivation is to be able to discover ci_command's on the PYTHONPATH
provided by packages outside of fireci.

Author: vkryachko

ci/fireci/fireci/__init__.py

@@ -11,3 +11,5 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+
+from .internal import ci_command

ci/fireci/fireci/commands.py

@@ -16,7 +16,7 @@
 import os
 
 from . import gradle
-from .internal import ci_command
+from . import ci_command
 
 
 @click.argument('task', required=True, nargs=-1)

ci/fireci/fireci/gradle.py

@@ -22,6 +22,11 @@
 ADB_INSTALL_TIMEOUT = '5'
 
 
+def P(name, value):
+  """Returns name and value in the format of gradle's project property cli argument."""
+  return '-P{}={}'.format(name, value)
+
+
 def run(*args, gradle_opts='', workdir=None):
   """Invokes gradle with specified args and gradle_opts."""
   new_env = dict(os.environ)

ci/fireci/fireci/main.py

@@ -15,11 +15,14 @@
 import logging
 
 from . import commands
+from . import plugins
 from .internal import main
 
 logging.basicConfig(
     format='%(name)s: [%(levelname)s] %(message)s',
     level=logging.DEBUG,
 )
 
+plugins.discover()
+
 cli = main

ci/fireci/fireci/plugins.py

@@ -0,0 +1,33 @@
+# Copyright 2018 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import importlib
+import pkgutil
+import fireciplugins
+
+
+def discover():
+  """Discovers fireci plugins available on PYTHONPATH under firebaseplugins subpackages.
+
+     Discovery works by importing all direct subpackages of firebaseplugins and importing them,
+     plugins are supposed to register ci_command's with fireci in their __init__.py files directly
+     or by importing from their own subpackages.
+
+     Note: plugins *must* define the `firebaseplugins` package as a namespace package.
+           See: https://packaging.python.org/guides/packaging-namespace-packages/
+  """
+  modules = pkgutil.iter_modules(fireciplugins.__path__,
+                                 fireciplugins.__name__ + ".")
+  for module in modules:
+    importlib.import_module(module.name)

ci/fireci/fireciplugins/__init__.py

@@ -0,0 +1,14 @@
+# Copyright 2018 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+__path__ = __import__('pkgutil').extend_path(__path__, __name__)