Use stevedore for plugin config of block device

This patch introduces stevedore plugin mechanism for use
with the block device layer.  This makes it possible that
other projects pass in their own block device plugins.

Change-Id: Id3ea56aaf75f5a20a4e1b6ac2a68adb12c56b574
Signed-off-by: Andreas Florath <andreas@florath.net>

diskimage_builder/block_device/blockdevice.py

@@ -15,13 +15,12 @@
 import codecs
 from diskimage_builder.block_device.blockdevicesetupexception \
     import BlockDeviceSetupException
-from diskimage_builder.block_device.level0 import LocalLoop
-from diskimage_builder.block_device.level1 import Partitioning
 from diskimage_builder.graph.digraph import Digraph
 import json
 import logging
 import os
 import shutil
+from stevedore import extension
 import sys
 import yaml
 
@@ -85,13 +84,6 @@ class BlockDevice(object):
 #  type: ext4
 #  mount_point: /
 
-    # A dictionary to map sensible names to internal implementation.
-    cfg_type_map = {
-        'local_loop': LocalLoop,
-        'partitioning': Partitioning,
-        'mkfs': 'not yet implemented',
-    }
-
     def __init__(self, block_device_config, build_dir,
                  default_image_size, default_image_dir):
         logger.debug("Creating BlockDevice object")
@@ -109,6 +101,9 @@ class BlockDevice(object):
                                       "states/block-device")
         self.state_json_file_name \
             = os.path.join(self.state_dir, "state.json")
+        self.plugin_manager = extension.ExtensionManager(
+            namespace='diskimage_builder.block_device.plugin',
+            invoke_on_load=False)
 
     def write_state(self, result):
         logger.debug("Write state [%s]" % self.state_json_file_name)
@@ -138,11 +133,11 @@ class BlockDevice(object):
 
             # As the first step the configured objects are created
             # (if it exists)
-            if cfg_obj_name not in BlockDevice.cfg_type_map:
+            if cfg_obj_name not in self.plugin_manager:
                 logger.error("Configured top level element [%s] "
                              "does not exists." % cfg_obj_name)
                 return 1
-            cfg_obj = BlockDevice.cfg_type_map[cfg_obj_name](
+            cfg_obj = self.plugin_manager[cfg_obj_name].plugin(
                 cfg_obj_val, default_config)
             # At this point it is only possible to add the nodes:
             # adding the edges needs all nodes first.

diskimage_builder/block_device/level0/localloop.py

@@ -14,6 +14,7 @@
 
 from diskimage_builder.block_device.blockdevicesetupexception \
     import BlockDeviceSetupException
+from diskimage_builder.block_device.plugin_base import NodePluginBase
 from diskimage_builder.block_device.utils import parse_abs_size_spec
 from diskimage_builder.graph.digraph import Digraph
 import logging
@@ -24,15 +25,13 @@ import subprocess
 logger = logging.getLogger(__name__)
 
 
-class LocalLoop(Digraph.Node):
+class LocalLoop(NodePluginBase):
     """Level0: Local loop image device handling.
 
     This class handles local loop devices that can be used
     for VM image installation.
     """
 
-    type_string = "local_loop"
-
     def __init__(self, config, default_config):
         logger.debug("Creating LocalLoop object; config [%s] "
                      "default_config [%s]" % (config, default_config))
@@ -50,9 +49,6 @@ class LocalLoop(Digraph.Node):
         Digraph.Node.__init__(self, self.name)
         self.filename = os.path.join(self.image_dir, self.name + ".raw")
 
-    def insert_nodes(self, dg):
-        dg.add_node(self)
-
     def insert_edges(self, dg):
         """Because this is created without base, there are no edges."""
         pass

diskimage_builder/block_device/level1/partitioning.py

@@ -16,6 +16,7 @@ import collections
 from diskimage_builder.block_device.blockdevicesetupexception \
     import BlockDeviceSetupException
 from diskimage_builder.block_device.level1.mbr import MBR
+from diskimage_builder.block_device.plugin_base import PluginBase
 from diskimage_builder.block_device.utils import parse_abs_size_spec
 from diskimage_builder.block_device.utils import parse_rel_size_spec
 from diskimage_builder.graph.digraph import Digraph
@@ -67,9 +68,7 @@ class Partition(Digraph.Node):
         pass
 
 
-class Partitioning(object):
-
-    type_string = "partitioning"
+class Partitioning(PluginBase):
 
     flag_boot = 1
     flag_primary = 2

diskimage_builder/block_device/plugin_base.py

@@ -0,0 +1,51 @@
+# Copyright 2017 Andreas Florath (andreas@florath.net)
+#
+# 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 abc
+from diskimage_builder.graph.digraph import Digraph
+import six
+
+
+@six.add_metaclass(abc.ABCMeta)
+class PluginBase(object):
+    """Abstract base class for block device plugins"""
+
+    def __init__(self, name):
+        """All plugins must have a name"""
+        self.name = name
+
+    @abc.abstractmethod
+    def create(self, state, rollback):
+        """Create the block device plugin
+
+        :param state: a dictionary to store results for this plugin.
+                      These are used in two scenarios: other plugins
+                      can use this to get information about the
+                      result of the plugin and it can be used in
+                      later runs of dib-block-device for cleaning up.
+        :type state: dict(str:?)
+
+        :param rollback: a list of python functions that will be
+                         called in reversed order to cleanup if there
+                         occurs an error later within the same
+                         dib-block-device run.
+        :type rollback: list(function)
+        """
+
+
+class NodePluginBase(PluginBase, Digraph.Node):
+
+    def insert_nodes(self, dg):
+        """Adds self as a node to the given digraph"""
+        dg.add_node(self)

requirements.txt

@@ -6,3 +6,4 @@ pbr>=2.0.0 # Apache-2.0
 PyYAML>=3.10.0 # MIT
 flake8<2.6.0,>=2.5.4 # MIT
 six>=1.9.0 # MIT
+stevedore>=1.20.0 # Apache-2.0

setup.cfg

@@ -56,3 +56,7 @@ console_scripts =
     disk-image-create = diskimage_builder.disk_image_create:main
     ramdisk-image-create = diskimage_builder.disk_image_create:main
     dib-run-parts = diskimage_builder.dib_run_parts:main
+
+diskimage_builder.block_device.plugin =
+    local_loop = diskimage_builder.block_device.level0.localloop:LocalLoop
+    partitioning = diskimage_builder.block_device.level1.partitioning:Partitioning