wic: refactor pluginbase

Wic plugin machinery implemented using metaclasses.

Reimplemented plugin machinery using this advice from
https://wiki.python.org/moin/PortingToPy3k/BilingualQuickRef
Syntax for creating instances with different metaclasses is very
different between Python 2 and 3. Use the ability to call type instances
as a way to portably create such instances.

Now it should work under both Python 2 and Python 3.

[YOCTO #9412]

(From OE-Core rev: e62fe5a41bdcdd72b9b257fecff7ccdc59c76d33)

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/lib/wic/pluginbase.py

@@ -15,34 +15,26 @@
 # with this program; if not, write to the Free Software Foundation, Inc., 59
 # Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 
+__all__ = ['ImagerPlugin', 'SourcePlugin', 'get_plugins']
+
+import sys
+from collections import defaultdict
+
 from wic import msger
 
-class _Plugin(object):
-    class __metaclass__(type):
-        def __init__(cls, name, bases, attrs):
-            if not hasattr(cls, 'plugins'):
-                cls.plugins = {}
+class PluginMeta(type):
+    plugins = defaultdict(dict)
+    def __new__(cls, name, bases, attrs):
+        class_type = type.__new__(cls, name, bases, attrs)
+        if 'name' in attrs:
+            cls.plugins[class_type.wic_plugin_type][attrs['name']] = class_type
 
-            elif 'wic_plugin_type' in attrs:
-                if attrs['wic_plugin_type'] not in cls.plugins:
-                    cls.plugins[attrs['wic_plugin_type']] = {}
+        return class_type
 
-            elif hasattr(cls, 'wic_plugin_type') and 'name' in attrs:
-                cls.plugins[cls.wic_plugin_type][attrs['name']] = cls
-
-        def show_plugins(cls):
-            for cls in cls.plugins[cls.wic_plugin_type]:
-                print cls
-
-        def get_plugins(cls):
-            return cls.plugins
-
-
-class ImagerPlugin(_Plugin):
+class ImagerPlugin(PluginMeta("Plugin", (), {})):
     wic_plugin_type = "imager"
 
-
-class SourcePlugin(_Plugin):
+class SourcePlugin(PluginMeta("Plugin", (), {})):
     wic_plugin_type = "source"
     """
     The methods that can be implemented by --source plugins.
@@ -99,10 +91,4 @@ class SourcePlugin(_Plugin):
         msger.debug("SourcePlugin: do_prepare_partition: part: %s" % part)
 
 def get_plugins(typen):
-    plugins = ImagerPlugin.get_plugins()
-    if typen in plugins:
-        return plugins[typen]
-    else:
-        return None
-
-__all__ = ['ImagerPlugin', 'SourcePlugin', 'get_plugins']
+    return PluginMeta.plugins.get(typen)