[entropy.spm] make SPM backend user configurable

conf/entropy.conf

@@ -77,3 +77,18 @@ system-name|Sabayon Linux
 #
 # Entropy processes nice level
 # nice-level|0
+
+#
+#  syntax for spm-backend:
+#
+#    spm-backend: Change Entropy Source Package Manager backend interface.
+#        Currently supported is "portage". Pay attention to edit this
+#        parameter, it could cause your Entropy to fail loading!
+#
+#    spm-backend|[supported backend identifier]
+#
+#    example:
+#    spm-backend|portage
+#
+# Default SPM backend value:
+# spm-backend|portage

libraries/entropy/core.py

@@ -1016,10 +1016,12 @@ class SystemSettings(Singleton):
         @rtype: dict
         """
 
-        data = {}
-        data['proxy'] = etpConst['proxy'].copy()
-        data['name'] = etpConst['systemname']
-        data['log_level'] = etpConst['entropyloglevel']
+        data = {
+            'proxy': etpConst['proxy'].copy(),
+            'name': etpConst['systemname'],
+            'log_level': etpConst['entropyloglevel'],
+            'spm_backend': etpConst['spm']['backend'],
+        }
 
         etp_conf = self.__setting_files['system']
         if not os.path.isfile(etp_conf) and \
@@ -1081,6 +1083,11 @@ class SystemSettings(Singleton):
 
                 data['name'] = split_line[1].strip()
 
+            elif line.startswith("spm-backend|") and \
+                (split_line_len == 2):
+
+                data['spm_backend'] = split_line[1].strip()
+
             elif line.startswith("nice-level|") and \
                 (split_line_len == 2):
 

libraries/entropy/spm/plugins/factory.py

@@ -12,6 +12,8 @@
 import os
 import sys
 from entropy.const import etpConst
+from entropy.core import SystemSettings
+from entropy.i18n import _
 from entropy.spm.plugins.skel import SpmPlugin
 PLUGIN_SUFFIX = "_plugin"
 _AVAILABLE_CACHE = None
@@ -70,10 +72,18 @@ def get_default_class():
     """
     Return currently configured Entropy Source Package Manager plugin class.
     """
-    backend = etpConst['spm']['backend']
+    settings = SystemSettings()
+    backend = settings['system'].get('spm_backend', etpConst['spm']['backend'])
     available = get_available_plugins()
+    klass = available.get(backend)
+    if klass is None:
+        import warnings
+        warnings.warn("%s: %s" % (
+            _("selected SPM backend not available"), backend,))
+        klass = available.get(etpConst['spm']['backend'])
     return available[backend]
 
+
 def get_default_instance(output_interface):
     """
     Return the currently configured Entropy SPM interface instance.