input: allow to remove builtin white list entries

The new "whitelistRemove" key in default.yaml remove entries from the
whitelist. It has a higher precedence than adding whitelist entries. It
is not an error to remove non-existent whitelist entries.

pym/bob/input.py

@@ -277,6 +277,8 @@ def onFinish(self, env, properties):
 
 
 class PluginSetting:
+    priority = 50
+
     """Base class for plugin settings.
 
     Plugins can be configured in the user configuration of a project. The
@@ -361,10 +363,11 @@ def newFun(args, **kwargs):
 class BuiltinSetting(PluginSetting):
     """Tiny wrapper to define Bob built-in settings"""
 
-    def __init__(self, schema, updater, mangle = False):
+    def __init__(self, schema, updater, mangle = False, priority=50):
         self.__schema = schema
         self.__updater = updater
         self.__mangle = mangle
+        self.priority = priority
 
     def merge(self, other):
         self.__updater(self.__schema.validate(other) if self.__mangle else other)
@@ -3132,6 +3135,14 @@ def updateWhiteList(x):
             else:
                 self.__whiteList.update(x)
 
+        def removeWhiteList(x):
+            if self.__platform == "win32":
+                # Convert to upper case on Windows. The Python interpreter does that
+                # too and the variables are considered case insensitive by Windows.
+                self.__whiteList.difference_update(i.upper() for i in x)
+            else:
+                self.__whiteList.difference_update(x)
+
         self.__settings = {
             "alias" : BuiltinSetting(
                 schema.Schema({ schema.Regex(r'^[0-9A-Za-z_-]+$') : str }),
@@ -3221,6 +3232,11 @@ def updateWhiteList(x):
                 schema.Schema([ schema.Regex(r'^[^=]*$') ]),
                 updateWhiteList
             ),
+            "whitelistRemove" : BuiltinSetting(
+                schema.Schema([ schema.Regex(r'^[^=]*$') ]),
+                removeWhiteList,
+                priority=100
+            ),
         }
 
     def __addRecipe(self, recipe):
@@ -3627,7 +3643,8 @@ def __parseUserConfig(self, fileName, relativeIncludes=None):
         if relativeIncludes is None:
             relativeIncludes = self.getPolicy("relativeIncludes")
         cfg = self.loadYaml(fileName, self.__userConfigSchema)
-        for (name, value) in cfg.items():
+        # merge settings by priority
+        for (name, value) in sorted(cfg.items(), key=lambda i: self.__settings.get(i[0], PluginSetting).priority):
             if name != "include" and name != "require": self.__settings[name].merge(value)
         for p in cfg.get("require", []):
             p = (os.path.join(os.path.dirname(fileName), p) if relativeIncludes else p) + ".yaml"