avocado.core.tree: Cache TreeNode.environment

Usually we create single tree and then multiplex it's values. This
generates lots of TreeNode.environment calls. This patch caches
TreeNode.environment calls and returns last known version.

There is one drawback, in case you modify tree structure after calling
TreeNode.environment you must call TreeNode.set_environment_dirty,
otherwise you get the outdated results.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

Conflicts:
	avocado/core/tree.py

avocado/core/tree.py

@@ -55,6 +55,7 @@ class TreeNode(object):
         self.value = value
         self.parent = parent
         self.children = []
+        self._environment = None
         for child in children:
             self.add_child(child)
 
@@ -142,21 +143,24 @@ class TreeNode(object):
         return self.get_environment()
 
     def get_environment(self):
-        def update_or_extend(target, source):
-            for k, _ in source.items():
-                if k in target and isinstance(target[k], list):
-                    target[k].extend(source[k])
-                else:
-                    if isinstance(source[k], list):
-                        target[k] = source[k][:]
+        if self._environment is None:
+            self._environment = (self.parent.environment.copy()
+                                 if self.parent else {})
+            for key, value in self.value.iteritems():
+                if isinstance(value, list):
+                    if (key in self._environment
+                            and isinstance(self._environment[key], list)):
+                        self._environment[key] = self._environment[key] + value
                     else:
-                        target[k] = source[k]
-        env = {}
-        rev_parents = reversed(self.get_parents())
-        for parent in rev_parents:
-            update_or_extend(env, parent.value)
-        update_or_extend(env, self.value)
-        return env
+                        self._environment[key] = value
+                else:
+                    self._environment[key] = value
+        return self._environment
+
+    def set_environment_dirty(self):
+        for child in self.children:
+            child.set_environment_dirty()
+        self._environment = None
 
     def iter_children_preorder(self, node=None):
         q = collections.deque()