Sandbox.py: Rename __root to _root.

This is to allow to allow its use by subclasses.

Since access to get_directories is now blocked for some plugins,
and the subclasses of Sandbox do not have configuration defined
by YAML files, they need another way to get at the root directory.

buildstream/sandbox/_sandboxchroot.py

@@ -90,7 +90,7 @@ class SandboxChroot(Sandbox):
             # Nonetheless a better solution could perhaps be found.
 
             rootfs = stack.enter_context(utils._tempdir(dir='/var/run/buildstream'))
-            stack.enter_context(self.create_devices(self.get_directory(), flags))
+            stack.enter_context(self.create_devices(self._root, flags))
             stack.enter_context(self.mount_dirs(rootfs, flags, stdout, stderr))
 
             if flags & SandboxFlags.INTERACTIVE:

buildstream/sandbox/sandbox.py

@@ -97,11 +97,12 @@ class Sandbox():
         self.__stdout = kwargs['stdout']
         self.__stderr = kwargs['stderr']
 
-        # Setup the directories
+        # Setup the directories. Root should be available to subclasses, hence
+        # being single-underscore. The others are private to this class.
+        self._root = os.path.join(directory, 'root')
         self.__directory = directory
-        self.__root = os.path.join(self.__directory, 'root')
         self.__scratch = os.path.join(self.__directory, 'scratch')
-        for directory_ in [self.__root, self.__scratch]:
+        for directory_ in [self._root, self.__scratch]:
             os.makedirs(directory_, exist_ok=True)
 
     def get_directory(self):
@@ -116,7 +117,7 @@ class Sandbox():
 
         """
         if self.__allow_real_directory:
-            return self.__root
+            return self._root
         else:
             raise BstError("You can't use get_directory")