refactor: Refactor BothLibraries logic

This commit introduces a new type of `HoldableObject`: The
`SecondLevelHolder`. The primary purpose of this class is
to handle cases where two (or more) `HoldableObject`s are
stored at the same time (with one default object). The
best (and currently only) example here is the `BothLibraries`
class.

1 files changed, 9 insertions, 0 deletions

diff --git a/mesonbuild/mesonlib/universal.py b/mesonbuild/mesonlib/universal.py
index 4b4eb6e..0ed99a5 100644
--- a/mesonbuild/mesonlib/universal.py
+++ b/mesonbuild/mesonlib/universal.py
@@ -48,6 +48,7 @@ __all__ = [
     'python_command',
     'project_meson_versions',
     'HoldableObject',
+    'SecondLevelHolder',
     'File',
     'FileMode',
     'GitException',
@@ -273,6 +274,14 @@ class HoldableObject(metaclass=abc.ABCMeta):
     ''' Dummy base class for all objects that can be
         held by an interpreter.baseobjects.ObjectHolder '''
 
+class SecondLevelHolder(HoldableObject, metaclass=abc.ABCMeta):
+    ''' A second level object holder. The primary purpose
+        of such objects is to hold multiple objects with one
+        default option. '''
+
+    @abc.abstractmethod
+    def get_default_object(self) -> HoldableObject: ...
+
 class FileMode:
     # The first triad is for owner permissions, the second for group permissions,
     # and the third for others (everyone else).