objectstore: make objects identifyable

This adds a new `id` property to the ObjectStore.Object, that is
meant to reflect the identifer of the Stage to build the contents
of it. This will help to transparently access objects that have
been built but not committed to the store.
Setting the `base_id` of an object will also set its `id`. When
the object is then modified via write() the `id` will be set to
None, since no the content and the id are out of sync. In the
same way, restting an object will reset its `id` to None.

osbuild/objectstore.py

@@ -49,6 +49,7 @@ class Object:
         self._base = None
         self._workdir = None
         self._tree = None
+        self.id = None
         self.store = store
         self.reset()
 
@@ -72,6 +73,7 @@ class Object:
     def base(self, base_id: Optional[str]):
         self._init = not base_id
         self._base = base_id
+        self.id = base_id
 
     @property
     def treesum(self) -> str:
@@ -97,6 +99,7 @@ class Object:
         self._check_readers()
         self._check_writer()
         self.init()
+        self.id = None
         with self.tempdir("writer") as target:
             mount(self._path, target, ro=False)
             try:
@@ -153,6 +156,7 @@ class Object:
         if self._workdir:
             self._workdir.cleanup()
             self._workdir = None
+        self.id = None
 
     def _check_readers(self):
         """Internal: Raise a ValueError if there are readers"""