objectstore: Object stores it base id not path

Instead of storing the base path, store the object id of its base
and resolve it to the path via the ObjectStore whenever needed.

osbuild/objectstore.py

@@ -30,7 +30,7 @@ def suppress_oserror(*errnos):
 class Object:
     def __init__(self, store: "ObjectStore"):
         self._init = True
-        self._base_path = None
+        self._base = None
         self._workdir = None
         self._tree = None
         self.store = store
@@ -41,20 +41,20 @@ class Object:
         if self._init:
             return
 
-        source = self._base_path
+        source = self.store.resolve_ref(self._base)
         subprocess.run(["cp", "--reflink=auto", "-a",
                         f"{source}/.", self._tree],
                        check=True)
         self._init = True
 
     @property
-    def base_path(self) -> Optional[str]:
-        return self._base_path
+    def base(self) -> Optional[str]:
+        return self._base
 
-    @base_path.setter
-    def base_path(self, path: Optional[str]):
-        self._init = not path
-        self._base_path = path
+    @base.setter
+    def base(self, base_id: Optional[str]):
+        self._init = not base_id
+        self._base = base_id
 
     @property
     def path(self) -> str:
@@ -73,8 +73,8 @@ class Object:
     @contextlib.contextmanager
     def open(self):
         """Open the directory and return the file descriptor"""
-        if self._base_path and not self._init:
-            path = self._base_path
+        if self._base and not self._init:
+            path = self.store.resolve_ref(self._base)
         else:
             path = self._tree
 
@@ -105,7 +105,7 @@ class Object:
         self._workdir = self.store.tempdir(suffix="object")
         self._tree = os.path.join(self._workdir.name, "tree")
         os.makedirs(self._tree, mode=0o755, exist_ok=True)
-        self._init = not self._base_path
+        self._init = not self._base
 
     def cleanup(self):
         if self._workdir:
@@ -175,10 +175,10 @@ class ObjectStore:
             # on success as object_id
 
             if base_id:
-                # if we were given a base id, resolve its path and set it
-                # as the base_path of the object
-                # NB: `obj` does not get initialized explicitly here
-                obj.base_path = self.resolve_ref(base_id)
+                # if we were given a base id then this is the base for the
+                # new object
+                # NB: its initialization is deferred to the first write
+                obj.base = base_id
 
             yield obj
 
@@ -219,11 +219,11 @@ class ObjectStore:
 
         # the reference that is pointing to `treesum_hash` is now the base
         # of `obj`. It is not actively initialized but any subsequent calls
-        # to `obj.write()` or `obj.path`will initialize it again
+        # to `obj.write()` or `obj.path` will initialize it again
         # NB: in the case that an object with the same treesum as `obj`
         # already existed in the store obj.store_tree() will not actually
         # have written anything to the store. In this case `obj` will then
         # be initialized with the content of the already existing object.
-        obj.base_path = self.resolve_ref(object_id)
+        obj.base = object_id
 
         return treesum_hash