objectstore: Object knows its base

When a new Object is created it can have a `base`, i.e. another
object that is already committed to the store, which is then used
to initialize the tree of the new object. That is, the contents
of the new Object will be based on the contents of the existing.
The initialization of an Object with its base (if any) was done
by the ObjectStore. Move all of that logic inside `Object`:
The Object will store its base, which `Object.init` will use to
initialize itself. Additionally, if `Object.path` is accessed
`init` is being called as well to make sure it is properly
initialized, i.e. the tree initialized with the base content.

osbuild/objectstore.py

@@ -29,19 +29,36 @@ def suppress_oserror(*errnos):
 
 class Object:
     def __init__(self, store: "ObjectStore"):
+        self._init = True
+        self._base_path = None
         self._workdir = None
         self._tree = None
         self.store = store
         self.reset()
 
-    def init(self, source: str) -> None:
-        """Initialize the object with source content"""
+    def init(self) -> None:
+        """Initialize the object with content of its base"""
+        if self._init:
+            return
+
+        source = self._base_path
         subprocess.run(["cp", "--reflink=auto", "-a",
-                        f"{source}/.", self.path],
+                        f"{source}/.", self._tree],
                        check=True)
+        self._init = True
+
+    @property
+    def base_path(self) -> Optional[str]:
+        return self._base_path
+
+    @base_path.setter
+    def base_path(self, path: Optional[str]):
+        self._init = not path
+        self._base_path = path
 
     @property
     def path(self) -> str:
+        self.init()
         return self._tree
 
     @property
@@ -78,6 +95,7 @@ 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
 
     def cleanup(self):
         if self._workdir:
@@ -147,10 +165,10 @@ class ObjectStore:
             # on success as object_id
 
             if base_id:
-                # the base, the working tree and the output dir are all
-                # on the same fs, so attempt a lightweight copy if the
-                # fs supports it
-                obj.init(self.resolve_ref(base_id))
+                # if we were given a base id, resolve its path and set it
+                # as the base_path of the object
+                obj.base_path = self.resolve_ref(base_id)
+                obj.init()
 
             yield obj
 
@@ -174,7 +192,8 @@ class ObjectStore:
         # the latter with the contents of `obj.path` and commit
         # it to the store
         with Object(self) as tmp:
-            tmp.init(obj.path)
+            tmp.base_path = obj.path
+            tmp.init()
             return self.commit(tmp, object_id)
 
     def commit(self, obj: Object, object_id: str) -> str: