objectstore: integrate metadata object

Integrate the new `Metadata` object as `meta` property on `Object`.
Use it to actually store metadata after a successful stage run.
A new class `PathAdapter` is introduce which is in turned used to
expose the base path of `Object` as `os.PathLike` so it can be
passed as path to `Metadata`. The advantage is that any changes
to the base path in `Object` will automatically be picked up by
`Metadata`; the prominent, and currently only, case where this is
happening in `Object` is `store_tree`.

osbuild/objectstore.py

@@ -5,7 +5,7 @@ import os
 import subprocess
 import tempfile
 import uuid
-from typing import Optional, Set
+from typing import Any, Optional, Set
 
 from osbuild.util import jsoncomm, rmrf
 from osbuild.util.mnt import mount, umount
@@ -18,6 +18,17 @@ __all__ = [
 ]
 
 
+class PathAdapter:
+    """Expose an object attribute as `os.PathLike`"""
+
+    def __init__(self, obj: Any, attr: str) -> None:
+        self.obj = obj
+        self.attr = attr
+
+    def __fspath__(self):
+        return getattr(self.obj, self.attr)
+
+
 class Object:
     class Mode(enum.Enum):
         READ = 0
@@ -111,6 +122,12 @@ class Object:
             tree = os.path.join(self._path, "tree")
             os.makedirs(tree)
 
+        # Expose our base path as `os.PathLike` via `PathAdater`
+        # so any changes to it, e.g. via `store_tree`, will be
+        # automatically picked up by `Metadata`.
+        wrapped = PathAdapter(self, "_path")
+        self._meta = self.Metadata(wrapped, folder="meta")
+
     @property
     def id(self) -> Optional[str]:
         return self._id
@@ -128,6 +145,10 @@ class Object:
     def tree(self) -> str:
         return os.path.join(self._path, "tree")
 
+    @property
+    def meta(self) -> Metadata:
+        return self._meta
+
     def store_tree(self):
         """Store the tree with a fresh name and close it