objectstore: add HostTree class to access host fs

Simple new object that should expose the root file system with the
same API as `objectstore.Object` but as read-only. This means that
the `read` call works exactly as for `Object` but `write` raises
an exception.
Add tests to specifically check the read-only properties.

osbuild/objectstore.py

@@ -203,6 +203,33 @@ class Object:
         return exc_type is None
 
 
+class HostTree:
+    """Read-only access to the host file system
+
+    An object that provides the same interface as
+    `objectstore.Object` that can be used to read
+    the host file-system.
+    """
+    def __init__(self, store):
+        self.store = store
+
+    @staticmethod
+    def write():
+        raise ValueError("Cannot write to host")
+
+    @contextlib.contextmanager
+    def read(self):
+        with self.store.tempdir() as tmp:
+            mount("/", tmp)
+            try:
+                yield tmp
+            finally:
+                umount(tmp)
+
+    def cleanup(self):
+        pass  # noop for the host
+
+
 class ObjectStore:
     def __init__(self, store):
         self.store = store

test/test_objectstore.py

@@ -263,6 +263,21 @@ class TestObjectStore(unittest.TestCase):
         assert os.path.exists(f"{object_store.refs}/b/A")
         assert os.path.exists(f"{object_store.refs}/b/B")
 
+    def test_host_tree(self):
+        object_store = objectstore.ObjectStore(self.store)
+        host = objectstore.HostTree(object_store)
+
+        # check we cannot call `write`
+        with self.assertRaises(ValueError):
+            with host.write() as _:
+                pass
+
+        # check we actually cannot write to the path
+        with host.read() as path:
+            p = Path(f"{path}/osbuild-test-file")
+            with self.assertRaises(OSError):
+                p.touch()
+
 
 if __name__ == "__main__":
     unittest.main()