doc(services): explain the Owned Path reason d'etre after trying to remove it

selfprivacy_api/services/owned_path.py

@@ -14,8 +14,21 @@ class BindError(Exception):
     pass
 
 
-# May be deprecated because of Binds
 class OwnedPath(BaseModel):
+    """
+    A convenient interface for explicitly defining ownership of service folders.
+    One overrides Service.get_owned_paths() for this.
+
+    Why this exists?:
+    One could use Bind to define ownership but then one would need to handle drive which
+    is unnecessary and produces code duplication.
+
+    It is also somewhat semantically wrong to include Owned Path into Bind
+    instead of user and group. Because owner and group in Bind are applied to
+    the original folder on the drive, not to the binding path. But maybe it is
+    ok since they are technically both owned. Idk yet.
+    """
+
     path: str
     owner: str
     group: str

selfprivacy_api/services/service.py

@@ -411,11 +411,27 @@ class Service(ABC):
 
     @classmethod
     def owned_path(cls, path: str):
-        """A default guess on folder ownership"""
+        """Default folder ownership"""
+        service_name = cls.get_display_name()
+
+        try:
+            owner = cls.get_user()
+            if owner is None:
+                # TODO: assume root?
+                # (if we do not want to do assumptions, maybe not declare user optional?)
+                raise LookupError(f"no user for service: {service_name}")
+            group = cls.get_group()
+            if group is None:
+                raise LookupError(f"no group for service: {service_name}")
+        except Exception as error:
+            raise LookupError(
+                f"when deciding a bind for folder {path} of service {service_name}, error: {str(error)}"
+            )
+
         return OwnedPath(
             path=path,
-            owner=cls.get_user(),
+            owner=owner,
-            group=cls.get_group(),
+            group=group,
         )
 
     def pre_backup(self):