Issue Description

Type: bug report

Describe what happened

java.lang.IllegalStateException: failed to req API:/nacos/v1/ns/instance/beat after all servers([minion-register:8848]) tried: failed to req API:minion-register:8848/nacos/v1/ns/instance/beat. code:400 msg: java.lang.IllegalArgumentException: dom name can only have these characters: 0-9a-zA-Z-._:, current: DEFAULT_GROU

Description
I get the following warnings once a minute:

2020-06-23 23:11:13,900 [salt.loaded.int.fileserver.roots:180 ][WARNING ][24993] Skipped invalid cache mtime entry in /var/cache/salt/master/roots/mtime_map: /srv/salt/a/b/c/ifcfg-eth0:0:1533517049.0

2020-06-23 23:11:13,901 [salt.loaded.int.fileserver.roots:180 ][WARNING ][24993] Skipped invalid cache mtime entry in /var/cache/salt

Basically we're in the DAG, so make sure we've let all the other resources (like files) get built (that might be used for the file server) before the server sends anything out. We could either block or just not return anything yet.

As an aside, does the rf.ReadFrom bit not get cancelled properly on close??

Hi,
thank you for the library. It really improved the way I approach configuration files.
Finally I have types and proper sections.
What I am missing is schema: I would love to be able to enforce that some parameters are obligatory, their type and provide a description, also raise an exception if there is some unknown parameter is configured.
I think I could use something like marshmallow

Similar to the Email validator.