@mcepl the least complicated part of 9p synthetic filesystems is the locking. Fids can be used for ensuring a pseudo client connection to the file where data is multiplexed using a mapping system.
The most complicated part of 9p synthetic filesystems is on demand filesystem generation. Clients must not assume they can cache the contents of a filesystem as this may cause major issues. A notable issue being the client thinks a file still exists when it does not. There is also the unlikely issue of running out of Qids on the server side. The 9p specification requires that servers never reuse Qids as clients may keep track of them for whatever reason they desire. In order to adhere to the specification servers must generate a new Qid for each file they generate on demand, though this is a potential issue when taking a functional programming approach. For imperative or object oriented the problem becomes more complicated.