At some point in future, I would like to understand how in the case of clients sharing a single connection, the “agent managing the sharing arranges that no two clients chose the same fid”.
In the Plan 9 kernel, one important such agent is the mount driver itself (devmnt.c), which converts operations derived from system calls into 9P requests.
It relies on the fid in each Chan being unique in the kernel, so when several processes act as 9P clients on a shared connection, the fid space is properly managed
(ie, each client process effectively has its own fid space).