Livelock if attaching a session in a circular way

[whisperity]

1. Create and attach to session A
2. From within A, create and attach to session B
3. From within B, attach to session A

Both clients' and the server's CPU use will spike once it starts communicating data between the sessions recursively, resulting in a livelock of the affected sessions. On powerful enough systems, it seems that scheduling and queueing allow for other sessions to survive this ordeal.

Note: It is enough if session A recursively attaches to session A.

It is not trivial to detect and keep track of what session is attached to what other session, and while it could technically be possible, it is not worth the effort. A blanket denial of recursive Monomux attaches should be implemented instead. (This is how tmux does it, by the way: "[...] sessions should be nested with care, unset TMUX and try again [...]".)