Nick collision FNC
Jilles Tjoelker <jilles -at- stack.nl>
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Nick collision FNC performs a forced nick change to the user's UID instead
of a kill. The criteria for which user may keep the nick are the same as
before. Server notices will say the clients are being "saved" instead of
"killed". The client will get a 043 numeric, like this:

:<server> 043 <nick> <uid> :Nick collision, forcing nick change to your unique ID

The following conditions must be fulfilled:

- All servers on the network must allow remote nicks starting with a digit.
  This is not checked; if this is not fulfilled, users will be killed right
  after being FNCed.

- All servers on the path between the two clients must support TS6 and
  nick collision FNC (SAVE capab). If this is not fulfilled, the collision is
  resolved with kills as before. (This uses the ENCAP GCAP data for remotes.)

- The general::collision_fnc option must be enabled on the server(s) that
  detect the collision.

Technical details:

The following message is used to propagate the nick change coming from the
server that detected the collision:

:<sid> SAVE <uid> <ts>

The TS is compared to the current nick TS for the user; if it is not equal,
the message is dropped. This prevents nick desyncs if the user changed their
nick after being collided. A SAVE message also generates a server notice to
+k.

The SAVE message is used for propagation to the target's server, and also
in several other cases if the destination supports SAVE. In other cases, a
normal nick change or introduction with the UID as nick is sent.

The TS of the UID nick is 100. This is necessary to ensure nick TS is always
the same on all servers for each nick-user pair, also if a user with a UID
nick changes their nick but is collided again (the server detecting the
collision will not propagate the nick change further).

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