bug#45872: 27.1; rcirc nick tracking

[Ken Raeburn <raeburn@redhat.com>]

One of my IRC contacts uses frequent nick changes to indicate away
status, e.g., "johnsmith" when available, "johnsmith|away",
"johnsmith|vacation", whatever. I've noticed that sometimes rcirc will
fail to rename the buffer used for private messages between us, and so
I'll wind up with a buffer "johnsmith|away" showing something along the
lines of:

  ... *** johnsmith NICK johnsmith|away
  ... *** johnsmith|away NICK johnsmith

but the buffer won't have been renamed back to "johnsmith@<server>", and
if I try sending him a message, it'll try sending to johnsmith|away and
will fail. I can use "/msg johnsmith...", or he can send messages to me,
and a new buffer will be created.

If, after this new buffer has been created, he issues a NICK command,
rcirc will attempt a rename, and may error out if it conflicts with the
existing johnsmith|away buffer.

The issue seems to be that rcirc-handler-NICK looks up the chat buffer
using rcirc-get-buffer, which uses rcirc-buffer-alist, but doesn't
update that alist, so a second invocation of rcirc-get-buffer using the
new nick won't find that buffer.

But fixing the assoc list won't address buffer renaming conflicts caused
by nick changes happening while I'm offline; if rcirc renames a buffer
to "johnsmith|away@server" and then I get disconnected, and when I
reconnect I see "johnsmith" active and start exchanging messages (in a
new buffer), and later I receive a NICK message causing another rename
to "johnsmith|away@server", it'll conflict with the existing one. If the
server isn't authenticating, we can't guarantee that the old
"johnsmith|away" and the new "johnsmith" really are the same user, so
I'm not sure if we should blithely try combining them or make them
unique; erroring out during a rename is an ugly failure mode though.




In GNU Emacs 27.1 (build 1, x86_64-pc-linux-gnu, X toolkit, Xaw scroll bars)
 of 2020-11-10 built on crash
Windowing system distributor 'Fedora Project', version 11.0.12006000
System Description: Fedora 31 (Workstation Edition)

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