Allowing users to follow links for various application protocols is
typically done by presenting an interface for external client handlers.
One such interface is `url-irc-function', needed by url.el's `url-irc'
loader. One such handler is `erc-handle-irc-url', tasked by ERC with
teleporting users to the right channel on the right server.

When conditions are ideal, things mostly work as designed. But when the
going gets tough and multiple asynchronous flows are in play, the user
experience suffers. In some cases, things don't work at all (and haven't
since at least Emacs 27). For example, clicking on a link with a channel
only works when an existing connection can be found. Otherwise, a hybrid
server/channel buffer is created, which violates many a foundational
invariant. (That it doesn't blow up right away is troublesome in its own
right and needs addressing stat, IMO.)

The attached changes are mainly meant to shore up the existing
implementation where it's lacking (and otherwise stay out of the way).
Once everything's firing, we should be able to:

  M-x browse-url-at-point RET on any irc:// link anywhere:

    (add-to-list 'browse-url-default-handlers
                 '("\\`ircs?://" . erc--handle-ircs-url))

  Follow irc:// links in eww, gnus, and beyond:

    (setq eww-use-browse-url
          (concat eww-use-browse-url "\\|\\`ircs?:"))

    (push '("\\bircs?://[a-z.@_+0-9%=?&/#-]+"
            0 t erc--handle-ircs-url 0)
          gnus-button-alist)

  Click on org links featuring the nonstandard "/chan/user" syntax:

    (erc--org-init)

Most importantly, we ought to be able to do these things without sharing
anything inadvertently, sensitive or not. A number of minor, supporting
changes that help provide such assurances are also included, such as
ensuring `erc-tls' defaults to a secure port when called from lisp code.

(Note that nothing's yet autoloaded for the snippets above, so you'll
have to require all the players beforehand if trying them out with
a real endpoint, such as "ircs://testnet.ergo.chat/#test".)

In the end, I'm hoping other folks will step forward who may be more
familiar with the libraries mentioned so that nicer renditions can
emerge.

Thanks,
J.P.


In GNU Emacs 29.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.34, cairo version 1.17.6)
 of 2022-07-06 built on localhost
Repository revision: e6504c3eda12c72268d2db6598764f043b74c24d
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12014000
System Description: Fedora Linux 36 (Workstation Edition)

Configured using:
 'configure --enable-check-lisp-object-type --enable-checking=yes,glyphs
 'CFLAGS=-O0 -g3'
 PKG_CONFIG_PATH=:/usr/lib64/pkgconfig:/usr/share/pkgconfig'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
JSON LCMS2 LIBOTF LIBSELINUX LIBSYSTEMD LIBXML2 M17N_FLT MODULES NOTIFY
INOTIFY PDUMPER PNG RSVG SECCOMP SOUND SQLITE3 THREADS TIFF
TOOLKIT_SCROLL_BARS WEBP X11 XDBE XIM XINPUT2 XPM GTK3 ZLIB

Important settings:
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  show-paren-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  line-number-mode: t
  indent-tabs-mode: t
  transient-mark-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message mailcap yank-media puny dired
dired-loaddefs rfc822 mml mml-sec password-cache epa derived epg rfc6068
epg-config gnus-util text-property-search time-date subr-x mm-decode
mm-bodies mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader
cl-loaddefs cl-lib sendmail rfc2047 rfc2045 ietf-drums mm-util
mail-prsvr mail-utils rmc iso-transl tooltip eldoc paren electric
uniquify ediff-hook vc-hooks lisp-float-type elisp-mode mwheel
term/x-win x-win term/common-win x-dnd tool-bar dnd fontset image
regexp-opt fringe tabulated-list replace newcomment text-mode lisp-mode
prog-mode register page tab-bar menu-bar rfn-eshadow isearch easymenu
timer select scroll-bar mouse jit-lock font-lock syntax font-core
term/tty-colors frame minibuffer nadvice seq simple cl-generic
indonesian philippine cham georgian utf-8-lang misc-lang vietnamese
tibetan thai tai-viet lao korean japanese eucjp-ms cp51932 hebrew greek
romanian slovak czech european ethiopic indian cyrillic chinese
composite emoji-zwj charscript charprop case-table epa-hook
jka-cmpr-hook help abbrev obarray oclosure cl-preloaded button loaddefs
faces cus-face macroexp files window text-properties overlay sha1 md5
base64 format env code-pages mule custom widget keymap
hashtable-print-readable backquote threads dbusbind inotify lcms2
dynamic-setting system-font-setting font-render-setting cairo
move-toolbar gtk x-toolkit xinput2 x multi-tty make-network-process
emacs)

Memory information:
((conses 16 35545 5688)
 (symbols 48 5073 0)
 (strings 32 13303 1546)
 (string-bytes 1 429956)
 (vectors 16 9197)
 (vector-slots 8 145428 11407)
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 (buffers 992 10))