Warn of absent networks module in ERC

* doc/misc/erc.texi: Add linkable note in Modules chapter about some
modules being required.  Also tweak markup in auth-source section.
* etc/ERC-NEWS: Mention the special role of `networks'.
* lisp/erc/erc-backend.el (erc--server-post-connect-hook): Add
internal hook for core modules to perform post-network-process,
pre-protocol config validation even when they haven't been loaded.
(erc--register-connection): Run `erc--server-post-connect-hook'.
* lisp/erc/erc-networks.el (erc-networks--bouncer-targets,
erc-networks-on-MOTD-end): Fix comments and doc strings.  Also change
former from constant to internal variable in case adjustment needed
between releases.
(erc-networks--warn-on-connect): New function to warn about the
`networks' module being absent from `erc-modules'.  This could
probably run at any time up to and including when the logical IRC
connection is established, but doing so at the process/protocol
boundary seems ideal.
* lisp/erc/erc-sasl.el (erc--register-connection): Defer to base
method instead of calling `erc-login' explicitly.
* lisp/erc/erc.el (erc-generate-new-buffer-name): Don't reconcile
buffer names when networks module not in play.
(erc-format-target-and/or-network): Don't assume networks module
loaded.
* test/lisp/erc/erc-scenarios-base-unstable.el:
(erc-scenarios-networks-no-module): New test.
* test/lisp/erc/resources/networks/no-module/basic.eld: New test data
file.  (Bug#60331.)

2 files changed, 97 insertions, 1 deletions

diff --git a/test/lisp/erc/erc-scenarios-base-unstable.el b/test/lisp/erc/erc-scenarios-base-unstable.el
index f5b8df6f4a1..e6db40c5efb 100644
--- a/test/lisp/erc/erc-scenarios-base-unstable.el
+++ b/test/lisp/erc/erc-scenarios-base-unstable.el
@@ -24,7 +24,7 @@
   (let ((load-path (cons (ert-resource-directory) load-path)))
     (require 'erc-scenarios-common)))
 
-(eval-when-compile (require 'erc-join))
+(eval-when-compile (require 'erc-join) (require 'warnings))
 
 ;; Not unstable, but stashed here for now
 
@@ -132,4 +132,56 @@
               (not (setq failed (zerop (cl-decf tries)))))))
     (should-not failed)))
 
+;; The `erc-networks' library has slowly become a hard dependency of
+;; the interactive client since its incorporation in 2006.  But its
+;; module, which was added in ERC 5.3 (2008) and thereafter loaded by
+;; default, only became quasi-required in ERC 5.5 (2022).  Despite
+;; this, a basic connection should still always succeed, at least long
+;; enough to warn users that their setup is abnormal.  Of course,
+;; third-party code intentionally omitting the module will have to
+;; override various erc-server-*-functions to avoid operating in a
+;; degraded state, which has likely been the case for a while.
+
+(ert-deftest erc-scenarios-networks-no-module ()
+  :tags '(:expensive-test)
+  (erc-scenarios-common-with-cleanup
+      ((erc-scenarios-common-dialog "networks/no-module")
+       (erc-server-flood-penalty 0.1)
+       (erc-networks-mode-orig erc-networks-mode)
+       (dumb-server (erc-d-run "localhost" t 'basic))
+       (port (process-contact dumb-server :service))
+       (erc-modules (remq 'networks erc-modules))
+       (warning-suppress-log-types '((erc)))
+       (expect (erc-d-t-make-expecter)))
+
+    (erc-networks-mode -1)
+    (ert-info ("Connect and retain dialed name")
+      (with-current-buffer (erc :server "127.0.0.1"
+                                :port port
+                                :nick "tester"
+                                :user "tester"
+                                :full-name "tester")
+        (funcall expect 10 "Required module `networks' not loaded")
+        (funcall expect 10 "This server is in debug mode")
+        ;; Buffer not named after network
+        (should (string= (buffer-name) (format "127.0.0.1:%d" port)))
+        (erc-cmd-JOIN "#chan")))
+
+    (ert-info ("Join #chan, change nick, query op")
+      (with-current-buffer (erc-d-t-wait-for 10 (get-buffer "#chan"))
+        (funcall expect 20 "Even at thy teat thou")
+        (erc-cmd-NICK "dummy")
+        (funcall expect 10 "Your new nickname is dummy")
+        (erc-scenarios-common-say "/msg alice hi")))
+
+    (ert-info ("Switch to query and quit")
+      (with-current-buffer (erc-d-t-wait-for 10 (get-buffer "alice"))
+        (funcall expect 20 "bye"))
+
+      (with-current-buffer (format "127.0.0.1:%d" port)
+        (erc-cmd-QUIT "")
+        (funcall expect 10 "finished")))
+    (when erc-networks-mode-orig
+      (erc-networks-mode +1))))
+
 ;;; erc-scenarios-base-unstable.el ends here
diff --git a/test/lisp/erc/resources/networks/no-module/basic.eld b/test/lisp/erc/resources/networks/no-module/basic.eld
new file mode 100644
index 00000000000..f1bdbd1219f
--- /dev/null
+++ b/test/lisp/erc/resources/networks/no-module/basic.eld
@@ -0,0 +1,44 @@
+;; -*- mode: lisp-data; -*-
+((nick 10 "NICK tester"))
+((user 1 "USER tester 0 * :tester")
+ (0.00 ":irc.foonet.org 001 tester :Welcome to the foonet IRC Network tester")
+ (0.00 ":irc.foonet.org 002 tester :Your host is irc.foonet.org, running version ergo-v2.8.0")
+ (0.00 ":irc.foonet.org 003 tester :This server was created Mon, 12 Dec 2022 01:25:38 UTC")
+ (0.00 ":irc.foonet.org 004 tester irc.foonet.org ergo-v2.8.0 BERTZios CEIMRUabefhiklmnoqstuv Iabefhkloqv")
+ (0.00 ":irc.foonet.org 005 tester AWAYLEN=390 BOT=B CASEMAPPING=ascii CHANLIMIT=#:100 CHANMODES=Ibe,k,fl,CEMRUimnstu CHANNELLEN=64 CHANTYPES=# ELIST=U EXCEPTS EXTBAN=,m FORWARD=f INVEX KICKLEN=390 :are supported by this server")
+ (0.00 ":irc.foonet.org 005 tester MAXLIST=beI:60 MAXTARGETS=4 MODES MONITOR=100 NETWORK=foonet NICKLEN=32 PREFIX=(qaohv)~&@%+ STATUSMSG=~&@%+ TARGMAX=NAMES:1,LIST:1,KICK:,WHOIS:1,USERHOST:10,PRIVMSG:4,TAGMSG:4,NOTICE:4,MONITOR:100 TOPICLEN=390 UTF8MAPPING=rfc8265 UTF8ONLY WHOX :are supported by this server")
+ (0.01 ":irc.foonet.org 005 tester draft/CHATHISTORY=100 :are supported by this server")
+ (0.00 ":irc.foonet.org 251 tester :There are 0 users and 4 invisible on 1 server(s)")
+ (0.00 ":irc.foonet.org 252 tester 0 :IRC Operators online")
+ (0.00 ":irc.foonet.org 253 tester 0 :unregistered connections")
+ (0.00 ":irc.foonet.org 254 tester 1 :channels formed")
+ (0.00 ":irc.foonet.org 255 tester :I have 4 clients and 0 servers")
+ (0.00 ":irc.foonet.org 265 tester 4 4 :Current local users 4, max 4")
+ (0.01 ":irc.foonet.org 266 tester 4 4 :Current global users 4, max 4")
+ (0.00 ":irc.foonet.org 422 tester :MOTD File is missing"))
+
+((mode 10 "MODE tester +i")
+ (0.00 ":irc.foonet.org 221 tester +i")
+ (0.00 ":irc.foonet.org NOTICE tester :This server is in debug mode and is logging all user I/O. If you do not wish for everything you send to be readable by the server owner(s), please disconnect."))
+
+((join 10 "JOIN #chan")
+ (0.03 ":tester!~u@z5d6jyn8pwxge.irc JOIN #chan"))
+
+((~nick 10 "NICK dummy")
+ (0.01 ":tester!~u@z5d6jyn8pwxge.irc NICK dummy"))
+
+((mode-1 10 "MODE #chan")
+ (0.01 ":irc.foonet.org 353 tester = #chan :@alice bob foonet tester")
+ (0.00 ":irc.foonet.org 366 tester #chan :End of NAMES list")
+ (0.03 ":irc.foonet.org 324 tester #chan +nt")
+ (0.00 ":irc.foonet.org 329 tester #chan 1670808354")
+ (0.00 ":bob!~u@d6ftaiqzk8x2k.irc PRIVMSG #chan :tester, welcome!")
+ (0.00 ":alice!~u@d6ftaiqzk8x2k.irc PRIVMSG #chan :tester, welcome!")
+ (0.03 ":bob!~u@d6ftaiqzk8x2k.irc PRIVMSG #chan :alice: Forbear it therefore; give your cause to heaven.")
+ (0.01 ":alice!~u@d6ftaiqzk8x2k.irc PRIVMSG #chan :bob: Even at thy teat thou hadst thy tyranny."))
+
+((privmsg 10 "PRIVMSG alice :hi")
+ (0.00 ":alice!~u@d6ftaiqzk8x2k.irc PRIVMSG dummy :bye"))
+
+((quit 10 "QUIT :\2ERC\2")
+ (0.03 ":dummy!~u@z5d6jyn8pwxge.irc QUIT :Quit: \2ERC\2"))