Sort and dedupe when loading modules in erc-open

* doc/misc/erc.texi: Add new subheading "Module Loading" under the
"Modules" chapter.
* lisp/erc/erc.el (erc--sort-modules): New utility function to sort
and dedupe modules.
(erc-modules): In `custom-set' function, factor out collation into
separate utility `erc--sort-modules'.
(erc-update-modules): Call `erc--update-modules' with an argument, the
current value of `erc-modules'.
(erc--aberrant-modules): New variable, a list of symbols whose modules
ERC suspects of being incorrectly defined.
(erc--warn-about-aberrant-modules): New function to print an error
message and emit a warning prior to connecting when
`erc--aberrant-modules' is non-nil.
(erc--find-mode): Make heuristic more robust by always checking for a
mode activation command rather than just a state variable.  This fixes
a compatibility bug, new in 5.6, affecting third-party modules that
autoload module definitions instead of their corresponding
mode-activation commands.
(erc--update-modules): Add new positional argument `modules'.
(erc--setup-buffer-hook): Add new default member,
`erc--warn-about-aberrant-modules'.
(erc-open): Pass sorted `erc-modules' to `erc--update-modules'.
* test/lisp/erc/erc-tests.el (erc--sort-modules): New test.
(erc-tests--update-modules): New fixture.
(erc--update-modules): Remove and rework as three separate tests
dedicated to specific contexts.  The existing one had poor coverage
and was difficult, if not impossible, to follow.
(erc--update-modules/unknown, erc--update-modules/local,
erc--update-modules/realistic): New tests.  (Bug#57955)

3 files changed, 184 insertions, 67 deletions

diff --git a/doc/misc/erc.texi b/doc/misc/erc.texi
index 3297d8b17f0..3bfa240cacc 100644
--- a/doc/misc/erc.texi
+++ b/doc/misc/erc.texi
@@ -653,6 +653,41 @@ And unlike global toggles, none of these ever mutates
 @code{erc-modules}.
 
 
+@anchor{Module Loading}
+@subheading Module Loading
+@cindex module loading
+
+ERC loads internal modules in alphabetical order and third-party
+modules as they appear in @code{erc-modules}.  When defining your own
+module, take care to ensure ERC can find it.  An easy way to do that
+is by mimicking the example in the doc string for
+@code{define-erc-module}.  For historical reasons, ERC also falls back
+to @code{require}ing features.  For example, if some module
+@code{<mymod>} in @code{erc-modules} lacks a corresponding
+@code{erc-<mymod>-mode} command, ERC will attempt to load the library
+@code{erc-<mymod>} prior to connecting.  If this fails, ERC signals an
+error.  Users wanting to define modules in an init files should
+@code{(provide 'erc-<my-mod>)} somewhere to placate ERC.  Dynamically
+generating modules on the fly is not supported.
+
+Sometimes, packages attempt to autoload a module's definition instead
+of its minor-mode command, which breaks the link between the library
+and the module.  This means that enabling the mode by invoking its
+command toggle isn't enough to load its defining library.  Such
+packages should instead only supply autoload cookies featuring an
+explicit @code{autoload} form for their module's minor-mode command.
+As mentioned above, packages can also usually avoid autoload cookies
+entirely so long as their module's prefixed name matches that of its
+defining library and the latter's provided feature.
+
+Packages have also been seen to specify unnecessary top-level
+@code{eval-after-load} forms, which end up being ineffective in most
+cases.  Another unfortunate practice is mutating @code{erc-modules}
+itself in an autoloaded form.  Doing this tricks Customize into
+displaying the widget for @code{erc-modules} incorrectly, with
+built-in modules moved from the predefined checklist to the
+user-provided free-form area.
+
 @c PRE5_4: Document every option of every module in its own subnode
 
 
diff --git a/lisp/erc/erc.el b/lisp/erc/erc.el
index 16651b41eef..87abe2a133b 100644
--- a/lisp/erc/erc.el
+++ b/lisp/erc/erc.el
@@ -2004,6 +2004,14 @@ buffer rather than a server buffer.")
   ;; each item is in the format '(old . new)
   (delete-dups (mapcar #'erc--normalize-module-symbol mods)))
 
+(defun erc--sort-modules (modules)
+  "Return a copy of MODULES, deduped and led by sorted built-ins."
+  (let (built-in third-party)
+    (dolist (mod modules)
+      (setq mod (erc--normalize-module-symbol mod))
+      (cl-pushnew mod (if (get mod 'erc--module) built-in third-party)))
+    `(,@(sort built-in #'string-lessp) ,@(nreverse third-party))))
+
 (defcustom erc-modules '( autojoin button completion fill imenu irccontrols
                           list match menu move-to-prompt netsplit
                           networks noncommands readonly ring stamp track)
@@ -2039,16 +2047,10 @@ removed from the list will be disabled."
                                           (when (symbol-value f)
                                             (funcall f 0))
                                           (kill-local-variable f)))))))))
-         (let (built-in third-party)
-           (dolist (v val)
-             (setq v (erc--normalize-module-symbol v))
-             (if (get v 'erc--module)
-                 (push v built-in)
-               (push v third-party)))
-           ;; Calling `set-default-toplevel-value' complicates testing
-           (set sym (append (sort built-in #'string-lessp)
-                            (nreverse third-party))))
+         ;; Calling `set-default-toplevel-value' complicates testing.
+         (set sym (erc--sort-modules val))
          ;; this test is for the case where erc hasn't been loaded yet
+         ;; FIXME explain how this ^ can occur or remove comment.
          (when (fboundp 'erc-update-modules)
            (unless erc--inside-mode-toggle-p
              (erc-update-modules))))
@@ -2112,15 +2114,29 @@ removed from the list will be disabled."
 (defun erc-update-modules ()
   "Enable minor mode for every module in `erc-modules'.
 Except ignore all local modules, which were introduced in ERC 5.5."
-  (erc--update-modules)
+  (erc--update-modules erc-modules)
   nil)
 
+(defvar erc--aberrant-modules nil
+  "Modules suspected of being improperly loaded.")
+
+(defun erc--warn-about-aberrant-modules ()
+  (when (and erc--aberrant-modules (not erc--target))
+    (erc-button--display-error-notice-with-keys-and-warn
+     "The following modules exhibited strange loading behavior: "
+     (mapconcat (lambda (s) (format "`%s'" s)) erc--aberrant-modules ", ")
+     ". Please contact ERC with \\[erc-bug] if you believe this to be untrue."
+     " See Info:\"(erc) Module Loading\" for more.")
+    (setq erc--aberrant-modules nil)))
+
 (defun erc--find-mode (sym)
   (setq sym (erc--normalize-module-symbol sym))
-  (if-let* ((mode (intern-soft (concat "erc-" (symbol-name sym) "-mode")))
-            ((or (boundp mode)
-                 (and (fboundp mode)
-                      (autoload-do-load (symbol-function mode) mode)))))
+  (if-let ((mode (intern-soft (concat "erc-" (symbol-name sym) "-mode")))
+           ((and (fboundp mode)
+                 (autoload-do-load (symbol-function mode) mode)))
+           ((or (get sym 'erc--module)
+                (symbol-file mode)
+                (ignore (cl-pushnew sym erc--aberrant-modules)))))
       mode
     (and (require (or (get sym 'erc--feature)
                       (intern (concat "erc-" (symbol-name sym))))
@@ -2129,9 +2145,9 @@ Except ignore all local modules, which were introduced in ERC 5.5."
          (fboundp mode)
          mode)))
 
-(defun erc--update-modules ()
+(defun erc--update-modules (modules)
   (let (local-modes)
-    (dolist (module erc-modules local-modes)
+    (dolist (module modules local-modes)
       (if-let ((mode (erc--find-mode module)))
           (if (custom-variable-p mode)
               (funcall mode 1)
@@ -2158,7 +2174,7 @@ realizes it's missing some required module \"foo\", it can
 confidently call (erc-foo-mode 1) without having to learn
 anything about the dependency's implementation.")
 
-(defvar erc--setup-buffer-hook nil
+(defvar erc--setup-buffer-hook '(erc--warn-about-aberrant-modules)
   "Internal hook for module setup involving windows and frames.")
 
 (defvar erc--display-context nil
@@ -2315,7 +2331,8 @@ Returns the buffer for the given server or channel."
     (setq old-point (point))
     (setq delayed-modules
           (erc--merge-local-modes (let ((erc--updating-modules-p t))
-                                    (erc--update-modules))
+                                    (erc--update-modules
+                                     (erc--sort-modules erc-modules)))
                                   (or erc--server-reconnecting
                                       erc--target-priors)))
 
diff --git a/test/lisp/erc/erc-tests.el b/test/lisp/erc/erc-tests.el
index 64b503832f3..0b88ad9cfa9 100644
--- a/test/lisp/erc/erc-tests.el
+++ b/test/lisp/erc/erc-tests.el
@@ -2293,65 +2293,130 @@
   (should (eq (erc--find-group 'smiley nil) 'erc))
   (should (eq (erc--find-group 'unmorse nil) 'erc)))
 
-(ert-deftest erc--update-modules ()
-  (let (calls
-        erc-modules
-        erc-kill-channel-hook erc-kill-server-hook erc-kill-buffer-hook)
+(ert-deftest erc--sort-modules ()
+  (should (equal (erc--sort-modules '(networks foo fill bar fill stamp bar))
+                 ;; Third-party mods appear in original order.
+                 '(fill networks stamp foo bar))))
+
+(defun erc-tests--update-modules (fn)
+  (let* ((calls nil)
+         (custom-modes nil)
+         (on-load nil)
+
+         (get-calls (lambda () (prog1 (nreverse calls) (setq calls nil))))
+
+         (add-onload (lambda (m k v)
+                       (put (intern m) 'erc--feature k)
+                       (push (cons k (lambda () (funcall v m))) on-load)))
 
-    ;; This `lbaz' module is unknown, so ERC looks for it via the
-    ;; symbol proerty `erc--feature' and, failing that, by
-    ;; `require'ing its "erc-" prefixed symbol.
-    (should-not (intern-soft "erc-lbaz-mode"))
+         (mk-cmd (lambda (module)
+                   (let ((mode (intern (format "erc-%s-mode" module))))
+                     (fset mode (lambda (n) (push (cons mode n) calls))))))
+
+         (mk-builtin (lambda (module-string)
+                       (let ((s (intern module-string)))
+                         (put s 'erc--module s))))
+
+         (mk-global (lambda (module)
+                      (push (intern (format "erc-%s-mode" module))
+                            custom-modes))))
 
     (cl-letf (((symbol-function 'require)
                (lambda (s &rest _)
-                 (when (eq s 'erc--lbaz-feature)
-                   (fset (intern "erc-lbaz-mode") ; local module
-                         (lambda (n) (push (cons 'lbaz n) calls))))
-                 (push s calls)))
-
-              ;; Local modules
-              ((symbol-function 'erc-lbar-mode)
-               (lambda (n) (push (cons 'lbar n) calls)))
-              ((get 'lbaz 'erc--feature) 'erc--lbaz-feature)
-
-              ;; Global modules
-              ((symbol-function 'erc-gfoo-mode)
-               (lambda (n) (push (cons 'gfoo n) calls)))
-              ((get 'erc-gfoo-mode 'standard-value) 'ignore)
+                 ;; Simulate library being loaded, things defined.
+                 (when-let ((h (alist-get s on-load))) (funcall h))
+                 (push (cons 'req s) calls)))
+
+              ;; Spoof global module detection.
+              ((symbol-function 'custom-variable-p)
+               (lambda (v) (memq v custom-modes))))
+
+      (funcall fn get-calls add-onload mk-cmd mk-builtin mk-global))
+    (should-not erc--aberrant-modules)))
+
+(ert-deftest erc--update-modules/unknown ()
+  (erc-tests--update-modules
+
+   (lambda (get-calls _ mk-cmd _ mk-global)
+
+     (ert-info ("Baseline")
+       (let* ((erc-modules '(foo))
+              (obarray (obarray-make))
+              (err (should-error (erc--update-modules erc-modules))))
+         (should (equal (cadr err) "`foo' is not a known ERC module"))
+         (should (equal (funcall get-calls)
+                        `((req . ,(intern-soft "erc-foo")))))))
+
+     ;; Module's mode command exists but lacks an associated file.
+     (ert-info ("Bad autoload flagged as suspect")
+       (should-not erc--aberrant-modules)
+       (let* ((erc--aberrant-modules nil)
+              (obarray (obarray-make))
+              (erc-modules (list (intern "foo"))))
+
+         ;; Create a mode activation command.
+         (funcall mk-cmd "foo")
+
+         ;; Make the mode var global.
+         (funcall mk-global "foo")
+
+         ;; No local modules to return.
+         (should-not (erc--update-modules erc-modules))
+         (should (equal (mapcar #'prin1-to-string erc--aberrant-modules)
+                        '("foo")))
+         ;; ERC requires the library via prefixed module name.
+         (should (equal (mapcar #'prin1-to-string (funcall get-calls))
+                        `("(req . erc-foo)" "(erc-foo-mode . 1)"))))))))
+
+;; A local module (here, `lo2') lacks a mode toggle, so ERC tries to
+;; load its defining library, first via the symbol property
+;; `erc--feature', and then via an "erc-" prefixed symbol.
+(ert-deftest erc--update-modules/local ()
+  (erc-tests--update-modules
+
+   (lambda (get-calls add-onload mk-cmd mk-builtin mk-global)
+
+     (let* ((obarray (obarray-make 20))
+            (erc-modules (mapcar #'intern '("glo" "lo1" "lo2"))))
+
+       ;; Create a global and a local module.
+       (mapc mk-cmd '("glo" "lo1"))
+       (mapc mk-builtin '("glo" "lo1"))
+       (funcall mk-global "glo")
+       (funcall add-onload "lo2" 'explicit-feature-lib mk-cmd)
+
+       ;; Returns local modules.
+       (should (equal (mapcar #'symbol-name (erc--update-modules erc-modules))
+                      '("erc-lo2-mode" "erc-lo1-mode")))
+
+       ;; Requiring `erc-lo2' defines `erc-lo2-mode'.
+       (should (equal (mapcar #'prin1-to-string (funcall get-calls))
+                      `("(erc-glo-mode . 1)"
+                        "(req . explicit-feature-lib)")))))))
+
+(ert-deftest erc--update-modules/realistic ()
+  (let ((calls nil)
+        ;; Module `pcomplete' "resolves" to `completion'.
+        (erc-modules '(pcomplete autojoin networks)))
+    (cl-letf (((symbol-function 'require)
+               (lambda (s &rest _) (push (cons 'req s) calls)))
+
+              ;; Spoof global module detection.
+              ((symbol-function 'custom-variable-p)
+               (lambda (v)
+                 (memq v '(erc-autojoin-mode erc-networks-mode
+                                             erc-completion-mode))))
+              ;; Mock and spy real builtins.
               ((symbol-function 'erc-autojoin-mode)
                (lambda (n) (push (cons 'autojoin n) calls)))
-              ((get 'erc-autojoin-mode 'standard-value) 'ignore)
               ((symbol-function 'erc-networks-mode)
                (lambda (n) (push (cons 'networks n) calls)))
-              ((get 'erc-networks-mode 'standard-value) 'ignore)
               ((symbol-function 'erc-completion-mode)
-               (lambda (n) (push (cons 'completion n) calls)))
-              ((get 'erc-completion-mode 'standard-value) 'ignore))
-
-      (ert-info ("Unknown module")
-        (setq erc-modules '(lfoo))
-        (should-error (erc--update-modules))
-        (should (equal (pop calls) 'erc-lfoo))
-        (should-not calls))
+               (lambda (n) (push (cons 'completion n) calls))))
 
-      (ert-info ("Local modules")
-        (setq erc-modules '(gfoo lbar lbaz))
-        ;; Don't expose the mode here
-        (should (equal (mapcar #'symbol-name (erc--update-modules))
-                       '("erc-lbaz-mode" "erc-lbar-mode")))
-        ;; Lbaz required because unknown.
-        (should (equal (nreverse calls) '((gfoo . 1) erc--lbaz-feature)))
-        (fmakunbound (intern "erc-lbaz-mode"))
-        (unintern (intern "erc-lbaz-mode") obarray)
-        (setq calls nil))
-
-      (ert-info ("Global modules") ; `pcomplete' resolved to `completion'
-        (setq erc-modules '(pcomplete autojoin networks))
-        (should-not (erc--update-modules)) ; no locals
-        (should (equal (nreverse calls)
-                       '((completion . 1) (autojoin . 1) (networks . 1))))
-        (setq calls nil)))))
+      (should-not (erc--update-modules erc-modules)) ; no locals
+      (should (equal (nreverse calls)
+                     '((completion . 1) (autojoin . 1) (networks . 1)))))))
 
 (ert-deftest erc--merge-local-modes ()
   (cl-letf (((get 'erc-b-mode 'erc-module) 'b)