Respect user markers in erc-insert-timestamp-left

* lisp/erc/erc-stamp.el (erc-insert-timestamp-left): Convert to normal
function, a mere wrapper that defers to existing generic variants, in
order to dissuade users from adding their own methods, which could
complicate troubleshooting, etc.
(erc--insert-timestamp-left): Rename both methods using internal
double-hyphen convention.  In `erc-stamp--display-margin-mode'
implementation, don't displace third-party markers.
* test/lisp/erc/erc-scenarios-stamp.el: New file.  (Bug#60936)

2 files changed, 96 insertions, 4 deletions

diff --git a/lisp/erc/erc-stamp.el b/lisp/erc/erc-stamp.el
index c8fd7c35392..b515513dcb7 100644
--- a/lisp/erc/erc-stamp.el
+++ b/lisp/erc/erc-stamp.el
@@ -492,8 +492,11 @@ and `erc-stamp--margin-left-p', before activating the mode."
     (put-text-property erc-insert-marker (1- erc-input-marker)
                        'display `((margin left-margin) ,prompt))))
 
-(cl-defmethod erc-insert-timestamp-left (string)
+(defun erc-insert-timestamp-left (string)
   "Insert timestamps at the beginning of the line."
+  (erc--insert-timestamp-left string))
+
+(cl-defmethod erc--insert-timestamp-left (string)
   (goto-char (point-min))
   (let* ((ignore-p (and erc-timestamp-only-if-changed-flag
                         (string-equal string erc-timestamp-last-inserted)))
@@ -504,13 +507,12 @@ and `erc-stamp--margin-left-p', before activating the mode."
     (erc-put-text-property 0 len 'invisible erc-stamp--invisible-property s)
     (insert s)))
 
-(cl-defmethod erc-insert-timestamp-left
+(cl-defmethod erc--insert-timestamp-left
   (string &context (erc-stamp--display-margin-mode (eql t)))
   (unless (and erc-timestamp-only-if-changed-flag
                (string-equal string erc-timestamp-last-inserted))
     (goto-char (point-min))
-    (insert-before-markers-and-inherit
-     (setq erc-timestamp-last-inserted string))
+    (insert-and-inherit (setq erc-timestamp-last-inserted string))
     (dolist (p erc-stamp--inherited-props)
       (when-let ((v (get-text-property (point) p)))
         (put-text-property (point-min) (point) p v)))
diff --git a/test/lisp/erc/erc-scenarios-stamp.el b/test/lisp/erc/erc-scenarios-stamp.el
new file mode 100644
index 00000000000..d6b5d868ce5
--- /dev/null
+++ b/test/lisp/erc/erc-scenarios-stamp.el
@@ -0,0 +1,90 @@
+;;; erc-scenarios-stamp.el --- Misc `erc-stamp' scenarios -*- lexical-binding: t -*-
+
+;; Copyright (C) 2023 Free Software Foundation, Inc.
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Code:
+
+(require 'ert-x)
+(eval-and-compile
+  (let ((load-path (cons (ert-resource-directory) load-path)))
+    (require 'erc-scenarios-common)))
+
+(require 'erc-stamp)
+
+(defvar erc-scenarios-stamp--user-marker nil)
+
+(defun erc-scenarios-stamp--on-post-modify ()
+  (when-let (((erc--check-msg-prop 'erc-cmd 4)))
+    (set-marker erc-scenarios-stamp--user-marker (point-max))
+    (ert-info ("User marker correctly placed at `erc-insert-marker'")
+      (should (= ?\n (char-before erc-scenarios-stamp--user-marker)))
+      (should (= erc-scenarios-stamp--user-marker erc-insert-marker))
+      (save-excursion
+        (goto-char erc-scenarios-stamp--user-marker)
+        ;; The raw message ends in " Iabefhkloqv".  However,
+        ;; `erc-server-004' only prints up to the 5th parameter.
+        (should (looking-back "CEIMRUabefhiklmnoqstuv\n"))))))
+
+(ert-deftest erc-scenarios-stamp--left/display-margin-mode ()
+
+  (erc-scenarios-common-with-cleanup
+      ((erc-scenarios-common-dialog "base/reconnect")
+       (dumb-server (erc-d-run "localhost" t 'unexpected-disconnect))
+       (port (process-contact dumb-server :service))
+       (erc-scenarios-stamp--user-marker (make-marker))
+       (erc-stamp--current-time 704591940)
+       (erc-stamp--tz t)
+       (erc-server-flood-penalty 0.1)
+       (erc-timestamp-only-if-changed-flag nil)
+       (erc-insert-timestamp-function #'erc-insert-timestamp-left)
+       (erc-modules (cons 'fill-wrap erc-modules))
+       (erc-timestamp-only-if-changed-flag nil)
+       (expect (erc-d-t-make-expecter)))
+
+    (ert-info ("Connect")
+      (with-current-buffer (erc :server "127.0.0.1"
+                                :port port
+                                :full-name "tester"
+                                :nick "tester")
+
+        (add-hook 'erc-insert-post-hook #'erc-scenarios-stamp--on-post-modify
+                  nil t)
+        (funcall expect 5 "This server is in debug mode")
+
+        (ert-info ("Stamps appear in left margin and are invisible")
+          (should (eq 'erc-timestamp (field-at-pos (pos-bol))))
+          (should (= (pos-bol) (field-beginning (pos-bol))))
+          (should (eq 'msg (get-text-property (pos-bol) 'erc-msg)))
+          (should (eq 'NOTICE (get-text-property (pos-bol) 'erc-cmd)))
+          (should (= ?- (char-after (field-end (pos-bol)))))
+          (should (equal (get-text-property (1+ (field-end (pos-bol)))
+                                            'erc-speaker)
+                         "irc.foonet.org"))
+          (should (pcase (get-text-property (pos-bol) 'display)
+                    (`((margin left-margin) ,s)
+                     (eq 'timestamp (get-text-property 0 'invisible s))))))
+
+        ;; We set a third-party marker at the end of 004's message (on
+        ;; then "\n"), post-insertion.
+        (ert-info ("User markers untouched by subsequent message left stamp")
+          (save-excursion
+            (goto-char erc-scenarios-stamp--user-marker)
+            (should (looking-back "CEIMRUabefhiklmnoqstuv\n"))
+            (should (looking-at (rx "[")))))))))
+
+;;; erc-scenarios-stamp.el ends here