Don't insert prompt input before markers in ERC

* etc/ERC-NEWS: Update entry in 5.6 section announcing a persistent
prompt.
* lisp/erc/erc-dcc.el (erc-dcc-chat-setup): Use helper
`erc--initialize-markers' to set up prompt.
* lisp/erc/erc.el (erc--initialize-markers): Change
`erc-insert-marker' type to t.  Do this to spare modules that want to
remember the current message location from resorting to workarounds,
like leaving a marker one char before the end of a message, which is
not foolproof.
(erc--refresh-prompt): Temporarily change type of `erc-insert-marker'
when inserting prompt.
(erc-display-line-1): Insert incoming message normally instead of
forcing before markers, thus respecting markers left by third parties.
Strictly speaking, this is a breaking change.
(erc-display-msg): Insert normally instead of calling
`insert-before-markers', which was a regression introduced by 05f6fdb9
"Preserve ERC prompt and its bounding markers".
* test/lisp/erc/erc-dcc-tests.el
(erc-dcc-tests--dcc-handle-ctcp-send,
erc-dcc-tests--erc-dcc-do-GET-command): Use helper to initialize
prompt and markers.
* test/lisp/erc/erc-networks-tests.el
(erc-networks--ensure-announced,
erc-networks--rename-server-buffer--existing--reuse,
erc-networks--rename-server-buffer--existing--noreuse,
erc-networks--rename-server-buffer--reconnecting,
erc-networks--rename-server-buffer--id,
erc-networks--rename-server-buffer--existing--live,
erc-networks--rename-server-buffer--local-match,
erc-networks--rename-server-buffer--local-nomatch): Use helper to
initialize prompt and bounding markers.
* test/lisp/erc/erc-stamp-tests.el (erc-stamp-tests--insert-right):
Use `erc--initialize-markers' for setting up prompt.
* test/lisp/erc/erc-tests.el (erc--refresh-prompt): Adjust assertions
for expected prompt content.  (Bug#60936)

7 files changed, 58 insertions, 60 deletions

diff --git a/etc/ERC-NEWS b/etc/ERC-NEWS
index e4947d0c64d..68f1083621c 100644
--- a/etc/ERC-NEWS
+++ b/etc/ERC-NEWS
@@ -244,7 +244,14 @@ running hooks during message insertion, and the position of its
 "insert marker" (ERC's per-buffer process mark) was inconsistent
 during these spells.  To make insertion handling more predictable in
 preparation for incorporating various protocol extensions, the prompt
-and its bounding markers have become perennial fixtures.  In rare
+and its bounding markers have become perennial fixtures.  To effect
+this change, small behavioral differences in message insertion have
+been adopted.  Crucially, 'erc-insert-marker' now has an "insertion
+type" of t, and 'erc-display-line-1' now calls 'insert' instead of
+'insert-before-prompt'.  This allows user code to leave its own
+markers via 'erc-insert-modify-hook' and 'erc-insert-post-hook'
+instead of having to resort to workarounds.  Message insertion for
+outgoing messages, in 'erc-display-msg', remains as before.  In rare
 cases, these changes may mean third-party code needs tweaking, for
 example, requiring the use of 'insert-before-markers' instead of
 'insert'.  As always, users feeling unduly inconvenienced by these
diff --git a/lisp/erc/erc-dcc.el b/lisp/erc/erc-dcc.el
index 2012bcadae1..cc2dcc9a788 100644
--- a/lisp/erc/erc-dcc.el
+++ b/lisp/erc/erc-dcc.el
@@ -1213,9 +1213,8 @@ other client."
             erc-dcc-from nick
             erc-dcc-entry-data entry
             erc-dcc-unprocessed-output ""
-            erc-insert-marker (point-max-marker)
             erc-input-marker (make-marker))
-      (erc-display-prompt buffer (point-max))
+      (erc--initialize-markers (point) nil)
       (set-process-buffer proc buffer)
       (add-hook 'kill-buffer-hook #'erc-dcc-chat-buffer-killed nil t)
       (run-hook-with-args 'erc-dcc-chat-connect-hook proc))
diff --git a/lisp/erc/erc.el b/lisp/erc/erc.el
index c0fa48bd9bd..c1c5b7b5e9a 100644
--- a/lisp/erc/erc.el
+++ b/lisp/erc/erc.el
@@ -2214,8 +2214,8 @@ nil."
                     (alist-get 'erc-insert-marker continued-session))
         (set-marker erc-input-marker
                     (alist-get 'erc-input-marker continued-session))
-        (goto-char erc-insert-marker)
-        (cl-assert (= (field-end) erc-input-marker))
+        (set-marker-insertion-type erc-insert-marker t)
+        (cl-assert (= (field-end erc-insert-marker) erc-input-marker))
         (goto-char old-point)
         (erc--unhide-prompt))
     (cl-assert (not (get-text-property (point) 'erc-prompt)))
@@ -2226,6 +2226,7 @@ nil."
     (insert "\n\n")
     (set-marker erc-insert-marker (point))
     (erc-display-prompt)
+    (set-marker-insertion-type erc-insert-marker t)
     (cl-assert (= (point) (point-max)))))
 
 (defun erc-open (&optional server port nick full-name
@@ -2821,10 +2822,12 @@ this option to nil."
   (when (functionp erc-prompt)
     (save-excursion
       (goto-char erc-insert-marker)
+      (set-marker-insertion-type erc-insert-marker nil)
       ;; Avoid `erc-prompt' (the named function), which appends a
       ;; space, and `erc-display-prompt', which propertizes all but
       ;; that space.
       (insert-and-inherit (funcall erc-prompt))
+      (set-marker-insertion-type erc-insert-marker t)
       (delete-region (point) (1- erc-input-marker)))))
 
 (defun erc-display-line-1 (string buffer)
@@ -2860,7 +2863,7 @@ If STRING is nil, the function does nothing."
               (save-restriction
                 (widen)
                 (goto-char insert-position)
-                (insert-before-markers string)
+                (insert string)
                 (erc--assert-input-bounds)
                 ;; run insertion hook, with point at restored location
                 (save-restriction
@@ -6490,15 +6493,14 @@ Return non-nil only if we actually send anything."
   (when erc-insert-this
     (save-excursion
       (erc--assert-input-bounds)
-      (let ((insert-position (marker-position erc-insert-marker))
+      (let ((insert-position (marker-position (goto-char erc-insert-marker)))
             beg)
-        (goto-char insert-position)
-        (insert-before-markers (erc-format-my-nick))
+        (insert (erc-format-my-nick))
         (setq beg (point))
-        (insert-before-markers line)
+        (insert line)
         (erc-put-text-property beg (point) 'font-lock-face 'erc-input-face)
         (erc-put-text-property insert-position (point) 'erc-command 'PRIVMSG)
-        (insert-before-markers "\n")
+        (insert "\n")
         (save-restriction
           (narrow-to-region insert-position (point))
           (run-hooks 'erc-send-modify-hook)
diff --git a/test/lisp/erc/erc-dcc-tests.el b/test/lisp/erc/erc-dcc-tests.el
index 7fb5f82e784..f02ddf228a2 100644
--- a/test/lisp/erc/erc-dcc-tests.el
+++ b/test/lisp/erc/erc-dcc-tests.el
@@ -57,11 +57,8 @@
       (erc-mode)
       (setq erc-server-process
             (start-process "fake" (current-buffer) "sleep" "10")
-            erc-input-marker (make-marker)
-            erc-insert-marker (make-marker)
             erc-server-current-nick "dummy")
-      (erc-display-prompt)
-      (set-marker erc-insert-marker (pos-bol))
+      (erc--initialize-markers (point) nil)
       (set-process-query-on-exit-flag erc-server-process nil)
       (should-not erc-dcc-list)
       (erc-ctcp-query-DCC erc-server-process
@@ -121,17 +118,14 @@
            calls)
       (erc-mode)
       (setq erc-server-process proc
-            erc-input-marker (make-marker)
-            erc-insert-marker (make-marker)
             erc-server-current-nick "dummy")
+      (erc--initialize-markers (point) nil)
       (set-process-query-on-exit-flag proc nil)
       (cl-letf (((symbol-function 'read-file-name)
                  (lambda (&rest _) file))
                 ((symbol-function 'erc-dcc-get-file)
                  (lambda (&rest r) (push r calls))))
         (goto-char (point-max))
-        (set-marker erc-insert-marker (point-max))
-        (erc-display-prompt)
 
         (ert-info ("No turbo")
           (should-not (plist-member elt :turbo))
diff --git a/test/lisp/erc/erc-networks-tests.el b/test/lisp/erc/erc-networks-tests.el
index b9d216f217b..f0fcbbc81c6 100644
--- a/test/lisp/erc/erc-networks-tests.el
+++ b/test/lisp/erc/erc-networks-tests.el
@@ -1233,10 +1233,7 @@
                    :contents "MOTD File is missing")))
 
       (erc-mode) ; boilerplate displayable start (needs `erc-server-process')
-      (insert "\n\n")
-      (setq erc-input-marker (make-marker) erc-insert-marker (make-marker))
-      (set-marker erc-insert-marker (point-max))
-      (erc-display-prompt) ; boilerplate displayable end
+      (erc--initialize-markers (point) nil)
 
       (erc-networks--ensure-announced erc-server-process parsed)
       (goto-char (point-min))
@@ -1277,9 +1274,9 @@
     (with-current-buffer old-buf
       (erc-mode)
       (insert "*** Old buf")
+      (erc--initialize-markers (point) nil)
       (setq erc-network 'FooNet
             erc-server-current-nick "tester"
-            erc-insert-marker (set-marker (make-marker) (point-max))
             erc-server-process old-proc
             erc-networks--id (erc-networks--id-create nil)))
 
@@ -1328,10 +1325,10 @@
            erc-reuse-buffers)
       (with-current-buffer old-buf
         (erc-mode)
+        (erc--initialize-markers (point) nil)
         (insert "*** Old buf")
         (setq erc-network 'FooNet
               erc-server-current-nick "tester"
-              erc-insert-marker (set-marker (make-marker) (point-max))
               erc-server-process old-proc
               erc-networks--id (erc-networks--id-create nil)))
       (with-current-buffer (get-buffer-create "#chan")
@@ -1377,10 +1374,10 @@
 
     (with-current-buffer old-buf
       (erc-mode)
+      (erc--initialize-markers (point) nil)
       (insert "*** Old buf")
       (setq erc-network 'FooNet
             erc-server-current-nick "tester"
-            erc-insert-marker (set-marker (make-marker) (point-max))
             erc-server-process old-proc
             erc-networks--id (erc-networks--id-create nil)))
 
@@ -1415,10 +1412,10 @@
 
     (with-current-buffer old-buf
       (erc-mode)
+      (erc--initialize-markers (point) nil)
       (insert "*** Old buf")
       (setq erc-network 'FooNet
             erc-networks--id (erc-networks--id-create 'MySession)
-            erc-insert-marker (set-marker (make-marker) (point-max))
             erc-server-process old-proc))
 
     (with-current-buffer (get-buffer-create "#chan")
@@ -1454,10 +1451,10 @@
 
     (with-current-buffer old-buf
       (erc-mode)
+      (erc--initialize-markers (point) nil)
       (insert "*** Old buf")
       (setq erc-network 'FooNet
             erc-server-current-nick "tester"
-            erc-insert-marker (set-marker (make-marker) (point-max))
             erc-server-process old-proc
             erc-networks--id (erc-networks--id-create nil))
       (should (erc-server-process-alive)))
@@ -1473,15 +1470,12 @@
     (ert-info ("New buffer rejected, abandoned, not killed")
       (with-current-buffer (get-buffer-create "irc.foonet.org")
         (erc-mode)
+        (erc--initialize-markers (point) nil)
         (setq erc-network 'FooNet
               erc-server-current-nick "tester"
-              erc-insert-marker (make-marker)
-              erc-input-marker (make-marker)
               erc-server-process (erc-networks-tests--create-live-proc)
               erc-networks--id (erc-networks--id-create nil))
         (set-process-sentinel erc-server-process #'ignore)
-        (erc-display-prompt nil (point-max))
-        (set-marker erc-insert-marker (pos-bol))
         (erc-display-message nil 'notice (current-buffer) "notice")
         (with-silent-modifications
           (should-not (erc-networks--rename-server-buffer erc-server-process)))
@@ -1514,10 +1508,10 @@
     (with-current-buffer old-buf
       (erc-mode)
       (insert "*** Old buf")
+      (erc--initialize-markers (point) nil)
       (setq erc-network 'FooNet
             erc-server-current-nick "tester"
             erc-server-announced-name "us-east.foonet.org"
-            erc-insert-marker (set-marker (make-marker) (point-max))
             erc-server-process old-proc
             erc--isupport-params (make-hash-table)
             erc-networks--id (erc-networks--id-create nil))
@@ -1566,10 +1560,10 @@
     (with-current-buffer old-buf
       (erc-mode)
       (insert "*** Old buf")
+      (erc--initialize-markers (point) nil)
       (setq erc-network 'FooNet
             erc-server-current-nick "tester"
             erc-server-announced-name "us-west.foonet.org"
-            erc-insert-marker (set-marker (make-marker) (point-max))
             erc-server-process old-proc
             erc--isupport-params (make-hash-table)
             erc-networks--id (erc-networks--id-create nil))
diff --git a/test/lisp/erc/erc-stamp-tests.el b/test/lisp/erc/erc-stamp-tests.el
index 01e71e348e0..6da7ed4503d 100644
--- a/test/lisp/erc/erc-stamp-tests.el
+++ b/test/lisp/erc/erc-stamp-tests.el
@@ -43,13 +43,10 @@
     (with-current-buffer (get-buffer-create "*erc-stamp-tests--insert-right*")
       (erc-mode)
       (erc-munge-invisibility-spec)
+      (erc--initialize-markers (point) nil)
       (setq erc-server-process (start-process "p" (current-buffer)
-                                              "sleep" "1")
-            erc-input-marker (make-marker)
-            erc-insert-marker (make-marker))
+                                              "sleep" "1"))
       (set-process-query-on-exit-flag erc-server-process nil)
-      (set-marker erc-insert-marker (point-max))
-      (erc-display-prompt)
 
       (funcall test)
 
diff --git a/test/lisp/erc/erc-tests.el b/test/lisp/erc/erc-tests.el
index 7d1e20132e1..f3489a16386 100644
--- a/test/lisp/erc/erc-tests.el
+++ b/test/lisp/erc/erc-tests.el
@@ -294,16 +294,19 @@
         (set-process-query-on-exit-flag erc-server-process nil)
         ;; Incoming message redraws prompt
         (erc-display-message nil 'notice nil "Welcome")
+        (should (looking-at-p (rx "*** Welcome")))
+        (forward-line)
         (should (looking-at-p "ServNet 4>"))
         ;; Say something
-        (save-excursion (goto-char erc-input-marker)
-                        (insert "Howdy")
-                        (erc-send-current-line)
-                        (forward-line -1)
+        (goto-char erc-input-marker)
+        (insert "Howdy")
+        (erc-send-current-line)
+        (save-excursion (forward-line -1)
                         (should (looking-at "No target"))
                         (forward-line -1)
                         (should (looking-at "<tester> Howdy")))
-        (should (looking-at-p "ServNet 6>"))
+        (should (looking-back "ServNet 6> "))
+        (should (= erc-input-marker (point)))
         ;; Space after prompt is unpropertized
         (should (get-text-property (1- erc-input-marker) 'erc-prompt))
         (should-not (get-text-property erc-input-marker 'erc-prompt))
@@ -317,6 +320,7 @@
         (erc-tests--send-prep)
         (goto-char erc-insert-marker)
         (should (looking-at-p "#chan 9>"))
+        (goto-char erc-input-marker)
         (setq erc-server-process (buffer-local-value 'erc-server-process
                                                      (get-buffer "ServNet"))
               erc-networks--id (erc-with-server-buffer erc-networks--id)
@@ -328,18 +332,18 @@
         (erc-update-current-channel-member "tester" "tester")
         (erc-display-message nil nil (current-buffer)
                              (erc-format-privmessage "alice" "Hi" nil t))
-        (should (looking-at-p "#chan@ServNet 10>"))
-        (save-excursion (goto-char erc-input-marker)
-                        (insert "Howdy")
-                        (erc-send-current-line)
-                        (forward-line -1)
+        (should (looking-back "#chan@ServNet 10> "))
+        (goto-char erc-input-marker)
+        (insert "Howdy")
+        (erc-send-current-line)
+        (save-excursion (forward-line -1)
                         (should (looking-at "<tester> Howdy")))
-        (should (looking-at-p "#chan@ServNet 11>"))
-        (save-excursion (goto-char erc-input-marker)
-                        (insert "/query bob")
-                        (erc-send-current-line))
+        (should (looking-back "#chan@ServNet 11> "))
+        (should (= (point) erc-input-marker))
+        (insert "/query bob")
+        (erc-send-current-line)
         ;; Query does not redraw (nor /help, only message input)
-        (should (looking-at-p "#chan@ServNet 11>"))
+        (should (looking-back "#chan@ServNet 11> "))
         ;; No sign of old prompts
         (save-excursion
           (goto-char (point-min))
@@ -349,15 +353,16 @@
       (with-current-buffer (get-buffer "bob")
         (goto-char erc-insert-marker)
         (should (looking-at-p "bob@ServNet 14>"))
+        (goto-char erc-input-marker)
         (erc-display-message nil nil (current-buffer)
                              (erc-format-privmessage "bob" "Hi" nil t))
-        (should (looking-at-p "bob@ServNet 15>"))
-        (save-excursion (goto-char erc-input-marker)
-                        (insert "Howdy")
-                        (erc-send-current-line)
-                        (forward-line -1)
+        (should (looking-back "bob@ServNet 15> "))
+        (goto-char erc-input-marker)
+        (insert "Howdy")
+        (erc-send-current-line)
+        (save-excursion (forward-line -1)
                         (should (looking-at "<tester> Howdy")))
-        (should (looking-at-p "bob@ServNet 16>"))
+        (should (looking-back "bob@ServNet 16> "))
         ;; No sign of old prompts
         (save-excursion
           (goto-char (point-min))