Subject: bug#66073: 30.0.50; ERC 5.6: Improve handling of blank lines at ERC's prompt

From: "J.P." <[email-masked]>
Date: Fri, 22 Sep 2023 07:20:50 -0700

v2. Fix faulty interaction with `erc-inhibit-multiline-input'. Move
slash-command detection to input-review hook.

>From eeedb524c2686245da10aa827a6a9bfcbc54d046 Mon Sep 17 00:00:00 2001
From: "F. Jason Park" <[email-masked]>
Date: Fri, 22 Sep 2023 07:15:18 -0700
Subject: [PATCH 0/4] *** NOT A PATCH ***

*** BLURB HERE ***

F. Jason Park (4):
  ; Increase ERC test server queue size
  Exempt text-scale-mode from erc-scrolltobottom-all
  [5.6] Improve erc-warn-about-blank-lines behavior
  Detect slash commands in erc--input-review-functions

 etc/ERC-NEWS                                  |   8 +-
 lisp/erc/erc-common.el                        |   1 +
 lisp/erc/erc-goodies.el                       |   8 +-
 lisp/erc/erc.el                               | 166 ++++++++++-----
 test/lisp/erc/erc-scenarios-scrolltobottom.el |   4 +-
 test/lisp/erc/erc-tests.el                    | 189 +++++++++++++-----
 test/lisp/erc/resources/erc-d/erc-d.el        |  50 ++---
 .../erc/resources/erc-scenarios-common.el     |   3 +-
 8 files changed, 301 insertions(+), 128 deletions(-)

Interdiff:
diff --git a/etc/ERC-NEWS b/etc/ERC-NEWS
index 05e933930e2..fadd97b65df 100644
--- a/etc/ERC-NEWS
+++ b/etc/ERC-NEWS
@@ -221,6 +221,12 @@ atop any message.  The new companion option 'erc-echo-timestamp-zone'
 determines the default timezone when not specified with a prefix
 argument.
 
+** Option 'erc-warn-about-blank-lines' is more informative.
+Enabled by default, this option now produces more useful feedback
+whenever ERC rejects prompt input containing whitespace-only lines.
+When paired with option 'erc-send-whitespace-lines', ERC echoes a
+tally of blank lines padded and trailing blanks culled.
+
 ** Miscellaneous UX changes.
 Some minor quality-of-life niceties have finally made their way to
 ERC.  For example, fool visibility has become togglable with the new
@@ -281,7 +287,7 @@ For starters, the 'cursor-sensor-functions' property no longer
 contains unique closures and thus no longer proves effective for
 traversing messages.  To compensate, a new property, 'erc-timestamp',
 now spans message bodies but not the newlines delimiting them.  Also
-affecting the `stamp' module is the deprecation of the function
+affecting the 'stamp' module is the deprecation of the function
 'erc-insert-aligned' and its removal from client code.  Additionally,
 the module now merges its 'invisible' property with existing ones and
 includes all white space around stamps when doing so.
diff --git a/lisp/erc/erc-goodies.el b/lisp/erc/erc-goodies.el
index 6eb015fdd64..431264ec8b4 100644
--- a/lisp/erc/erc-goodies.el
+++ b/lisp/erc/erc-goodies.el
@@ -128,6 +128,11 @@ erc--scrolltobottom-post-force-commands
 That is, ERC recalculates the window's start instead of blindly
 restoring it.")
 
+;; Unfortunately, this doesn't work when `erc-scrolltobottom-relaxed'
+;; is enabled (scaling up still moves the prompt).
+(defvar erc--scrolltobottom-post-ignore-commands '(text-scale-adjust)
+  "Commands to skip instead of force-scroll on `post-command-hook'.")
+
 (defvar erc--scrolltobottom-relaxed-skip-commands
   '(recenter-top-bottom scroll-down-command)
   "Commands exempt from triggering a stash and restore of `window-start'.
@@ -158,7 +163,8 @@ erc--scrolltobottom-on-post-command
              ((= (nth 2 found)
                  (count-screen-lines (window-start) (point-max)))))
         (set-window-start (selected-window) (nth 1 found))
-      (erc--scrolltobottom-confirm))
+      (unless (memq this-command erc--scrolltobottom-post-ignore-commands)
+        (erc--scrolltobottom-confirm)))
     (setq erc--scrolltobottom-window-info nil)))
 
 (defun erc--scrolltobottom-on-pre-command-relaxed ()
diff --git a/lisp/erc/erc.el b/lisp/erc/erc.el
index 7165f38189e..b9ce6b8e53a 100644
--- a/lisp/erc/erc.el
+++ b/lisp/erc/erc.el
@@ -268,8 +268,8 @@ erc-send-whitespace-lines
 (defcustom erc-inhibit-multiline-input nil
   "When non-nil, conditionally disallow input consisting of multiple lines.
 Issue an error when the number of input lines submitted for
-sending exceeds this value.  The value t means disallow more
-than 1 line of input."
+sending meets or exceeds this value.  The value t is synonymous
+with a value of 2 and means disallow more than 1 line of input."
   :package-version '(ERC . "5.5")
   :group 'erc
   :type '(choice integer boolean))
@@ -1101,7 +1101,8 @@ 'erc--pre-send-split-functions
   'erc--input-review-functions "30.1")
 (defvar erc--input-review-functions '(erc--split-lines
                                       erc--run-input-validation-checks
-                                      erc--discard-trailing-multiline-nulls)
+                                      erc--discard-trailing-multiline-nulls
+                                      erc--inhibit-slash-cmd-insertion)
   "Special hook for reviewing and modifying prompt input.
 ERC runs this before clearing the prompt and before running any
 send-related hooks, such as `erc-pre-send-functions'.  Thus, it's
@@ -6476,6 +6477,9 @@ erc--count-blank-lines
       (setq pad 1))
     (list total pad strip)))
 
+(defvar erc--check-prompt-explanation nil
+  "List of strings to print if no validator returns non-nil.")
+
 (defun erc--check-prompt-input-for-multiline-blanks (_ lines)
   "Return non-nil when multiline prompt input has blank LINES.
 Consider newlines to be intervening delimiters, meaning the
@@ -6499,7 +6503,10 @@ erc--check-prompt-input-for-multiline-blanks
              (when msg
                (setf msg (nreverse msg)
                      (car msg) (capitalize (car msg))))
-             (and msg `(message ,(string-join msg " ") ,@(nreverse args)))))
+             (when msg
+               (push (apply #'format (string-join msg " ") (nreverse args))
+                     erc--check-prompt-explanation)
+               nil)))
           (erc-warn-about-blank-lines
            (concat (if (= total 1)
                        (if (zerop strip) "Blank" "Trailing")
@@ -6534,27 +6541,38 @@ erc--check-prompt-input-for-multiline-command
 (defvar erc--check-prompt-input-functions
   '(erc--check-prompt-input-for-point-in-bounds
     erc--check-prompt-input-for-something
+    erc--check-prompt-input-for-multiline-command
     erc--check-prompt-input-for-multiline-blanks
     erc--check-prompt-input-for-running-process
-    erc--check-prompt-input-for-excess-lines
-    erc--check-prompt-input-for-multiline-command)
+    erc--check-prompt-input-for-excess-lines)
   "Validators for user input typed at prompt.
-Called with latest input string submitted by user and the list of
-lines produced by splitting it.  If any member function returns
-non-nil, processing is abandoned and input is left untouched.
-When the returned value is a string, ERC passes it to `erc-error'.")
+Called with two arguments: the current input submitted by the
+user, as a string, along with the same input as a list of
+strings.  If any member function returns non-nil, ERC abandons
+processing and leaves pending input untouched in the prompt area.
+When the returned value is a string, ERC passes it to
+`user-error'.  Any other non-nil value tells ERC to abort
+silently.  If all members return nil, and the variable
+`erc--check-prompt-explanation' is a nonempty list of strings,
+ERC prints them as a single message joined by newlines.")
 
 (defun erc--run-input-validation-checks (state)
   "Run input checkers from STATE, an `erc--input-split' object."
-  (when-let ((msg (run-hook-with-args-until-success
-                   'erc--check-prompt-input-functions
-                   (erc--input-split-string state)
-                   (erc--input-split-lines state))))
-    (cond ((eq (car-safe msg) 'message)
-           (apply 'message (cdr msg)))
-          ((stringp msg)
-           (user-error msg))
-          (t (push msg (erc--input-split-abortp state))))))
+  (let* ((erc--check-prompt-explanation nil)
+         (msg (run-hook-with-args-until-success
+               'erc--check-prompt-input-functions
+               (erc--input-split-string state)
+               (erc--input-split-lines state))))
+    (cond ((stringp msg) (user-error msg))
+          (msg (push msg (erc--input-split-abortp state)))
+          (erc--check-prompt-explanation
+           (message "%s" (string-join (nreverse erc--check-prompt-explanation)
+                                      "\n"))))))
+
+(defun erc--inhibit-slash-cmd-insertion (state)
+  "Don't insert STATE object's message if it's a \"slash\" command."
+  (when (erc--input-split-cmdp state)
+    (setf (erc--input-split-insertp state) nil)))
 
 (defun erc-send-current-line ()
   "Parse current line and send it to IRC."
@@ -6664,9 +6682,8 @@ erc--send-input-lines
   "Send lines in `erc--input-split-lines' object LINES-OBJ."
   (when (erc--input-split-sendp lines-obj)
     (dolist (line (erc--input-split-lines lines-obj))
-      (unless (erc--input-split-cmdp lines-obj)
-        (when (erc--input-split-insertp lines-obj)
-          (erc-display-msg line)))
+      (when (erc--input-split-insertp lines-obj)
+        (erc-display-msg line))
       (erc-process-input-line (concat line "\n")
                               (null erc-flood-protect)
                               (not (erc--input-split-cmdp lines-obj))))))
diff --git a/test/lisp/erc/erc-scenarios-scrolltobottom.el b/test/lisp/erc/erc-scenarios-scrolltobottom.el
index dd0a8612388..206687ccab5 100644
--- a/test/lisp/erc/erc-scenarios-scrolltobottom.el
+++ b/test/lisp/erc/erc-scenarios-scrolltobottom.el
@@ -35,7 +35,7 @@ erc-scenarios-scrolltobottom--normal
 
   (should-not erc-scrolltobottom-all)
 
-  (erc-scenarios-scrolltobottom--normal
+  (erc-scenarios-common-scrolltobottom--normal
    (lambda ()
      (ert-info ("New insertion doesn't anchor prompt in other window")
        (let ((w (next-window)))
@@ -52,7 +52,7 @@ erc-scenarios-scrolltobottom--all
 
   (let ((erc-scrolltobottom-all t))
 
-    (erc-scenarios-scrolltobottom--normal
+    (erc-scenarios-common-scrolltobottom--normal
      (lambda ()
        (ert-info ("New insertion anchors prompt in other window")
          (let ((w (next-window)))
diff --git a/test/lisp/erc/erc-tests.el b/test/lisp/erc/erc-tests.el
index bb7e3259608..8a68eca6196 100644
--- a/test/lisp/erc/erc-tests.el
+++ b/test/lisp/erc/erc-tests.el
@@ -292,7 +292,7 @@ erc--refresh-prompt
                                (cl-incf counter))))
          erc-accidental-paste-threshold-seconds
          erc-insert-modify-hook
-         erc--input-review-functions
+         (erc--input-review-functions erc--input-review-functions)
          erc-send-completed-hook)
 
     (ert-info ("Server buffer")
@@ -357,6 +357,9 @@ erc--refresh-prompt
         (should (= (point) erc-input-marker))
         (insert "/query bob")
         (erc-send-current-line)
+        ;; Last command not inserted
+        (save-excursion (forward-line -1)
+                        (should (looking-at "<tester> Howdy")))
         ;; Query does not redraw (nor /help, only message input)
         (should (looking-back "#chan@ServNet 11> "))
         ;; No sign of old prompts
@@ -877,11 +880,12 @@ erc-ring-previous-command
   (with-current-buffer (get-buffer-create "*#fake*")
     (erc-mode)
     (erc-tests--send-prep)
+    (setq erc-server-current-nick "tester")
     (setq-local erc-last-input-time 0)
     (should-not (local-variable-if-set-p 'erc-send-completed-hook))
     (set (make-local-variable 'erc-send-completed-hook) nil) ; skip t (globals)
     ;; Just in case erc-ring-mode is already on
-    (setq-local erc--input-review-functions nil)
+    (setq-local erc--input-review-functions erc--input-review-functions)
     (add-hook 'erc--input-review-functions #'erc-add-to-input-ring)
     ;;
     (cl-letf (((symbol-function 'erc-process-input-line)
@@ -1256,8 +1260,9 @@ erc--check-prompt-input-for-multiline-blanks/explanations
                      (("" "a" "" "b" "" "c" "" "")
                       "Stripping (2) and padding (3) blank lines")))
       (ert-info ((format "Input: %S, Msg: %S" input msg))
-        (let ((rv (erc--check-prompt-input-for-multiline-blanks nil input)))
-          (should (equal (apply #'format (cdr rv)) msg))))))
+        (let (erc--check-prompt-explanation)
+          (should-not (erc--check-prompt-input-for-multiline-blanks nil input))
+          (should (equal (list msg) erc--check-prompt-explanation))))))
 
   (pcase-dolist (`(,input ,msg)
                  '((("") "Blank line detected")
@@ -1311,17 +1316,12 @@ erc-send-whitespace-lines
        (should-not (funcall next)))
 
      (ert-info ("Multiline command with trailing blank filtered")
-       (pcase-dolist (`(,p . ,q)
-                      '(("/a b\r" "/a b\n") ("/a b\n" "/a b\n")
-                        ("/a b\n\n" "/a b\n") ("/a b\r\n" "/a b\n")
-                        ("/a b\n\n\n" "/a b\n")))
+       (dolist (p '("/a b" "/a b\n" "/a b\n\n" "/a b\n\n\n"))
          (insert p)
          (erc-send-current-line)
          (erc-bol)
          (should (eq (point) (point-max)))
-         (while q
-           (should (pcase (funcall next)
-                     (`(,cmd ,_ nil) (equal cmd (pop q))))))
+         (should (pcase (funcall next) (`(,cmd ,_ nil) (equal cmd "/a b\n"))))
          (should-not (funcall next))))
 
      (ert-info ("Multiline command with non-blanks errors")
diff --git a/test/lisp/erc/resources/erc-d/erc-d.el b/test/lisp/erc/resources/erc-d/erc-d.el
index b86769220dd..f072c6b93b2 100644
--- a/test/lisp/erc/resources/erc-d/erc-d.el
+++ b/test/lisp/erc/resources/erc-d/erc-d.el
@@ -254,7 +254,7 @@ erc-d--initialize-client
          (ending (process-get process :dialog-ending))
          (dialog (make-erc-d-dialog :name name
                                     :process process
-                                    :queue (make-ring 5)
+                                    :queue (make-ring 10)
                                     :exchanges (make-ring 10)
                                     :match-handlers mat-h
                                     :server-fqdn fqdn)))
@@ -292,33 +292,27 @@ erc-d-load-replacement-dialog
 
 (defvar erc-d--m-debug (getenv "ERC_D_DEBUG"))
 
-(defmacro erc-d--m (process format-string &rest args)
-  "Output ARGS using FORMAT-STRING somewhere depending on context.
-PROCESS should be a client connection or a server network process."
-  `(let ((format-string (if erc-d--m-debug
-                            (concat (format-time-string "%s.%N: ")
-                                    ,format-string)
-                          ,format-string))
-         (want-insert (and ,process erc-d--in-process))
-         (buffer (process-buffer (process-get ,process :server))))
-     (when (and want-insert (buffer-live-p buffer))
-       (with-current-buffer buffer
-         (goto-char (point-max))
-         (insert (concat (format ,format-string ,@args) "\n"))))
-     (when (or erc-d--m-debug (not want-insert))
-       (message format-string ,@args))))
-
-(defmacro erc-d--log (process string &optional outbound)
-  "Log STRING sent to (OUTBOUND) or received from PROCESS peer."
-  `(let ((id (or (process-get ,process :log-id)
-                 (let ((port (erc-d-u--get-remote-port ,process)))
-                   (process-put ,process :log-id port)
-                   port)))
-         (name (erc-d-dialog-name (process-get ,process :dialog))))
-     (if ,outbound
-         (erc-d--m process "-> %s:%s %s" name id ,string)
-       (dolist (line (split-string ,string (process-get process :ending)))
-         (erc-d--m process "<- %s:%s %s" name id line)))))
+(defun erc-d--m (process format-string &rest args)
+  "Output ARGS using FORMAT-STRING to PROCESS's buffer or elsewhere."
+  (when erc-d--m-debug
+    (setq format-string (concat (format-time-string "%s.%N: ") format-string)))
+  (let ((insertp (and process erc-d--in-process))
+        (buffer (process-buffer (process-get process :server))))
+    (when (and insertp (buffer-live-p buffer))
+      (princ (concat (apply #'format format-string args) "\n") buffer))
+    (when (or erc-d--m-debug (not insertp))
+      (apply #'message format-string args))))
+
+(defun erc-d--log (process string &optional outbound)
+  "Log STRING received from or OUTBOUND to PROCESS peer."
+  (let ((id (or (process-get process :log-id)
+                (let ((port (erc-d-u--get-remote-port process)))
+                  (process-put process :log-id port) port)))
+        (name (erc-d-dialog-name (process-get process :dialog))))
+    (if outbound
+        (erc-d--m process "-> %s:%s %s" name id string)
+      (dolist (line (split-string string (process-get process :ending)))
+        (erc-d--m process "<- %s:%s %s" name id line)))))
 
 (defun erc-d--log-process-event (server process msg)
   (erc-d--m server "%s: %s" process (string-trim-right msg)))
diff --git a/test/lisp/erc/resources/erc-scenarios-common.el b/test/lisp/erc/resources/erc-scenarios-common.el
index 19f26bf08bd..5354b300b47 100644
--- a/test/lisp/erc/resources/erc-scenarios-common.el
+++ b/test/lisp/erc/resources/erc-scenarios-common.el
@@ -341,7 +341,7 @@ erc-scenarios-common--recenter-top-bottom
 
 ;;;; Fixtures
 
-(defun erc-scenarios-scrolltobottom--normal (test)
+(defun erc-scenarios-common-scrolltobottom--normal (test)
   (erc-scenarios-common-with-noninteractive-in-term
       ((erc-scenarios-common-dialog "scrolltobottom")
        (dumb-server (erc-d-run "localhost" t 'help))
@@ -402,6 +402,7 @@ erc-scenarios-scrolltobottom--normal
         (erc-cmd-MSG "NickServ help register")
         (save-excursion (erc-d-t-search-for 10 "End of NickServ"))
         (should (= 1 (point)))
+        (redisplay)
         (should (zerop (count-screen-lines (window-start) (window-point))))
         (should (erc-scenarios-common--prompt-past-win-end-p)))
 
-- 
2.41.0

>From 86ee2386d8474384e38c5023163098572f05d3fa Mon Sep 17 00:00:00 2001
From: "F. Jason Park" <[email-masked]>
Date: Mon, 18 Sep 2023 22:50:28 -0700
Subject: [PATCH 1/4] ; Increase ERC test server queue size

* test/lisp/erc/erc-scenarios-scrolltobottom.el
(erc-scenarios-scrolltobottom--normal,
erc-scenarios-scrolltobottom--all): Use update name for test fixture.
* test/lisp/erc/resources/erc-d/erc-d.el
(erc-d--initialize-client): For lengthy batches, `erc-d--filter' may
run multiple times before `erc-d--on-request' can pull from the queue,
which results in discarded incoming messages and match failures.
(erc-d--m, erc-d--log): Convert to ordinary functions.
* test/lisp/erc/resources/erc-scenarios-common.el
(erc-scenarios-scrolltobottom--normal,
erc-scenarios-common-scrolltobottom--normal): Rename test fixture from
former to latter and attempt to fix intermittent failure re
`count-screen-lines'.
---
 test/lisp/erc/erc-scenarios-scrolltobottom.el |  4 +-
 test/lisp/erc/resources/erc-d/erc-d.el        | 50 ++++++++-----------
 .../erc/resources/erc-scenarios-common.el     |  3 +-
 3 files changed, 26 insertions(+), 31 deletions(-)

diff --git a/test/lisp/erc/erc-scenarios-scrolltobottom.el b/test/lisp/erc/erc-scenarios-scrolltobottom.el
index dd0a8612388..206687ccab5 100644
--- a/test/lisp/erc/erc-scenarios-scrolltobottom.el
+++ b/test/lisp/erc/erc-scenarios-scrolltobottom.el
@@ -35,7 +35,7 @@ erc-scenarios-scrolltobottom--normal
 
   (should-not erc-scrolltobottom-all)
 
-  (erc-scenarios-scrolltobottom--normal
+  (erc-scenarios-common-scrolltobottom--normal
    (lambda ()
      (ert-info ("New insertion doesn't anchor prompt in other window")
        (let ((w (next-window)))
@@ -52,7 +52,7 @@ erc-scenarios-scrolltobottom--all
 
   (let ((erc-scrolltobottom-all t))
 
-    (erc-scenarios-scrolltobottom--normal
+    (erc-scenarios-common-scrolltobottom--normal
      (lambda ()
        (ert-info ("New insertion anchors prompt in other window")
          (let ((w (next-window)))
diff --git a/test/lisp/erc/resources/erc-d/erc-d.el b/test/lisp/erc/resources/erc-d/erc-d.el
index b86769220dd..f072c6b93b2 100644
--- a/test/lisp/erc/resources/erc-d/erc-d.el
+++ b/test/lisp/erc/resources/erc-d/erc-d.el
@@ -254,7 +254,7 @@ erc-d--initialize-client
          (ending (process-get process :dialog-ending))
          (dialog (make-erc-d-dialog :name name
                                     :process process
-                                    :queue (make-ring 5)
+                                    :queue (make-ring 10)
                                     :exchanges (make-ring 10)
                                     :match-handlers mat-h
                                     :server-fqdn fqdn)))
@@ -292,33 +292,27 @@ erc-d-load-replacement-dialog
 
 (defvar erc-d--m-debug (getenv "ERC_D_DEBUG"))
 
-(defmacro erc-d--m (process format-string &rest args)
-  "Output ARGS using FORMAT-STRING somewhere depending on context.
-PROCESS should be a client connection or a server network process."
-  `(let ((format-string (if erc-d--m-debug
-                            (concat (format-time-string "%s.%N: ")
-                                    ,format-string)
-                          ,format-string))
-         (want-insert (and ,process erc-d--in-process))
-         (buffer (process-buffer (process-get ,process :server))))
-     (when (and want-insert (buffer-live-p buffer))
-       (with-current-buffer buffer
-         (goto-char (point-max))
-         (insert (concat (format ,format-string ,@args) "\n"))))
-     (when (or erc-d--m-debug (not want-insert))
-       (message format-string ,@args))))
-
-(defmacro erc-d--log (process string &optional outbound)
-  "Log STRING sent to (OUTBOUND) or received from PROCESS peer."
-  `(let ((id (or (process-get ,process :log-id)
-                 (let ((port (erc-d-u--get-remote-port ,process)))
-                   (process-put ,process :log-id port)
-                   port)))
-         (name (erc-d-dialog-name (process-get ,process :dialog))))
-     (if ,outbound
-         (erc-d--m process "-> %s:%s %s" name id ,string)
-       (dolist (line (split-string ,string (process-get process :ending)))
-         (erc-d--m process "<- %s:%s %s" name id line)))))
+(defun erc-d--m (process format-string &rest args)
+  "Output ARGS using FORMAT-STRING to PROCESS's buffer or elsewhere."
+  (when erc-d--m-debug
+    (setq format-string (concat (format-time-string "%s.%N: ") format-string)))
+  (let ((insertp (and process erc-d--in-process))
+        (buffer (process-buffer (process-get process :server))))
+    (when (and insertp (buffer-live-p buffer))
+      (princ (concat (apply #'format format-string args) "\n") buffer))
+    (when (or erc-d--m-debug (not insertp))
+      (apply #'message format-string args))))
+
+(defun erc-d--log (process string &optional outbound)
+  "Log STRING received from or OUTBOUND to PROCESS peer."
+  (let ((id (or (process-get process :log-id)
+                (let ((port (erc-d-u--get-remote-port process)))
+                  (process-put process :log-id port) port)))
+        (name (erc-d-dialog-name (process-get process :dialog))))
+    (if outbound
+        (erc-d--m process "-> %s:%s %s" name id string)
+      (dolist (line (split-string string (process-get process :ending)))
+        (erc-d--m process "<- %s:%s %s" name id line)))))
 
 (defun erc-d--log-process-event (server process msg)
   (erc-d--m server "%s: %s" process (string-trim-right msg)))
diff --git a/test/lisp/erc/resources/erc-scenarios-common.el b/test/lisp/erc/resources/erc-scenarios-common.el
index 19f26bf08bd..5354b300b47 100644
--- a/test/lisp/erc/resources/erc-scenarios-common.el
+++ b/test/lisp/erc/resources/erc-scenarios-common.el
@@ -341,7 +341,7 @@ erc-scenarios-common--recenter-top-bottom
 
 ;;;; Fixtures
 
-(defun erc-scenarios-scrolltobottom--normal (test)
+(defun erc-scenarios-common-scrolltobottom--normal (test)
   (erc-scenarios-common-with-noninteractive-in-term
       ((erc-scenarios-common-dialog "scrolltobottom")
        (dumb-server (erc-d-run "localhost" t 'help))
@@ -402,6 +402,7 @@ erc-scenarios-scrolltobottom--normal
         (erc-cmd-MSG "NickServ help register")
         (save-excursion (erc-d-t-search-for 10 "End of NickServ"))
         (should (= 1 (point)))
+        (redisplay)
         (should (zerop (count-screen-lines (window-start) (window-point))))
         (should (erc-scenarios-common--prompt-past-win-end-p)))
 
-- 
2.41.0

>From 9abdac639b0037fcb11bd39df97a59c534b21fb6 Mon Sep 17 00:00:00 2001
From: "F. Jason Park" <[email-masked]>
Date: Wed, 20 Sep 2023 21:40:36 -0700
Subject: [PATCH 2/4] Exempt text-scale-mode from erc-scrolltobottom-all

* lisp/erc/erc-goodies.el (erc--scrolltobottom-post-ignore-commands):
New variable, a list of commands that should not trigger a re-scroll.
(erc--scrolltobottom-on-post-command): Don't `recenter' when the
current command appears in `erc--scrolltobottom-post-ignore-commands'.
---
 lisp/erc/erc-goodies.el | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/lisp/erc/erc-goodies.el b/lisp/erc/erc-goodies.el
index 6eb015fdd64..431264ec8b4 100644
--- a/lisp/erc/erc-goodies.el
+++ b/lisp/erc/erc-goodies.el
@@ -128,6 +128,11 @@ erc--scrolltobottom-post-force-commands
 That is, ERC recalculates the window's start instead of blindly
 restoring it.")
 
+;; Unfortunately, this doesn't work when `erc-scrolltobottom-relaxed'
+;; is enabled (scaling up still moves the prompt).
+(defvar erc--scrolltobottom-post-ignore-commands '(text-scale-adjust)
+  "Commands to skip instead of force-scroll on `post-command-hook'.")
+
 (defvar erc--scrolltobottom-relaxed-skip-commands
   '(recenter-top-bottom scroll-down-command)
   "Commands exempt from triggering a stash and restore of `window-start'.
@@ -158,7 +163,8 @@ erc--scrolltobottom-on-post-command
              ((= (nth 2 found)
                  (count-screen-lines (window-start) (point-max)))))
         (set-window-start (selected-window) (nth 1 found))
-      (erc--scrolltobottom-confirm))
+      (unless (memq this-command erc--scrolltobottom-post-ignore-commands)
+        (erc--scrolltobottom-confirm)))
     (setq erc--scrolltobottom-window-info nil)))
 
 (defun erc--scrolltobottom-on-pre-command-relaxed ()
-- 
2.41.0

>From 4241661eb4cf568196cf11f77ccbf56a8f23c1d6 Mon Sep 17 00:00:00 2001
From: "F. Jason Park" <[email-masked]>
Date: Fri, 15 Sep 2023 06:08:55 -0700
Subject: [PATCH 3/4] [5.6] Improve erc-warn-about-blank-lines behavior

* etc/ERC-NEWS: Mention more detailed feedback when option
`erc-warn-about-blank-lines' is non-nil.
* lisp/erc/erc-common.el (erc--input-split): Add `abortp' slot to
allow a premature exit while validating prompt input.
* lisp/erc/erc.el (erc-warn-about-blank-lines): Clarify meaning of
"blank lines" and mention interaction with
`erc-send-whitespace-lines'.
(erc-inhibit-multiline-input): Fix inaccurate description in doc
string.
(erc--input-review-functions): Move
`erc--discard-trailing-multiline-nulls' to end of list, after
`erc--run-input-validation-checks'.
(erc--blank-in-multiline-input-p): Remove unused internal function.
(erc--check-prompt-input-for-something): New trivial validation
function to check if the input is empty.
(erc--count-blank-lines): New function that tallies up the number of
blank and whitespace-only lines in the current input.
(erc--check-prompt-explanation): New variable.
(erc--check-prompt-input-for-multiline-blanks): Rework significantly
to provide more informative messages and more sensible behavior for
common cases with respect to relevant option values.
(erc--check-prompt-input-functions): Add new validation function
`erc--check-prompt-for-something'.
(erc--run-input-validation-checks): Set `abortp' slot of
`erc--input-split' when hooks return a non-string, rather than
generate an unhelpful fallback message.  Also print a message when
the variable `erc--check-prompt-explanation' is non-nil.
(erc-send-current-line): When the `abortp' slot of the current
`erc--input-split' object is non-nil, forgo normal input processing.
This fixes a regression in 5.6-git, which emits an error message when
it should instead just exit the command.
(erc--discard-trailing-multiline-nulls): Always run, regardless of
`erc-send-whitespace-lines', and leave a blank line behind when
stripping a message consisting of only blank lines.
(erc--run-send-hooks): Always run hooks and surrounding logic rather
than only when hooks are populated.
* test/lisp/erc/erc-tests.el (erc--refresh-prompt): Add assertion and
use default value for `erc--input-review-functions'.
(erc-ring-previous-command): Use default value for
`erc--input-review-functions'.
(erc--blank-in-multiline-input-p): Remove irrelevant test.
(erc--check-prompt-input-functions): Update expected message.
(erc--discard-trailing-multiline-nulls, erc--count-blank-lines): New
tests.
(erc-tests--check-prompt-input--expect,
erc-tests--check-prompt-input-messages): New helper variables.
(erc--check-prompt-input-for-multiline-blanks,
erc--check-prompt-input-for-multiline-blanks/explanations): New tests.
(erc-send-whitespace-lines): Expect hook-processing logic to pad empty
lines instead of deferring to `erc-send-input-line-function'.
(Bug#66073)
---
 etc/ERC-NEWS               |   8 +-
 lisp/erc/erc-common.el     |   1 +
 lisp/erc/erc.el            | 151 ++++++++++++++++++++++---------
 test/lisp/erc/erc-tests.el | 180 ++++++++++++++++++++++++++++---------
 4 files changed, 256 insertions(+), 84 deletions(-)

diff --git a/etc/ERC-NEWS b/etc/ERC-NEWS
index 05e933930e2..fadd97b65df 100644
--- a/etc/ERC-NEWS
+++ b/etc/ERC-NEWS
@@ -221,6 +221,12 @@ atop any message.  The new companion option 'erc-echo-timestamp-zone'
 determines the default timezone when not specified with a prefix
 argument.
 
+** Option 'erc-warn-about-blank-lines' is more informative.
+Enabled by default, this option now produces more useful feedback
+whenever ERC rejects prompt input containing whitespace-only lines.
+When paired with option 'erc-send-whitespace-lines', ERC echoes a
+tally of blank lines padded and trailing blanks culled.
+
 ** Miscellaneous UX changes.
 Some minor quality-of-life niceties have finally made their way to
 ERC.  For example, fool visibility has become togglable with the new
@@ -281,7 +287,7 @@ For starters, the 'cursor-sensor-functions' property no longer
 contains unique closures and thus no longer proves effective for
 traversing messages.  To compensate, a new property, 'erc-timestamp',
 now spans message bodies but not the newlines delimiting them.  Also
-affecting the `stamp' module is the deprecation of the function
+affecting the 'stamp' module is the deprecation of the function
 'erc-insert-aligned' and its removal from client code.  Additionally,
 the module now merges its 'invisible' property with existing ones and
 includes all white space around stamps when doing so.
diff --git a/lisp/erc/erc-common.el b/lisp/erc/erc-common.el
index 67c2cf8535b..8d896e663b5 100644
--- a/lisp/erc/erc-common.el
+++ b/lisp/erc/erc-common.el
@@ -60,6 +60,7 @@ erc-input
                                                      ((obsolete erc-send-this))
                                                    erc-send-this))))
   (lines nil :type (list-of string))
+  (abortp nil :type (list-of symbol))
   (cmdp nil :type boolean))
 
 (cl-defstruct (erc-server-user (:type vector) :named)
diff --git a/lisp/erc/erc.el b/lisp/erc/erc.el
index ec4fae548c7..76af17583da 100644
--- a/lisp/erc/erc.el
+++ b/lisp/erc/erc.el
@@ -249,7 +249,14 @@ erc-prompt-for-password
   :type 'boolean)
 
 (defcustom erc-warn-about-blank-lines t
-  "Warn the user if they attempt to send a blank line."
+  "Warn the user if they attempt to send a blank line.
+When non-nil, ERC signals a `user-error' upon encountering prompt
+input containing empty or whitespace-only lines.  When nil, ERC
+still inhibits sending but does so silently.  With the companion
+option `erc-send-whitespace-lines' enabled, ERC sends pending
+input and prints a message in the echo area indicating the amount
+of padding and/or stripping applied, if any.  Setting this option
+to nil suppresses such reporting."
   :group 'erc
   :type 'boolean)
 
@@ -261,8 +268,8 @@ erc-send-whitespace-lines
 (defcustom erc-inhibit-multiline-input nil
   "When non-nil, conditionally disallow input consisting of multiple lines.
 Issue an error when the number of input lines submitted for
-sending exceeds this value.  The value t means disallow more
-than 1 line of input."
+sending meets or exceeds this value.  The value t is synonymous
+with a value of 2 and means disallow more than 1 line of input."
   :package-version '(ERC . "5.5")
   :group 'erc
   :type '(choice integer boolean))
@@ -1092,9 +1099,9 @@ erc-pre-send-functions
 
 (define-obsolete-variable-alias 'erc--pre-send-split-functions
   'erc--input-review-functions "30.1")
-(defvar erc--input-review-functions '(erc--discard-trailing-multiline-nulls
-                                      erc--split-lines
-                                      erc--run-input-validation-checks)
+(defvar erc--input-review-functions '(erc--split-lines
+                                      erc--run-input-validation-checks
+                                      erc--discard-trailing-multiline-nulls)
   "Special hook for reviewing and modifying prompt input.
 ERC runs this before clearing the prompt and before running any
 send-related hooks, such as `erc-pre-send-functions'.  Thus, it's
@@ -6421,20 +6428,6 @@ erc--input-line-delim-regexp
 (defvar erc-command-regexp "^/\\([A-Za-z']+\\)\\(\\s-+.*\\|\\s-*\\)$"
   "Regular expression used for matching commands in ERC.")
 
-(defun erc--blank-in-multiline-input-p (lines)
-  "Detect whether LINES contains a blank line.
-When `erc-send-whitespace-lines' is in effect, return nil if
-LINES is multiline or the first line is non-empty.  When
-`erc-send-whitespace-lines' is nil, return non-nil when any line
-is empty or consists of one or more spaces, tabs, or form-feeds."
-  (catch 'return
-    (let ((multilinep (cdr lines)))
-      (dolist (line lines)
-        (when (if erc-send-whitespace-lines
-                  (and (string-empty-p line) (not multilinep))
-                (string-match (rx bot (* (in " \t\f")) eot) line))
-          (throw 'return t))))))
-
 (defun erc--check-prompt-input-for-excess-lines (_ lines)
   "Return non-nil when trying to send too many LINES."
   (when erc-inhibit-multiline-input
@@ -6454,13 +6447,78 @@ erc--check-prompt-input-for-excess-lines
                      (y-or-n-p (concat "Send input " msg "?")))
           (concat "Too many lines " msg))))))
 
-(defun erc--check-prompt-input-for-multiline-blanks (_ lines)
-  "Return non-nil when multiline prompt input has blank LINES."
-  (when (erc--blank-in-multiline-input-p lines)
+(defun erc--check-prompt-input-for-something (string _)
+  (when (string-empty-p string)
     (if erc-warn-about-blank-lines
         "Blank line - ignoring..."
       'invalid)))
 
+(defun erc--count-blank-lines (lines)
+  "Report on the number of whitespace-only and empty LINES.
+Return a list of (BLANKS TO-PAD TO-STRIP).  Expect caller to know
+that BLANKS includes non-empty whitespace-only lines and that no
+padding or stripping has yet occurred."
+  (let ((real 0) (total 0) (pad 0) (strip 0))
+    (dolist (line lines)
+      (if (string-match (rx bot (* (in " \t\f")) eot) line)
+          (progn
+            (cl-incf total)
+            (if (zerop (match-end 0))
+                (cl-incf strip)
+              (cl-incf pad strip)
+              (setq strip 0)))
+        (cl-incf real)
+        (unless (zerop strip)
+          (cl-incf pad strip)
+          (setq strip 0))))
+    (when (and (zerop real) (not (zerop total)) (= total (+ pad strip)))
+      (cl-incf strip (1- pad))
+      (setq pad 1))
+    (list total pad strip)))
+
+(defvar erc--check-prompt-explanation nil
+  "List of strings to print if no validator returns non-nil.")
+
+(defun erc--check-prompt-input-for-multiline-blanks (_ lines)
+  "Return non-nil when multiline prompt input has blank LINES.
+Consider newlines to be intervening delimiters, meaning the
+zero-width logical line between a trailing newline and `eob'
+constitutes a separate message."
+  (pcase-let ((`(,total ,pad ,strip)(erc--count-blank-lines lines)))
+    (cond ((zerop total) nil)
+          ((and erc-warn-about-blank-lines erc-send-whitespace-lines)
+           (let (msg args)
+             (unless (zerop strip)
+               (push "stripping (%d)" msg)
+               (push strip args))
+             (unless (zerop pad)
+               (when msg
+                 (push "and" msg))
+               (push "padding (%d)" msg)
+               (push pad args))
+             (when msg
+               (push "blank" msg)
+               (push (if (> (apply #'+ args) 1) "lines" "line") msg))
+             (when msg
+               (setf msg (nreverse msg)
+                     (car msg) (capitalize (car msg))))
+             (when msg
+               (push (apply #'format (string-join msg " ") (nreverse args))
+                     erc--check-prompt-explanation)
+               nil)))
+          (erc-warn-about-blank-lines
+           (concat (if (= total 1)
+                       (if (zerop strip) "Blank" "Trailing")
+                     (if (= total strip)
+                         (format "%d trailing" strip)
+                       (format "%d blank" total)))
+                   (and (> total 1) (/= total strip) (not (zerop strip))
+                        (format " (%d trailing)" strip))
+                   (if (= total 1) " line" " lines")
+                   " detected (see `erc-send-whitespace-lines')"))
+          (erc-send-whitespace-lines nil)
+          (t 'invalid))))
+
 (defun erc--check-prompt-input-for-point-in-bounds (_ _)
   "Return non-nil when point is before prompt."
   (when (< (point) (erc-beg-of-input-line))
@@ -6481,25 +6539,34 @@ erc--check-prompt-input-for-multiline-command
 
 (defvar erc--check-prompt-input-functions
   '(erc--check-prompt-input-for-point-in-bounds
+    erc--check-prompt-input-for-something
     erc--check-prompt-input-for-multiline-blanks
     erc--check-prompt-input-for-running-process
     erc--check-prompt-input-for-excess-lines
     erc--check-prompt-input-for-multiline-command)
   "Validators for user input typed at prompt.
-Called with latest input string submitted by user and the list of
-lines produced by splitting it.  If any member function returns
-non-nil, processing is abandoned and input is left untouched.
-When the returned value is a string, ERC passes it to `erc-error'.")
+Called with two arguments: the current input submitted by the
+user, as a string, along with the same input as a list of
+strings.  If any member function returns non-nil, ERC abandons
+processing and leaves pending input untouched in the prompt area.
+When the returned value is a string, ERC passes it to
+`user-error'.  Any other non-nil value tells ERC to abort
+silently.  If all members return nil, and the variable
+`erc--check-prompt-explanation' is a nonempty list of strings,
+ERC prints them as a single message joined by newlines.")
 
 (defun erc--run-input-validation-checks (state)
   "Run input checkers from STATE, an `erc--input-split' object."
-  (when-let ((msg (run-hook-with-args-until-success
-                   'erc--check-prompt-input-functions
-                   (erc--input-split-string state)
-                   (erc--input-split-lines state))))
-    (unless (stringp msg)
-      (setq msg (format "Input error: %S" msg)))
-    (user-error msg)))
+  (let* ((erc--check-prompt-explanation nil)
+         (msg (run-hook-with-args-until-success
+               'erc--check-prompt-input-functions
+               (erc--input-split-string state)
+               (erc--input-split-lines state))))
+    (cond ((stringp msg) (user-error msg))
+          (msg (push msg (erc--input-split-abortp state)))
+          (erc--check-prompt-explanation
+           (message "%s" (string-join (nreverse erc--check-prompt-explanation)
+                                      "\n"))))))
 
 (defun erc-send-current-line ()
   "Parse current line and send it to IRC."
@@ -6523,8 +6590,9 @@ erc-send-current-line
                                  str erc--input-line-delim-regexp)
                          :cmdp (string-match erc-command-regexp str))))
             (run-hook-with-args 'erc--input-review-functions state)
-            (let ((inhibit-read-only t)
-                  (old-buf (current-buffer)))
+            (when-let (((not (erc--input-split-abortp state)))
+                       (inhibit-read-only t)
+                       (old-buf (current-buffer)))
               (progn ; unprogn this during next major surgery
                 (erc-set-active-buffer (current-buffer))
                 ;; Kill the input and the prompt
@@ -6553,12 +6621,11 @@ erc-user-input
    (erc-end-of-input-line)))
 
 (defun erc--discard-trailing-multiline-nulls (state)
-  "Ensure last line of STATE's string is non-null.
-But only when `erc-send-whitespace-lines' is non-nil.  STATE is
-an `erc--input-split' object."
-  (when (and erc-send-whitespace-lines (erc--input-split-lines state))
+  "Remove trailing empty lines from STATE, an `erc--input-split' object.
+When all lines are empty, remove all but the first."
+  (when (erc--input-split-lines state)
     (let ((reversed (nreverse (erc--input-split-lines state))))
-      (while (and reversed (string-empty-p (car reversed)))
+      (while (and (cdr reversed) (string-empty-p (car reversed)))
         (setq reversed (cdr reversed)))
       (setf (erc--input-split-lines state) (nreverse reversed)))))
 
@@ -6578,7 +6645,7 @@ erc--run-send-hooks
 limits and pad empty ones, knowing full well that additional
 processing may still corrupt messages before they reach the send
 queue.  Expect LINES-OBJ to be an `erc--input-split' object."
-  (when (or erc-send-pre-hook erc-pre-send-functions)
+  (progn ; FIXME remove `progn' after code review.
     (with-suppressed-warnings ((lexical str) (obsolete erc-send-this))
       (defvar str) ; see note in string `erc-send-input'.
       (let* ((str (string-join (erc--input-split-lines lines-obj) "\n"))
diff --git a/test/lisp/erc/erc-tests.el b/test/lisp/erc/erc-tests.el
index 05d45b2d027..2da1f7b29c1 100644
--- a/test/lisp/erc/erc-tests.el
+++ b/test/lisp/erc/erc-tests.el
@@ -292,7 +292,7 @@ erc--refresh-prompt
                                (cl-incf counter))))
          erc-accidental-paste-threshold-seconds
          erc-insert-modify-hook
-         erc--input-review-functions
+         (erc--input-review-functions erc--input-review-functions)
          erc-send-completed-hook)
 
     (ert-info ("Server buffer")
@@ -357,6 +357,9 @@ erc--refresh-prompt
         (should (= (point) erc-input-marker))
         (insert "/query bob")
         (erc-send-current-line)
+        ;; Last command not inserted
+        (save-excursion (forward-line -1)
+                        (should (looking-at "<tester> Howdy")))
         ;; Query does not redraw (nor /help, only message input)
         (should (looking-back "#chan@ServNet 11> "))
         ;; No sign of old prompts
@@ -877,11 +880,12 @@ erc-ring-previous-command
   (with-current-buffer (get-buffer-create "*#fake*")
     (erc-mode)
     (erc-tests--send-prep)
+    (setq erc-server-current-nick "tester")
     (setq-local erc-last-input-time 0)
     (should-not (local-variable-if-set-p 'erc-send-completed-hook))
     (set (make-local-variable 'erc-send-completed-hook) nil) ; skip t (globals)
     ;; Just in case erc-ring-mode is already on
-    (setq-local erc--input-review-functions nil)
+    (setq-local erc--input-review-functions erc--input-review-functions)
     (add-hook 'erc--input-review-functions #'erc-add-to-input-ring)
     ;;
     (cl-letf (((symbol-function 'erc-process-input-line)
@@ -1056,43 +1060,6 @@ erc--input-line-delim-regexp
     (should (equal '("" "" "") (split-string "\n\n" p)))
     (should (equal '("" "" "") (split-string "\n\r" p)))))
 
-(ert-deftest erc--blank-in-multiline-input-p ()
-  (let ((check (lambda (s)
-                 (erc--blank-in-multiline-input-p
-                  (split-string s erc--input-line-delim-regexp)))))
-
-    (ert-info ("With `erc-send-whitespace-lines'")
-      (let ((erc-send-whitespace-lines t))
-        (should (funcall check ""))
-        (should-not (funcall check "\na"))
-        (should-not (funcall check "/msg a\n")) ; real /cmd
-        (should-not (funcall check "a\n\nb")) ; "" allowed
-        (should-not (funcall check "/msg a\n\nb")) ; non-/cmd
-        (should-not (funcall check " "))
-        (should-not (funcall check "\t"))
-        (should-not (funcall check "a\nb"))
-        (should-not (funcall check "a\n "))
-        (should-not (funcall check "a\n \t"))
-        (should-not (funcall check "a\n \f"))
-        (should-not (funcall check "a\n \nb"))
-        (should-not (funcall check "a\n \t\nb"))
-        (should-not (funcall check "a\n \f\nb"))))
-
-    (should (funcall check ""))
-    (should (funcall check " "))
-    (should (funcall check "\t"))
-    (should (funcall check "a\n\nb"))
-    (should (funcall check "a\n\nb"))
-    (should (funcall check "a\n "))
-    (should (funcall check "a\n \t"))
-    (should (funcall check "a\n \f"))
-    (should (funcall check "a\n \nb"))
-    (should (funcall check "a\n \t\nb"))
-
-    (should-not (funcall check "a\rb"))
-    (should-not (funcall check "a\nb"))
-    (should-not (funcall check "a\r\nb"))))
-
 (defun erc-tests--with-process-input-spy (test)
   (with-current-buffer (get-buffer-create "FakeNet")
     (let* ((erc--input-review-functions
@@ -1138,7 +1105,7 @@ erc--check-prompt-input-functions
        (delete-region (point) (point-max))
        (insert "one\n")
        (let ((e (should-error (erc-send-current-line))))
-         (should (equal "Blank line - ignoring..." (cadr e))))
+         (should (string-prefix-p "Trailing line detected" (cadr e))))
        (goto-char (point-max))
        (ert-info ("Input remains untouched")
          (should (save-excursion (goto-char erc-input-marker)
@@ -1180,6 +1147,137 @@ erc-send-current-line
 
      (should (consp erc-last-input-time)))))
 
+(ert-deftest erc--discard-trailing-multiline-nulls ()
+  (pcase-dolist (`(,input ,want) '((("") (""))
+                                   (("" "") (""))
+                                   (("a") ("a"))
+                                   (("a" "") ("a"))
+                                   (("" "a") ("" "a"))
+                                   (("" "a" "") ("" "a"))))
+    (ert-info ((format "Input: %S, want: %S" input want))
+      (let ((s (make-erc--input-split :lines input)))
+        (erc--discard-trailing-multiline-nulls s)
+        (should (equal (erc--input-split-lines s) want))))))
+
+(ert-deftest erc--count-blank-lines ()
+  (pcase-dolist (`(,input ,want) '((() (0 0 0))
+                                   (("") (1 1 0))
+                                   (("" "") (2 1 1))
+                                   (("" "" "") (3 1 2))
+                                   ((" " "") (2 0 1))
+                                   ((" " "" "") (3 0 2))
+                                   (("" " " "") (3 1 1))
+                                   (("" "" " ") (3 2 0))
+                                   (("a") (0 0 0))
+                                   (("a" "") (1 0 1))
+                                   (("a" " " "") (2 0 1))
+                                   (("a" "" "") (2 0 2))
+                                   (("a" "b") (0 0 0))
+                                   (("a" "" "b") (1 1 0))
+                                   (("a" " " "b") (1 0 0))
+                                   (("" "a") (1 1 0))
+                                   ((" " "a") (1 0 0))
+                                   (("" "a" "") (2 1 1))
+                                   (("" " " "a" "" " ") (4 2 0))
+                                   (("" " " "a" "" " " "") (5 2 1))))
+    (ert-info ((format "Input: %S, want: %S" input want))
+      (should (equal (erc--count-blank-lines input) want)))))
+
+;; Opt `wb': `erc-warn-about-blank-lines'
+;; Opt `sw': `erc-send-whitespace-lines'
+;; `s': " \n",`a': "a\n",`b': "b\n"
+(defvar erc-tests--check-prompt-input--expect
+  ;;  opts     ""  " "   "\n"  "\n "   " \n" "\n\n" "a\n" "a\n " "a\n \nb"
+  '(((+wb -sw) err err   err   err     err   err    err   err    err)
+    ((-wb -sw) nop nop   nop   nop     nop   nop    nop   nop    nop)
+    ((+wb +sw) err (s)   (0 s) (1 s s) (s)   (0 s)  (0 a) (a s)  (a s b))
+    ((-wb +sw) nop (s)   (s)   (s s)   (s)   (s)    (a)   (a s)  (a s b))))
+
+;; Help messages echoed (not IRC message) was emitted
+(defvar erc-tests--check-prompt-input-messages
+  '("Stripping" "Padding"))
+
+(ert-deftest erc--check-prompt-input-for-multiline-blanks ()
+  (erc-tests--with-process-input-spy
+   (lambda (next)
+     (erc-tests--set-fake-server-process "sleep" "1")
+     (should-not erc-send-whitespace-lines)
+     (should erc-warn-about-blank-lines)
+
+     (pcase-dolist (`((,wb ,sw) . ,ex) erc-tests--check-prompt-input--expect)
+       (let ((print-escape-newlines t)
+             (erc-warn-about-blank-lines (eq wb '+wb))
+             (erc-send-whitespace-lines (eq sw '+sw))
+             (samples '("" " " "\n" "\n " " \n" "\n\n"
+                        "a\n" "a\n " "a\n \nb")))
+         (setq ex `(,@ex (a) (a b)) ; baseline, same for all combos
+               samples `(,@samples "a" "a\nb"))
+         (dolist (input samples)
+           (insert input)
+           (ert-info ((format "Opts: %S, Input: %S, want: %S"
+                              (list wb sw) input (car ex)))
+             (ert-with-message-capture messages
+               (pcase-exhaustive (pop ex)
+                 ('err (let ((e (should-error (erc-send-current-line))))
+                         (should (string-match (rx (| "trailing" "blank"))
+                                               (cadr e))))
+                       (should (equal (erc-user-input) input))
+                       (should-not (funcall next)))
+                 ('nop (erc-send-current-line)
+                       (should (equal (erc-user-input) input))
+                       (should-not (funcall next)))
+                 ('clr (erc-send-current-line)
+                       (should (string-empty-p (erc-user-input)))
+                       (should-not (funcall next)))
+                 ((and (pred consp) v)
+                  (erc-send-current-line)
+                  (should (string-empty-p (erc-user-input)))
+                  (setq v (reverse v)) ; don't use `nreverse' here
+                  (while v
+                    (pcase (pop v)
+                      ((and (pred integerp) n)
+                       (should (string-search
+                                (nth n erc-tests--check-prompt-input-messages)
+                                messages)))
+                      ('s (should (equal " \n" (car (funcall next)))))
+                      ('a (should (equal "a\n" (car (funcall next)))))
+                      ('b (should (equal "b\n" (car (funcall next)))))))
+                  (should-not (funcall next))))))
+           (delete-region erc-input-marker (point-max))))))))
+
+(ert-deftest erc--check-prompt-input-for-multiline-blanks/explanations ()
+  (should erc-warn-about-blank-lines)
+  (should-not erc-send-whitespace-lines)
+
+  (let ((erc-send-whitespace-lines t))
+    (pcase-dolist (`(,input ,msg)
+                   '((("") "Padding (1) blank line")
+                     (("" " ") "Padding (1) blank line")
+                     ((" " "") "Stripping (1) blank line")
+                     (("a" "") "Stripping (1) blank line")
+                     (("" "") "Stripping (1) and padding (1) blank lines")
+                     (("" "" "") "Stripping (2) and padding (1) blank lines")
+                     (("" "a" "" "b" "" "c" "" "")
+                      "Stripping (2) and padding (3) blank lines")))
+      (ert-info ((format "Input: %S, Msg: %S" input msg))
+        (let (erc--check-prompt-explanation)
+          (should-not (erc--check-prompt-input-for-multiline-blanks nil input))
+          (should (equal (list msg) erc--check-prompt-explanation))))))
+
+  (pcase-dolist (`(,input ,msg)
+                 '((("") "Blank line detected")
+                   (("" " ") "2 blank lines detected")
+                   ((" " "") "2 blank (1 trailing) lines detected")
+                   (("a" "") "Trailing line detected")
+                   (("" "") "2 blank (1 trailing) lines detected")
+                   (("a" "" "") "2 trailing lines detected")
+                   (("" "a" "" "b" "" "c" "" "")
+                    "5 blank (2 trailing) lines detected")))
+    (ert-info ((format "Input: %S, Msg: %S" input msg))
+      (let ((rv (erc--check-prompt-input-for-multiline-blanks nil input)))
+        (should (equal (concat msg " (see `erc-send-whitespace-lines')")
+                       rv ))))))
+
 (ert-deftest erc-send-whitespace-lines ()
   (erc-tests--with-process-input-spy
    (lambda (next)
@@ -1196,7 +1294,7 @@ erc-send-whitespace-lines
          (erc-bol)
          (should (eq (point) (point-max))))
        (should (equal (funcall next) '("two\n" nil t)))
-       (should (equal (funcall next) '("\n" nil t)))
+       (should (equal (funcall next) '(" \n" nil t)))
        (should (equal (funcall next) '("one\n" nil t))))
 
      (ert-info ("Multiline hunk with trailing newline filtered")
-- 
2.41.0

>From eeedb524c2686245da10aa827a6a9bfcbc54d046 Mon Sep 17 00:00:00 2001
From: "F. Jason Park" <[email-masked]>
Date: Fri, 9 Jun 2023 21:00:03 -0700
Subject: [PATCH 4/4] Detect slash commands in erc--input-review-functions

* lisp/erc/erc.el (erc--input-review-functions): Add
`erc--inhibit-slash-cmd-insertion'.
(erc--check-prompt-input-functions): Move
`erc--check-prompt-input-for-multiline-command' above
`erc--check-prompt-input-for-multiline-blanks'.
(erc--inhibit-slash-cmd-insertion): New function to suppress insertion
of prompt input for slash commands.  Doesn't affect "meta" slash
commands like /SAY.
(erc--send-input-lines): Don't bother checking whether message is a
command.  Instead, trust verdict handed down by message-prep
functions.  This opens the door to optional insertion for debugging
purposes or when echoing command lines in a shell-like fashion.
* test/lisp/erc/erc-tests.el (erc-send-whitespace-lines): clean up
portion dealing with trimming slash commands.  (Bug#66073)
---
 lisp/erc/erc.el            | 17 +++++++++++------
 test/lisp/erc/erc-tests.el |  9 ++-------
 2 files changed, 13 insertions(+), 13 deletions(-)

diff --git a/lisp/erc/erc.el b/lisp/erc/erc.el
index 76af17583da..b9ce6b8e53a 100644
--- a/lisp/erc/erc.el
+++ b/lisp/erc/erc.el
@@ -1101,7 +1101,8 @@ 'erc--pre-send-split-functions
   'erc--input-review-functions "30.1")
 (defvar erc--input-review-functions '(erc--split-lines
                                       erc--run-input-validation-checks
-                                      erc--discard-trailing-multiline-nulls)
+                                      erc--discard-trailing-multiline-nulls
+                                      erc--inhibit-slash-cmd-insertion)
   "Special hook for reviewing and modifying prompt input.
 ERC runs this before clearing the prompt and before running any
 send-related hooks, such as `erc-pre-send-functions'.  Thus, it's
@@ -6540,10 +6541,10 @@ erc--check-prompt-input-for-multiline-command
 (defvar erc--check-prompt-input-functions
   '(erc--check-prompt-input-for-point-in-bounds
     erc--check-prompt-input-for-something
+    erc--check-prompt-input-for-multiline-command
     erc--check-prompt-input-for-multiline-blanks
     erc--check-prompt-input-for-running-process
-    erc--check-prompt-input-for-excess-lines
-    erc--check-prompt-input-for-multiline-command)
+    erc--check-prompt-input-for-excess-lines)
   "Validators for user input typed at prompt.
 Called with two arguments: the current input submitted by the
 user, as a string, along with the same input as a list of
@@ -6568,6 +6569,11 @@ erc--run-input-validation-checks
            (message "%s" (string-join (nreverse erc--check-prompt-explanation)
                                       "\n"))))))
 
+(defun erc--inhibit-slash-cmd-insertion (state)
+  "Don't insert STATE object's message if it's a \"slash\" command."
+  (when (erc--input-split-cmdp state)
+    (setf (erc--input-split-insertp state) nil)))
+
 (defun erc-send-current-line ()
   "Parse current line and send it to IRC."
   (interactive)
@@ -6676,9 +6682,8 @@ erc--send-input-lines
   "Send lines in `erc--input-split-lines' object LINES-OBJ."
   (when (erc--input-split-sendp lines-obj)
     (dolist (line (erc--input-split-lines lines-obj))
-      (unless (erc--input-split-cmdp lines-obj)
-        (when (erc--input-split-insertp lines-obj)
-          (erc-display-msg line)))
+      (when (erc--input-split-insertp lines-obj)
+        (erc-display-msg line))
       (erc-process-input-line (concat line "\n")
                               (null erc-flood-protect)
                               (not (erc--input-split-cmdp lines-obj))))))
diff --git a/test/lisp/erc/erc-tests.el b/test/lisp/erc/erc-tests.el
index 2da1f7b29c1..8a68eca6196 100644
--- a/test/lisp/erc/erc-tests.el
+++ b/test/lisp/erc/erc-tests.el
@@ -1316,17 +1316,12 @@ erc-send-whitespace-lines
        (should-not (funcall next)))
 
      (ert-info ("Multiline command with trailing blank filtered")
-       (pcase-dolist (`(,p . ,q)
-                      '(("/a b\r" "/a b\n") ("/a b\n" "/a b\n")
-                        ("/a b\n\n" "/a b\n") ("/a b\r\n" "/a b\n")
-                        ("/a b\n\n\n" "/a b\n")))
+       (dolist (p '("/a b" "/a b\n" "/a b\n\n" "/a b\n\n\n"))
          (insert p)
          (erc-send-current-line)
          (erc-bol)
          (should (eq (point) (point-max)))
-         (while q
-           (should (pcase (funcall next)
-                     (`(,cmd ,_ nil) (equal cmd (pop q))))))
+         (should (pcase (funcall next) (`(,cmd ,_ nil) (equal cmd "/a b\n"))))
          (should-not (funcall next))))
 
      (ert-info ("Multiline command with non-blanks errors")
-- 
2.41.0