From ad98e21545e5d248e13ef8b124b42ca4f0215f6e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Miha=20Rihtar=C5=A1i=C4=8D?= <miha@kamnitnik.top>
Date: Fri, 16 Jul 2021 17:08:12 +0200
Subject: [PATCH] Improve undoing of RET in comint and eshell

* lisp/comint.el (comint-send-input):
(comint-accumulate):
* lisp/eshell/esh-mode.el (eshell-send-input): Before sending input to
the process, delete it and reinsert it again.  Undoing this
insertion with 'C-/' will delete the region, moving the process mark
back to its original position.
---
 lisp/comint.el          | 24 +++++++++++++++++++++++-
 lisp/eshell/esh-mode.el |  8 ++++++++
 2 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/lisp/comint.el b/lisp/comint.el
index 9e406614b9..f24ec4b6bf 100644
--- a/lisp/comint.el
+++ b/lisp/comint.el
@@ -1890,6 +1890,14 @@ comint-send-input
                           (delete-region pmark start)
                           copy))))
 
+        ;; Delete and reinsert input.  This seems like a no-op, except
+        ;; for the resulting entries in the undo list: undoing this
+        ;; insertion will delete the region, moving the process mark
+        ;; back to its original position.
+        (let ((inhibit-read-only t))
+          (delete-region pmark (point))
+          (insert input))
+
         (unless no-newline
           (insert ?\n))
 
@@ -1933,7 +1941,7 @@ comint-send-input
         ;; in case we get output amidst sending the input.
         (set-marker comint-last-input-start pmark)
         (set-marker comint-last-input-end (point))
-        (set-marker (process-mark proc) (point))
+        (set-marker pmark (point))
         ;; clear the "accumulation" marker
         (set-marker comint-accum-marker nil)
         (let ((comint-input-sender-no-newline no-newline))
@@ -3489,6 +3497,20 @@ comint-accumulate
 The entire accumulated text becomes one item in the input history
 when you send it."
   (interactive)
+  (when-let* ((proc (get-buffer-process (current-buffer)))
+              (pmark (process-mark proc))
+              ((or (marker-position comint-accum-marker)
+                   (set-marker comint-accum-marker pmark)
+                   t))
+              ((>= (point) comint-accum-marker pmark)))
+    ;; Delete and reinsert input.  This seems like a no-op, except for
+    ;; the resulting entries in the undo list: undoing this insertion
+    ;; will delete the region, moving the accumulation marker back to
+    ;; its original position.
+    (let ((text (buffer-substring comint-accum-marker (point)))
+          (inhibit-read-only t))
+      (delete-region comint-accum-marker (point))
+      (insert text)))
   (insert "\n")
   (set-marker comint-accum-marker (point))
   (if comint-input-ring-index
diff --git a/lisp/eshell/esh-mode.el b/lisp/eshell/esh-mode.el
index f9dbce9770..92e1e9eb6a 100644
--- a/lisp/eshell/esh-mode.el
+++ b/lisp/eshell/esh-mode.el
@@ -614,6 +614,14 @@ eshell-send-input
 		  (and eshell-send-direct-to-subprocesses
 		       proc-running-p))
 	(insert-before-markers-and-inherit ?\n))
+      ;; Delete and reinsert input.  This seems like a no-op, except
+      ;; for the resulting entries in the undo list: undoing this
+      ;; insertion will delete the region, moving the process mark
+      ;; back to its original position.
+      (let ((text (buffer-substring eshell-last-output-end (point)))
+            (inhibit-read-only t))
+        (delete-region eshell-last-output-end (point))
+        (insert text))
       (if proc-running-p
 	  (progn
 	    (eshell-update-markers eshell-last-output-end)
-- 
2.32.0