Subject: Re: bug#66700: 30.0.50; Some history is missing with multiple eshell buffers

From: Liu Hui <[email-masked]>
Date: Thu, 7 Dec 2023 18:23:57 +0800

Sean Whitton <[email-masked]> 于2023年12月6日周三 23:33写道：
>
> On Thu 16 Nov 2023 at 05:47pm +08, Liu Hui wrote:
>
> > Jim Porter <[email-masked]> 于2023年11月15日周三 01:15写道：
> >>
> >> On 11/14/2023 4:43 AM, Sean Whitton wrote:
> >> > On Tue 14 Nov 2023 at 02:32pm +02, Eli Zaretskii wrote:
> >> >>
> >> >> IMO, this is too soon to change the default behavior in incompatible
> >> >> ways.  We need to wait for a while, perhaps at least one major
> >> >> release, before we do that.
> >> >
> >> > This is fair enough.  It would be really useful to have the default
> >> > behaviour now for those of used to it from other shells, though -- Liu,
> >> > perhaps you would be interested in adding a defcustom?
> >>
> >> Yeah, I think a defcustom would be good here. That would also make it
> >> easier for us to test out the new code and find any bugs.
> >
> > Thank you all for your suggestions. A defcustom has been added in the
> > attached patch without changing previous behavior.
>
> Thanks.  This looks good, but can you add a NEWS entry, please?
>
> Ideally it would also get documented in the Eshell manual.

Thanks for your feedback. I have updated the patch as suggested.
From acf054d4832ce248d8441dfbb0418b69d4ecfe98 Mon Sep 17 00:00:00 2001
From: Liu Hui <[email-masked]>
Date: Thu, 16 Nov 2023 11:45:28 +0800
Subject: [PATCH] Add option `eshell-history-append'

* lisp/eshell/em-hist.el (eshell-history-append): New option.
(eshell--save-history): New function.
(eshell-hist-initialize):
(eshell-save-some-history): Replace eshell-write-history with
eshell--save-history, which respects the new option.
* doc/misc/eshell.texi (History): Update.
* etc/NEWS: Announce the change.  (Bug#66700)
---
 doc/misc/eshell.texi   | 18 +++++++++++++-----
 etc/NEWS               |  6 ++++++
 lisp/eshell/em-hist.el | 16 +++++++++++++---
 3 files changed, 32 insertions(+), 8 deletions(-)

diff --git a/doc/misc/eshell.texi b/doc/misc/eshell.texi
index e8aa8cdc6a3..389b4cbbc21 100644
--- a/doc/misc/eshell.texi
+++ b/doc/misc/eshell.texi
@@ -1237,11 +1237,19 @@ History
 beginning with @code{foo} is accessible by @code{!foo:n}.
 
 @vindex eshell-history-file-name
-The history ring is loaded from a file at the start of every session,
-and written back to the file at the end of every session.  The file path
-is specified in @code{eshell-history-file-name}.  Unlike other shells,
-such as Bash, Eshell can not be configured to keep a history ring of a
-different size than that of the history file.
+@vindex eshell-history-append
+The history is loaded to the history ring from a file at the start of
+every session, and saved to the file at the end of every session.  The
+default history saving behavior is overwriting the file with the whole
+history ring of the session.  By setting @code{eshell-history-append}
+to non-@code{nil}, the history will be saved by appending new entries
+of that session to the file, which could prevent potential history
+loss with multiple Eshell sessions.  The file path is specified in
+@code{eshell-history-file-name}.  Unlike other shells, such as Bash,
+Eshell can not be configured to control the size of the history file.
+Especially, when @code{eshell-history-append} is non-@code{nil}, the
+size of file will keep increasing and the recommended way to truncate
+the file is to run the @samp{history -w} command in an Eshell session.
 
 Since the default buffer navigation and searching key-bindings are
 still present in the Eshell buffer, the commands for history
diff --git a/etc/NEWS b/etc/NEWS
index c55719416d3..e14ab4aed27 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -617,6 +617,12 @@ calling external rgrep.
 +++
 *** If a command exits abnormally, the Eshell prompt now shows its exit code.
 
++++
+*** New user option 'eshell-history-append'.
+If non-nil, each Eshell session will save history by appending new
+entries of that session to the history file rather than overwriting
+the file with the whole history of the session.  The default is nil.
+
 ** Minibuffer and Completions
 
 *** New commands 'previous-line-completion' and 'next-line-completion'.
diff --git a/lisp/eshell/em-hist.el b/lisp/eshell/em-hist.el
index cf03f8399a6..79336204847 100644
--- a/lisp/eshell/em-hist.el
+++ b/lisp/eshell/em-hist.el
@@ -116,6 +116,12 @@ eshell-save-history-on-exit
 		 (const :tag "Ask" ask)
 		 (const :tag "Always save" t)))
 
+(defcustom eshell-history-append nil
+  "If non-nil, append new entries to the history file when saving history."
+  :type '(choice (const :tag "Overwrite history file" nil)
+		 (const :tag "Append new entries to file" t))
+  :version "30.1")
+
 (defcustom eshell-input-filter 'eshell-input-filter-default
   "Predicate for filtering additions to input history.
 Takes one argument, the input.  If non-nil, the input may be saved on
@@ -294,17 +300,21 @@ eshell-hist-initialize
     (if eshell-history-file-name
 	(eshell-read-history nil t))
 
-    (add-hook 'eshell-exit-hook #'eshell-write-history nil t))
+    (add-hook 'eshell-exit-hook #'eshell--save-history nil t))
 
   (unless eshell-history-ring
     (setq eshell-history-ring (make-ring eshell-history-size)))
 
-  (add-hook 'eshell-exit-hook #'eshell-write-history nil t)
+  (add-hook 'eshell-exit-hook #'eshell--save-history nil t)
 
   (add-hook 'kill-emacs-query-functions #'eshell-save-some-history)
 
   (add-hook 'eshell-input-filter-functions #'eshell-add-to-history nil t))
 
+(defun eshell--save-history ()
+  "Save the history for current Eshell buffer."
+  (eshell-write-history nil eshell-history-append))
+
 (defun eshell-save-some-history ()
   "Save the history for any open Eshell buffers."
   (dolist (buf (buffer-list))
@@ -318,7 +328,7 @@ eshell-save-some-history
 			(format-message
 			 "Save input history for Eshell buffer `%s'? "
 			 (buffer-name buf)))))
-	      (eshell-write-history)))))
+	      (eshell--save-history)))))
   t)
 
 (defun eshell/history (&rest args)
-- 
2.25.1