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): Document the change.
* etc/NEWS: Announce the change.  (Bug#66700)

doc/misc/eshell.texi

@@ -1237,11 +1237,20 @@ command containing @code{foo}.  The n-th argument of the last command
 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 the file
+@code{eshell-history-file-name} at the start of every session, and
+saved to that file at the end of every session.  The default history
+saving behavior is to overwrite the history file with the whole
+history ring of the session.  If @code{eshell-history-append} is
+non-@code{nil}, the history will instead be saved by appending new
+entries from the session to the history file, which could prevent
+potential history loss with multiple Eshell sessions.  Unlike other
+shells, such as Bash, Eshell cannot currently be configured to control
+the size of the history file.  In particular, when
+@code{eshell-history-append} is non-@code{nil}, the size of the 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

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'.

lisp/eshell/em-hist.el

@@ -116,6 +116,12 @@ If set to t, history will always be saved, silently."
 		 (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 @@ Returns nil if INPUT is prepended by blank space, otherwise non-nil."
     (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 @@ Returns nil if INPUT is prepended by blank space, otherwise non-nil."
 			(format-message
 			 "Save input history for Eshell buffer `%s'? "
 			 (buffer-name buf)))))
-	      (eshell-write-history)))))
+	      (eshell--save-history)))))
   t)
 
 (defun eshell/history (&rest args)