Add 'server-eval-args-left' to server.el

Passing arbitrary arguments to functions through
"emacsclient --eval" sometimes requires complicated escaping
to avoid them being parsed as Lisp (as seen in
emacsclient-mail.desktop before this change).

The new variable 'server-eval-args-left' allows access to the
arguments before they are parsed as Lisp.  By removing
arguments from the variable before they're parsed, a snippet
of Lisp can consume arguments, as in emacsclient-mail.desktop.

org-protocol might be able to use this as well, which might allow it
to drop its current advice on server-visit-files.

* etc/emacsclient-mail.desktop: Use 'server-eval-args-left'.
* lisp/server.el (server-eval-args-left): New variable.
(server-process-filter, server-execute): Make '-eval' arguments
available through 'server-eval-args-left'.
* lisp/startup.el (argv): Mention 'server-eval-args-left' in
docstring.
* etc/NEWS: Announce 'server-eval-args-left'.
* doc/emacs/misc.texi (emacsclient Options): Document
'server-eval-args-left'.  (Bug#65902)

doc/emacs/misc.texi

@@ -2078,6 +2078,15 @@ files.  When this option is given, the arguments to
 @command{emacsclient} are interpreted as a list of expressions to
 evaluate, @emph{not} as a list of files to visit.
 
+@vindex server-eval-args-left
+If you have arbitrary data which you want to provide as input to one
+of your expressions, you can pass the data as another argument to
+@command{emacsclient} and use @var{server-eval-args-left} in the
+expression to access the data.  Be careful to have your expression
+remove the data from @var{server-eval-args-left} regardless of whether
+your code succeeds, such as by using @code{pop}, otherwise Emacs will
+attempt to evaluate the data as a Lisp expression.
+
 @item -f @var{server-file}
 @itemx --server-file=@var{server-file}
 Specify a server file (@pxref{TCP Emacs server}) for connecting to an

etc/NEWS

@@ -233,6 +233,16 @@ to enter the file you want to modify.
 It can be used to customize the look of the appointment notification
 displayed on the mode line when 'appt-display-mode-line' is non-nil.
 
+** Emacs Server and Client
+
+---
+*** 'server-eval-args-left' can be used to pop subsequent eval args
+When '--eval' is passed to emacsclient and Emacs is evaluating each
+argument, this variable is set to those which have not yet been
+evaluated.  It can be used to 'pop' arguments to prevent them from
+being evaluated, which is useful when those arguments contain
+arbitrary data.
+
 
 * Editing Changes in Emacs 30.1
 

etc/emacsclient-mail.desktop

@@ -1,10 +1,7 @@
 [Desktop Entry]
 Categories=Network;Email;
 Comment=GNU Emacs is an extensible, customizable text editor - and more
-# We want to pass the following commands to the shell wrapper:
-# u=$(echo "$1" | sed 's/[\"]/\\&/g'); exec emacsclient --alternate-editor= --display="$DISPLAY" --eval "(message-mailto \"$u\")"
-# Special chars '"', '$', and '\' must be escaped as '\\"', '\\$', and '\\\\'.
-Exec=sh -c "u=\\$(echo \\"\\$1\\" | sed 's/[\\\\\\"]/\\\\\\\\&/g'); exec emacsclient --alternate-editor= --display=\\"\\$DISPLAY\\" --eval \\"(message-mailto \\\\\\"\\$u\\\\\\")\\"" sh %u
+Exec=emacsclient --alternate-editor= --eval "(message-mailto (pop server-eval-args-left))" %u
 Icon=emacs
 Name=Emacs (Mail, Client)
 MimeType=x-scheme-handler/mailto;
@@ -16,7 +13,7 @@ Actions=new-window;new-instance;
 
 [Desktop Action new-window]
 Name=New Window
-Exec=sh -c "u=\\$(echo \\"\\$1\\" | sed 's/[\\\\\\"]/\\\\\\\\&/g'); exec emacsclient --alternate-editor= --create-frame --eval \\"(message-mailto \\\\\\"\\$u\\\\\\")\\"" sh %u
+Exec=emacsclient --alternate-editor= --create-frame --eval "(message-mailto (pop server-eval-args-left))" %u
 
 [Desktop Action new-instance]
 Name=New Instance

lisp/server.el

@@ -1199,6 +1199,7 @@ The following commands are accepted by the client:
 		parent-id  ; Window ID for XEmbed
 		dontkill   ; t if client should not be killed.
 		commands
+		evalexprs
 		dir
 		use-current-frame
 		frame-parameters  ;parameters for newly created frame
@@ -1332,8 +1333,7 @@ The following commands are accepted by the client:
                  (let ((expr (pop args-left)))
                    (if coding-system
                        (setq expr (decode-coding-string expr coding-system)))
-                   (push (lambda () (server-eval-and-print expr proc))
-                         commands)
+                   (push expr evalexprs)
                    (setq filepos nil)))
 
                 ;; -env NAME=VALUE:  An environment variable.
@@ -1358,7 +1358,7 @@ The following commands are accepted by the client:
 	    ;; arguments, use an existing frame.
 	    (and nowait
 		 (not (eq tty-name 'window-system))
-		 (or files commands)
+		 (or files commands evalexprs)
 		 (setq use-current-frame t))
 
 	    (setq frame
@@ -1407,7 +1407,7 @@ The following commands are accepted by the client:
                  (let ((default-directory
                          (if (and dir (file-directory-p dir))
                              dir default-directory)))
-                   (server-execute proc files nowait commands
+                   (server-execute proc files nowait commands evalexprs
                                    dontkill frame tty-name)))))
 
             (when (or frame files)
@@ -1417,22 +1417,35 @@ The following commands are accepted by the client:
     ;; condition-case
     (t (server-return-error proc err))))
 
-(defun server-execute (proc files nowait commands dontkill frame tty-name)
+(defvar server-eval-args-left nil
+  "List of eval args not yet processed.
+
+Adding or removing strings from this variable while the Emacs
+server is processing a series of eval requests will affect what
+Emacs evaluates.
+
+See also `argv' for a similar variable which works for
+invocations of \"emacs\".")
+
+(defun server-execute (proc files nowait commands evalexprs dontkill frame tty-name)
   ;; This is run from timers and process-filters, i.e. "asynchronously".
   ;; But w.r.t the user, this is not really asynchronous since the timer
   ;; is run after 0s and the process-filter is run in response to the
   ;; user running `emacsclient'.  So it is OK to override the
-  ;; inhibit-quit flag, which is good since `commands' (as well as
+  ;; inhibit-quit flag, which is good since `evalexprs' (as well as
   ;; find-file-noselect via the major-mode) can run arbitrary code,
   ;; including code that needs to wait.
   (with-local-quit
     (condition-case err
         (let ((buffers (server-visit-files files proc nowait)))
           (mapc 'funcall (nreverse commands))
+          (let ((server-eval-args-left (nreverse evalexprs)))
+            (while server-eval-args-left
+              (server-eval-and-print (pop server-eval-args-left) proc)))
 	  ;; If we were told only to open a new client, obey
 	  ;; `initial-buffer-choice' if it specifies a file
           ;; or a function.
-          (unless (or files commands)
+          (unless (or files commands evalexprs)
             (let ((buf
                    (cond ((stringp initial-buffer-choice)
 			  (find-file-noselect initial-buffer-choice))

lisp/startup.el

@@ -120,7 +120,10 @@ the remaining command-line args are in the variable `command-line-args-left'.")
     "List of command-line args not yet processed.
 This is a convenience alias, so that one can write (pop argv)
 inside of --eval command line arguments in order to access
-following arguments."))
+following arguments.
+
+See also `server-eval-args-left' for a similar variable which
+works for invocations of \"emacsclient --eval\"."))
 (internal-make-var-non-special 'argv)
 
 (defvar command-line-args-left nil