bug#65902: 29.0.92; emacsclient-mail.desktop fails due to complicated escaping

[sbaugh@catern.com]

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Eli Zaretskii <eliz@gnu.org> writes:

>> From: sbaugh@catern.com
>> Date: Fri, 22 Sep 2023 01:36:47 +0000 (UTC)
>> Cc: Spencer Baugh <sbaugh@janestreet.com>, jporterbugs@gmail.com,
>> 	65902@debbugs.gnu.org
>> 
>> > Currently, when emacsclient is invoked like this:
>> >
>> >   $ emacsclient --eval '(func args)' foo bar
>> >
>> > we send to the server the following commands:
>> >
>> >   -eval (func args)
>> >   -eval foo
>> >   -eval bar
>> >
>> > IOW, every command-line argument after --eval is treated as being
>> > implicitly preceded with --eval.
>> >
>> > With your proposal, how will the server know that some of "-eval foo"
>> > commands should cause foo to be added to server-eval-args-left instead
>> > of being evaluated as it does now?
>> 
>> As in the attached patch.
>
> I wish you'd accompanied the patch with some plain-text description of
> the idea, to make it easier to understand and to avoid unnecessary
> misunderstandings.

Ah, well, the basic idea is that for each -eval request we receive in
the protocol, we accumulate the argument as a string into a single list
(instead of immediately turning it into a closure which calls
server-eval-and-print).  Then we let-bind server-eval-args-left to that
list around the calls to to server-eval-and-print, and consume each
argument from the list one by one.

> IIUC, this kind of solution is fine by me, but the protocol of
> accessing and using server-eval-args-left in the Lisp expressions
> specified on the emacsclient command line should be well-documented to
> avoid any confusion and UB.

Added a docstring and included it in NEWS.  I would have put it in the
manual, but it seems rather niche to put in the Emacs manual and I
didn't see a natural place to put it in the Emacs Lisp manual.


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From cf7fcda1374491cc73a751d5636f36a95b2e8669 Mon Sep 17 00:00:00 2001
From: Spencer Baugh <sbaugh@catern.com>
Date: Thu, 21 Sep 2023 21:35:50 -0400
Subject: [PATCH] Add server-eval-args-left to server.el

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

This 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. (bug#65902)
* lisp/server.el (server-eval-args-left): Add.
(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.
---
 etc/NEWS                     | 10 ++++++++++
 etc/emacsclient-mail.desktop |  7 ++-----
 lisp/server.el               | 35 ++++++++++++++++++++++++++++-------
 lisp/startup.el              |  5 ++++-
 4 files changed, 44 insertions(+), 13 deletions(-)

diff --git a/etc/NEWS b/etc/NEWS
index cd4c414a64c..de1d79fcb98 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -121,6 +121,16 @@ Anything following the symbol 'mode-line-format-right-align' in
 right-aligned to is controlled by the new user option
 'mode-line-right-align-edge'.
 
+** 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.
+
 \f
 * Editing Changes in Emacs 30.1
 
diff --git a/etc/emacsclient-mail.desktop b/etc/emacsclient-mail.desktop
index 0a2420ddead..5962fa1764c 100644
--- a/etc/emacsclient-mail.desktop
+++ b/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
diff --git a/lisp/server.el b/lisp/server.el
index c3325e5a24c..65f28cdea23 100644
--- a/lisp/server.el
+++ b/lisp/server.el
@@ -1189,6 +1189,7 @@ server-process-filter
 		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
@@ -1319,8 +1320,7 @@ server-process-filter
                  (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.
@@ -1345,7 +1345,7 @@ server-process-filter
 	    ;; 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
@@ -1394,7 +1394,7 @@ server-process-filter
                  (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)
@@ -1404,22 +1404,43 @@ server-process-filter
     ;; 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.
+
+When the server receives a request to eval one or more strings,
+it stores them in this variable.  Then, until this variable is
+nil, it `pop's a string from this variable and evaluates it with
+`server-eval-and-print'.  Adding or removing strings from this
+variable during this process will affect what is evaluated.
+
+This allows an expression passed to \"emacsclient --eval\" to
+consume one or more subsequent arguments before they're parsed or
+evaluated, with (pop server-eval-args-left).  This is useful if
+those arguments are arbitrary strings which should not be
+evaluated.
+
+See also `command-line-args-left' for a similar variable which
+works for command line 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))
diff --git a/lisp/startup.el b/lisp/startup.el
index 7f601668369..f2c84751211 100644
--- a/lisp/startup.el
+++ b/lisp/startup.el
@@ -120,7 +120,10 @@ command-switch-alist
     "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
-- 
2.41.0