From 7fd6fa4a8b71615fa21dc990503736d4405193c6 Mon Sep 17 00:00:00 2001
From: Sean Whitton <spwhitton@spwhitton.name>
Date: Sun, 8 Feb 2026 16:52:12 +0000
Subject: [PATCH] Revert "Extend emacs server protocol for empty arguments"

This reverts this change:

  Author:     Andreas Schwab <schwab@linux-m68k.org>
  AuthorDate: Sun Feb 8 12:34:02 2026 +0100

    Extend emacs server protocol for empty arguments

    An empty argument is represented by &0.  On the receiving side, &0 is
    replaced by nothing.

    * lisp/server.el (server-unquote-arg): Replace "&0" by nothing.
    (server-quote-arg): Produce "&0" for an empty string.
    * lib-src/emacsclient.c (quote_argument): Produce "&0" for an
    empty string.
    (unquote_argument): Replace "&0" by nothing.   (Bug#80356)

The bug in question was already fixed by this change:

  Author:     Sean Whitton <spwhitton@spwhitton.name>
  AuthorDate: Fri Nov 7 12:33:21 2025 +0000

    Don't discard empty string arguments from emacsclient

    * lisp/server.el (server--process-filter-1): Don't discard empty
    string arguments from emacsclient.
    (server-eval-args-left):
    * doc/emacs/misc.texi (emacsclient Options):
    * etc/NEWS: Document the change.
---
 lib-src/emacsclient.c |  6 +-----
 lisp/server.el        | 20 ++++++++------------
 2 files changed, 9 insertions(+), 17 deletions(-)

diff --git a/lib-src/emacsclient.c b/lib-src/emacsclient.c
index ba08d7dd6d3..4f3215ea6b1 100644
--- a/lib-src/emacsclient.c
+++ b/lib-src/emacsclient.c
@@ -867,10 +867,8 @@ send_to_emacs (HSOCKET s, const char *data)
 static void
 quote_argument (HSOCKET s, const char *str)
 {
-  char *copy = xmalloc (strlen (str) * 2 + 3);
+  char *copy = xmalloc (strlen (str) * 2 + 1);
   char *q = copy;
-  if (*str == '\0')
-    *q++ = '&', *q++ = '0';
   if (*str == '-')
     *q++ = '&', *q++ = *str++;
   for (; *str; str++)
@@ -912,8 +910,6 @@ unquote_argument (char *str)
 	    c = ' ';
 	  else if (c == 'n')
 	    c = '\n';
-	  else if (c == '0')
-	    continue;
 	}
       *q++ = c;
     }
diff --git a/lisp/server.el b/lisp/server.el
index 88b370d15a2..fcfc6c01972 100644
--- a/lisp/server.el
+++ b/lisp/server.el
@@ -531,7 +531,6 @@ See `server-quote-arg' and `server-process-filter'."
 	    (?& "&")
 	    (?- "-")
 	    (?n "\n")
-	    (?0 "")
 	    (_ " ")))
    arg t t))
 
@@ -539,19 +538,16 @@ See `server-quote-arg' and `server-process-filter'."
   "In ARG, insert a & before each &, each space, each newline, and -.
 Change spaces to underscores, too, so that the return value never
 contains a space.
-An empty ARG is represented by &0.
 
 See `server-unquote-arg' and `server-process-filter'."
-  (if (equal arg "")
-      "&0"
-    (replace-regexp-in-string
-     "[-&\n ]" (lambda (s)
-	         (pcase (aref s 0)
-		   (?& "&&")
-		   (?- "&-")
-		   (?\n "&n")
-		   (?\s "&_")))
-     arg t t)))
+  (replace-regexp-in-string
+   "[-&\n ]" (lambda (s)
+	       (pcase (aref s 0)
+		 (?& "&&")
+		 (?- "&-")
+		 (?\n "&n")
+		 (?\s "&_")))
+   arg t t))
 
 (defun server-send-string (proc string)
   "A wrapper around `process-send-string' for logging."