New emacsclient option to either create or reuse an existing frame.

* doc/emacs/misc.texi (emacsclient Options): Document the new option.

* doc/man/emacsclient.1: Mention the new option (bug#51374).
* etc/NEWS: Mention the new option.

* lib-src/emacsclient.c (reuse_frame): New variable.
(longopts): New option.
(decode_options): Decode the new option.
(print_help_and_exit): Document the new option.
(main): Use the new option.

doc/emacs/misc.texi

@@ -1992,6 +1992,11 @@ the new frame displays the @file{*scratch*} buffer by default.  You
 can customize this behavior with the variable @code{initial-buffer-choice}
 (@pxref{Entering Emacs}).
 
+@item -r
+@itemx --reuse-frame
+Create a new graphical @dfn{client frame} if none exists, otherwise
+use an existing Emacs frame.
+
 @item -F @var{alist}
 @itemx --frame-parameters=@var{alist}
 Set the parameters for a newly-created graphical frame

doc/man/emacsclient.1

@@ -69,6 +69,9 @@ start Emacs in daemon mode, and try to connect to it.
 .B -c, \-\-create-frame
 Create a new frame instead of trying to use the current Emacs frame.
 .TP
+.B -r \-\-reuse-frame
+Reuse an existing frame if one exists, otherwise create a new frame.
+.TP
 .B \-F, \-\-frame-parameters=ALIST
 Set the parameters of a newly-created frame.
 .TP

etc/NEWS

@@ -102,6 +102,14 @@ highlights segments of buffer text whose reordering for display is
 suspicious and could be malicious.
 
 
+
+** Emacs server and client changes
+
++++
+*** New command-line option '-r' for emacsclient.
+With this command-line option, Emacs reuses an existing graphical client
+frame if one exists; otherwise a new frame is created.
+
 * Editing Changes in Emacs 29.1
 
 ---

lib-src/emacsclient.c

@@ -116,6 +116,9 @@ static bool eval;
 /* True means open a new frame.  --create-frame etc.  */
 static bool create_frame;
 
+/* True means reuse a frame if it already exists.  */
+static bool reuse_frame;
+
 /* The display on which Emacs should work.  --display.  */
 static char const *display;
 
@@ -165,6 +168,7 @@ static struct option const longopts[] =
   { "tty",	no_argument,       NULL, 't' },
   { "nw",	no_argument,       NULL, 't' },
   { "create-frame", no_argument,   NULL, 'c' },
+  { "reuse-frame", no_argument,   NULL, 'r' },
   { "alternate-editor", required_argument, NULL, 'a' },
   { "frame-parameters", required_argument, NULL, 'F' },
 #ifdef SOCKETS_IN_FILE_SYSTEM
@@ -551,6 +555,11 @@ decode_options (int argc, char **argv)
 	  create_frame = true;
           break;
 
+	case 'r':
+	  create_frame = true;
+	  reuse_frame = true;
+	  break;
+
 	case 'p':
 	  parent_id = optarg;
 	  create_frame = true;
@@ -647,6 +656,8 @@ The following OPTIONS are accepted:\n\
 -nw, -t, --tty 		Open a new Emacs frame on the current terminal\n\
 -c, --create-frame    	Create a new frame instead of trying to\n\
 			use the current Emacs frame\n\
+-r, --reuse-frame	Create a new frame if none exists, otherwise\n\
+			use the current Emacs frame\n\
 ", "\
 -F ALIST, --frame-parameters=ALIST\n\
 			Set the parameters of a new frame\n\
@@ -1941,7 +1952,7 @@ main (int argc, char **argv)
   if (nowait)
     send_to_emacs (emacs_socket, "-nowait ");
 
-  if (!create_frame)
+  if (!create_frame || reuse_frame)
     send_to_emacs (emacs_socket, "-current-frame ");
 
   if (display)