Enable/disable 'server-mode' when starting/stopping the server

* lisp/server.el (server-mode-map): New keymap...
(server-mode): ... use it.
(server-start): Update the 'server-mode' variable (and sync to
'global-minor-modes') when starting/stopping the server.

* test/lisp/server-tests.el: New file (bug#58909).

lisp/server.el

@@ -670,7 +670,6 @@ the `server-process' variable."
                             "/tmp/")
                  (ignore-errors
                    (delete-directory (file-name-directory server-file))))))
-         (setq server-mode nil) ;; already set by the minor mode code
          (display-warning
           'server
           (concat "Unable to start the Emacs server.\n"
@@ -688,7 +687,9 @@ server or call `\\[server-force-delete]' to forcibly disconnect it."))
       (if leave-dead
 	  (progn
 	    (unless (eq t leave-dead) (server-log (message "Server stopped")))
-	    (setq server-process nil))
+            (setq server-mode nil
+                  global-minor-modes (delq 'server-mode global-minor-modes)
+                  server-process nil))
 	;; Make sure there is a safe directory in which to place the socket.
 	(server-ensure-safe-dir server-dir)
 	(when server-process
@@ -728,6 +729,8 @@ server or call `\\[server-force-delete]' to forcibly disconnect it."))
 			       :plist '(:authenticated t)))))
 	  (unless server-process (error "Could not start server process"))
 	  (process-put server-process :server-file server-file)
+          (setq server-mode t)
+          (push 'server-mode global-minor-modes)
 	  (when server-use-tcp
 	    (let ((auth-key (server-get-auth-key)))
 	      (process-put server-process :auth-key auth-key)
@@ -796,6 +799,10 @@ by the current Emacs process, use the `server-process' variable."
 	t)
     (file-error nil)))
 
+;; This keymap is empty, but allows users to define keybindings to use
+;; when `server-mode' is active.
+(defvar-keymap server-mode-map)
+
 ;;;###autoload
 (define-minor-mode server-mode
   "Toggle Server mode.
@@ -805,6 +812,7 @@ Server mode runs a process that accepts commands from the
 `server-start' for details."
   :global t
   :version "22.1"
+  :keymap server-mode-map
   ;; Fixme: Should this check for an existing server socket and do
   ;; nothing if there is one (for multiple Emacs sessions)?
   (server-start (not server-mode)))

test/lisp/server-tests.el

@@ -0,0 +1,41 @@
+;;; server-tests.el --- Emacs server test suite  -*- lexical-binding:t -*-
+
+;; Copyright (C) 2022 Free Software Foundation, Inc.
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Code:
+
+(require 'ert)
+(require 'server)
+
+;;; Tests:
+
+(ert-deftest server-test/server-start-sets-minor-mode ()
+  "Ensure that calling `server-start' also sets `server-mode' properly."
+  (server-start)
+  (unwind-protect
+      (progn
+        ;; Make sure starting the server activates the minor mode.
+        (should (eq server-mode t))
+        (should (memq 'server-mode global-minor-modes)))
+    ;; Always stop the server, even if the above checks fail.
+    (server-start t))
+  ;; Make sure stopping the server deactivates the minor mode.
+  (should (eq server-mode nil))
+  (should-not (memq 'server-mode global-minor-modes)))
+
+;;; server-tests.el ends here