services/emacs: Prevent deletion of socket file

When running a socket-activated emacs service, we don't want emacs to
remove the socket file after exiting, because then subsequent
invocations of `emacsclient` won't be able to use the socket to start
emacs.service again.

modules/services/emacs.nix

@@ -35,7 +35,8 @@ let
 
   # Match the default socket path for the Emacs version so emacsclient continues
   # to work without wrapping it.
-  socketPath = "%t/emacs/server";
+  socketDir = "%t/emacs";
+  socketPath = "${socketDir}/server";
 
 in {
   meta.maintainers = [ maintainers.tadfisher ];
@@ -112,6 +113,13 @@ in {
           SuccessExitStatus = 15;
 
           Restart = "on-failure";
+        } // optionalAttrs (cfg.socketActivation.enable) {
+          # Use read-only directory permissions to prevent emacs from
+          # deleting systemd's socket file before exiting.
+          ExecStartPost =
+            "${pkgs.coreutils}/bin/chmod --changes -w ${socketDir}";
+          ExecStopPost =
+            "${pkgs.coreutils}/bin/chmod --changes +w ${socketDir}";
         };
       } // optionalAttrs (!cfg.socketActivation.enable) {
         Install = { WantedBy = [ "default.target" ]; };

tests/modules/services/emacs/emacs-socket-27.nix

@@ -31,7 +31,7 @@ in {
       assertFileContent home-files/.config/systemd/user/emacs.service \
                         ${
                           pkgs.substituteAll {
-                            inherit (pkgs) runtimeShell;
+                            inherit (pkgs) runtimeShell coreutils;
                             src = ./emacs-socket-emacs.service;
                           }
                         }

tests/modules/services/emacs/emacs-socket-28.nix

@@ -31,7 +31,7 @@ in {
       assertFileContent home-files/.config/systemd/user/emacs.service \
                         ${
                           pkgs.substituteAll {
-                            inherit (pkgs) runtimeShell;
+                            inherit (pkgs) runtimeShell coreutils;
                             src = ./emacs-socket-emacs.service;
                           }
                         }

tests/modules/services/emacs/emacs-socket-emacs.service

@@ -1,5 +1,7 @@
 [Service]
 ExecStart=@runtimeShell@ -l -c "@emacs@/bin/emacs --fg-daemon='%t/emacs/server'"
+ExecStartPost=@coreutils@/bin/chmod --changes -w %t/emacs
+ExecStopPost=@coreutils@/bin/chmod --changes +w %t/emacs
 Restart=on-failure
 SuccessExitStatus=15
 Type=notify