gnome-flashback: add module support to gnome-panel for installing applets

Co-authored-by: Jan Tojnar <[e-mail]>

nixos/modules/services/x11/desktop-managers/gnome.nix

···
           default = [];
           description = "Other GNOME Flashback sessions to enable.";
         };
+
+        panelModulePackages = mkOption {
+          default = [ pkgs.gnome.gnome-applets ];
+          type = types.listOf types.path;
+          description = ''
+            Packages containing modules that should be made available to <literal>gnome-panel</literal> (usually for applets).
+
+            If you're packaging something to use here, please install the modules in <literal>$out/lib/gnome-panel/modules</literal>.
+          '';
+          example = literalExample "[ pkgs.gnome.gnome-applets ]";
+        };
       };
     };
 
···
             (wm:
               pkgs.gnome.gnome-flashback.mkSessionForWm {
                 inherit (wm) wmName wmLabel wmCommand enableGnomePanel;
+                inherit (cfg.flashback) panelModulePackages;
               }
             ) flashbackWms;
 

pkgs/desktops/gnome/misc/gnome-flashback/default.nix

···
 , pam
 , wrapGAppsHook
 , writeTextFile
-, writeShellScriptBin
 , xkeyboard_config
 , xorg
 , runCommand
+, buildEnv
 }:
 let
   pname = "gnome-flashback";
···
         versionPolicy = "odd-unstable";
       };
 
-      mkSessionForWm = { wmName, wmLabel, wmCommand, enableGnomePanel }:
+      mkSessionForWm = { wmName, wmLabel, wmCommand, enableGnomePanel, panelModulePackages }:
         let
           wmApplication = writeTextFile {
             name = "gnome-flashback-${wmName}-wm";
···
             '';
           };
 
-          executable = writeShellScriptBin "gnome-flashback-${wmName}" ''
-            if [ -z $XDG_CURRENT_DESKTOP ]; then
-              export XDG_CURRENT_DESKTOP="GNOME-Flashback:GNOME"
-            fi
+          # gnome-panel will only look for applets in a single directory so symlink them into here.
+          panelModulesEnv = buildEnv {
+            name = "gnome-panel-modules-env";
+            # We always want to find the built-in panel applets.
+            paths = [ gnome-panel gnome-flashback ] ++ panelModulePackages;
+            pathsToLink = [ "/lib/gnome-panel/modules" ];
+          };
 
-            export XDG_DATA_DIRS=${wmApplication}/share:${gnomeSession}/share:${gnome-flashback}/share:${gnome-panel}/share:$XDG_DATA_DIRS
+          executable = stdenv.mkDerivation {
+            name = "gnome-flashback-${wmName}";
+            nativeBuildInputs = [ glib wrapGAppsHook ];
+            buildInputs = [ gnome-flashback ] ++ lib.optionals enableGnomePanel ([ gnome-panel ] ++ panelModulePackages);
 
-            exec ${gnome-session}/bin/gnome-session --session=gnome-flashback-${wmName} "$@"
-          '';
+            # We want to use the wrapGAppsHook mechanism to wrap gnome-session
+            # with the environment that gnome-flashback and gnome-panel need to
+            # run, including the configured applet packages. This is only possible
+            # in the fixup phase, so turn everything else off.
+            dontUnpack = true;
+            dontConfigure = true;
+            dontBuild = true;
+            dontInstall = true;
+            dontWrapGApps = true; # We want to do the wrapping ourselves.
+
+            # gnome-flashback and gnome-panel need to be added to XDG_DATA_DIRS so that their .desktop files can be found by gnome-session.
+            preFixup = ''
+              makeWrapper ${gnome-session}/bin/gnome-session $out \
+                --add-flags "--session=gnome-flashback-${wmName}" \
+                --set-default XDG_CURRENT_DESKTOP 'GNOME-Flashback:GNOME' \
+                --prefix XDG_DATA_DIRS : '${lib.makeSearchPath "share" ([ wmApplication gnomeSession gnome-flashback ] ++ lib.optional enableGnomePanel gnome-panel)}' \
+                "''${gappsWrapperArgs[@]}" \
+                ${lib.optionalString enableGnomePanel "--set NIX_GNOME_PANEL_MODULESDIR '${panelModulesEnv}/lib/gnome-panel/modules'"}
+            '';
+          };
 
         in
         writeTextFile
···
               [Desktop Entry]
               Name=GNOME Flashback (${wmLabel})
               Comment=This session logs you into GNOME Flashback with ${wmLabel}
-              Exec=${executable}/bin/gnome-flashback-${wmName}
+              Exec=${executable}
               TryExec=${wmCommand}
               Type=Application
               DesktopNames=GNOME-Flashback;GNOME;

pkgs/desktops/gnome/misc/gnome-panel/default.nix

···
     hash = "sha256-nxNQde3GZs8rnKkd41xnA+KxdxwQp3B0FPtlbCilmzs=";
   };
 
+  patches = [
+    # Load modules from path in `NIX_GNOME_PANEL_MODULESDIR` environment variable
+    # instead of gnome-panel’s libdir so that the NixOS module can make gnome-panel
+    # load modules from other packages as well.
+    ./modulesdir-env-var.patch
+  ];
+
   # make .desktop Exec absolute
   postPatch = ''
     patch -p0 <<END_PATCH

pkgs/desktops/gnome/misc/gnome-panel/modulesdir-env-var.patch

···
+diff --git a/gnome-panel/gp-module-manager.c b/gnome-panel/gp-module-manager.c
+index 58447fd84..7af99de7d 100644
+--- a/gnome-panel/gp-module-manager.c
++++ b/gnome-panel/gp-module-manager.c
+@@ -49,8 +49,16 @@ load_modules (GpModuleManager *self)
+ {
+   GDir *dir;
+   const gchar *name;
++  const gchar *modules_dir;
+ 
+-  dir = g_dir_open (MODULESDIR, 0, NULL);
++  modules_dir = g_getenv ("NIX_GNOME_PANEL_MODULESDIR");
++
++  if (!modules_dir) {
++    g_warning ("The NIX_GNOME_PANEL_MODULESDIR environment variable was not set, modules will not be loaded.");
++    return;
++  }
++
++  dir = g_dir_open (modules_dir, 0, NULL);
+   if (!dir)
+     return;
+ 
+@@ -63,7 +71,7 @@ load_modules (GpModuleManager *self)
+       if (!g_str_has_suffix (name, ".so"))
+         continue;
+ 
+-      path = g_build_filename (MODULESDIR, name, NULL);
++      path = g_build_filename (modules_dir, name, NULL);
+       module = gp_module_new_from_path (path);
+       g_free (path);
+