sway: make it possible to alter conf in preCheckConfig

Some situations may require altering the config file itself to remove or
adjust things. While this of course reduces the correctness of the
check, it may be better to lose a little than be forced to disable the
check altogether.

modules/services/window-managers/i3-sway/sway.nix

@@ -269,8 +269,9 @@ let
     checkPhase = lib.optionalString cfg.checkConfig ''
       export DBUS_SESSION_BUS_ADDRESS=/dev/null
       export XDG_RUNTIME_DIR=$(mktemp -d)
+      cp "$target" sway.conf
       ${cfg.preCheckConfig}
-      ${pkgs.xvfb-run}/bin/xvfb-run ${swayPackage}/bin/sway --config "$target" --validate --unsupported-gpu
+      ${pkgs.xvfb-run}/bin/xvfb-run ${swayPackage}/bin/sway --config sway.conf --validate --unsupported-gpu
     '';
 
     text = concatStringsSep "\n"
@@ -483,12 +484,15 @@ in {
       type = types.lines;
       default = "";
       example = ''
+        sed 's/foo/bar/g' -i sway.conf
         export HOME=$(mktemp -d)
         touch ~/background.png
       '';
       description = ''
         Script to run before running `sway --validate` on the generated config
-        file. Only used if `checkConfig = true`.
+        file. The config file is available as `sway.conf`, and can be
+        manipulated without affecting the build output. Only used if
+        `checkConfig = true`.
       '';
     };
 

tests/modules/services/window-managers/sway/sway-check-config-precheck.nix

@@ -6,12 +6,15 @@ lib.mkIf config.test.enableBig {
     checkConfig = true;
     preCheckConfig = ''
       export HOME=$(mktemp -d)
-      touch ~/mybg.png
+      sed 's/mybg/otherbg/g' -i sway.conf
+      touch ~/otherbg.png
     '';
     config.output."*".background = "~/mybg.png fill";
   };
 
   nmt.script = ''
     assertFileExists home-files/.config/sway/config
+    assertFileRegex home-files/.config/sway/config 'mybg\.png'
+    assertFileNotRegex home-files/.config/sway/config 'otherbg\.png'
   '';
 }