Give a proper type to the sourceFileset option

deployment/check/cli/nixosTest.nix

@@ -16,7 +16,7 @@ in
 
   name = "deployment-cli";
 
-  extraSourceFileset = lib.fileset.unions [
+  sourceFileset = lib.fileset.unions [
     # REVIEW: I would like to be able to grab all of `/deployment` minus
     # `/deployment/check`, but I can't because there is a bunch of other files
     # in `/deployment`. Maybe we can think of a reorg making things more robust

deployment/check/common/nixosTest.nix

@@ -13,6 +13,7 @@ let
     toJSON
     ;
   inherit (lib)
+    types
     fileset
     mkOption
     genAttrs
@@ -27,23 +28,6 @@ let
 
   forConcat = xs: f: concatStringsSep "\n" (map f xs);
 
-  ## A subset of the repository that is necessary for this test. It will be
-  ## copied inside the test. The smaller this set, the faster our CI, because we
-  ## won't need to re-run when things change outside of it.
-  src = fileset.toSource {
-    root = ../../..;
-    fileset = fileset.unions [
-      # NOTE: not the flake itself; it will be overridden.
-      ../../../mkFlake.nix
-      ../../../flake.lock
-      ../../../npins
-
-      ./.
-      config.pathToCwd
-      config.extraSourceFileset
-    ];
-  };
-
   ## We will need to override some inputs by the empty flake, so we make one.
   emptyFlake = runCommandNoCC "empty-flake" { } ''
     mkdir $out
@@ -63,21 +47,35 @@ in
     ## `mkOrder` to put this module's string before something else.
     extraTestScript = mkOption { };
 
-    extraSourceFileset = mkOption {
-      ## FIXME: type? what is the type of a fileset? cf
-      ## https://moduscreate.slack.com/archives/C0T4L4QPR/p1752048299160429
+    sourceFileset = mkOption {
+      ## REVIEW: Upstream to nixpkgs?
+      type = types.mkOptionType {
+        name = "fileset";
+        description = "fileset";
+        descriptionClass = "noun";
+        check = (x: (builtins.tryEval (fileset.unions [ x ])).success);
+        merge = (_: defs: fileset.unions (map (x: x.value) defs));
+      };
       description = ''
         A fileset that will be copied to the deployer node in the current
         working directory. This should contain all the files that are
         necessary to run that particular test, such as the NixOS
-        modules necessary to evaluate a deployment. The common architecture
-        of the tests is already included.
+        modules necessary to evaluate a deployment.
       '';
-      default = (fileset.unions [ ]); # the empty file set
     };
   };
 
   config = {
+    sourceFileset = fileset.unions [
+      # NOTE: not the flake itself; it will be overridden.
+      ../../../mkFlake.nix
+      ../../../flake.lock
+      ../../../npins
+
+      ./.
+      config.pathToCwd
+    ];
+
     acmeNodeIP = config.nodes.acme.networking.primaryIPAddress;
 
     nodes =
@@ -125,8 +123,16 @@ in
         ${n}.wait_for_unit("multi-user.target")
       '')}
 
+      ## A subset of the repository that is necessary for this test. It will be
+      ## copied inside the test. The smaller this set, the faster our CI, because we
+      ## won't need to re-run when things change outside of it.
       with subtest("Unpacking"):
-        deployer.succeed("cp -r --no-preserve=mode ${src}/* .")
+        deployer.succeed("cp -r --no-preserve=mode ${
+          fileset.toSource {
+            root = ../../..;
+            fileset = config.sourceFileset;
+          }
+        }/* .")
 
       with subtest("Configure the network"):
         ${forConcat config.targetMachines (

deployment/check/panel/nixosTest.nix

@@ -125,7 +125,7 @@ in
 
   name = "deployment-panel";
 
-  extraSourceFileset = lib.fileset.unions [
+  sourceFileset = lib.fileset.unions [
     # REVIEW: I would like to be able to grab all of `/deployment` minus
     # `/deployment/check`, but I can't because there is a bunch of other files
     # in `/deployment`. Maybe we can think of a reorg making things more robust