lib/jsonschema: render defaults for submodule options

this relaxes the constraint that options of type `submodule` are always
required, and will render benign default values.

lib/jsonschema/default.nix

@@ -425,7 +425,19 @@ rec {
     # return jsonschema property definition for submodule
     # then (lib.attrNames (option.type.getSubOptions option.loc).opt)
     then
-      exposedModuleInfo // example // description // parseSubOptions { inherit option; }
+      (lib.recursiveUpdate exposedModuleInfo (
+        if (default ? default) then
+          default
+        else
+          {
+            "$exportedModuleInfo" = {
+              required = true;
+            };
+          }
+      ))
+      // example
+      // description
+      // parseSubOptions { inherit option; }
     # throw error if option type is not supported
     else
       notSupported option;

lib/jsonschema/example-interface.nix

@@ -20,7 +20,7 @@
       default = false;
       description = "Is the user an admin?";
     };
-    # a submodule option
+    # a submodule option without default
     services = lib.mkOption {
       type = lib.types.submodule {
         options.opt = lib.mkOption {
@@ -30,6 +30,17 @@
         };
       };
     };
+    # a submodule option with default
+    programs = lib.mkOption {
+      type = lib.types.submodule {
+        options.opt = lib.mkOption {
+          type = lib.types.str;
+          default = "bar";
+          description = "Another submodule option";
+        };
+      };
+      default = { };
+    };
     # attrs of int
     userIds = lib.mkOption {
       type = lib.types.attrsOf lib.types.int;

lib/jsonschema/example-schema.json

@@ -92,6 +92,20 @@
         }
       }
     },
+    "programs": {
+      "$exportedModuleInfo": { "path": ["programs"] },
+      "type": "object",
+      "additionalProperties": false,
+      "properties": {
+        "opt": {
+          "$exportedModuleInfo": { "path": ["programs", "opt"] },
+          "type": "string",
+          "default": "bar",
+          "description": "Another submodule option"
+        }
+      },
+      "default": {}
+    },
     "destinations": {
       "$exportedModuleInfo": { "path": ["destinations"] },
       "additionalProperties": {

lib/jsonschema/test_parseOption.nix

@@ -324,6 +324,7 @@ in
             };
           };
         };
+        default = { };
       };
     };
 
@@ -354,6 +355,7 @@ in
             };
           };
         };
+        default = { };
         required = [ "opt" ];
       };
     };