dev(extensions): improved configuration support

Extensions now read configuration objects under the "extensions" block
in the backend configuration. Additionally, a `config.json` for default
values directly in the extension's directory is now supported.

extensions/hellodriver/config.json

@@ -0,0 +1,3 @@
+{
+    "test": "yes I am a test"
+}

extensions/hellodriver/hellodriver.js

@@ -48,6 +48,11 @@ extension.on('create.interfaces', event => {
  * permission to access this driver:
  *
  *   service:no-frills:ii:hello-world
+ *
+ * The value of `<subject>` can be one of many "special" values
+ * to demonstrate capabilities of drivers or extensions, including:
+ * - `%fail%`: simulate an error response from a driver
+ * - `%config%`: return the effective configuration object
  */
 extension.on('create.drivers', event => {
     event.createDriver('hello-world', 'no-frills', {
@@ -55,6 +60,9 @@ extension.on('create.drivers', event => {
             if ( subject === '%fail%' ) {
                 throw new Error('failing on purpose');
             }
+            if ( subject === '%config%' ) {
+                return JSON.stringify(config ?? null);
+            }
             return `Hello, ${subject ?? 'World'}!`;
         },
     });

extensions/hellodriver/package.json

@@ -0,0 +1,5 @@
+{
+    "name": "hellodriver",
+    "main": "hellodriver.js",
+    "type": "module"
+}

src/backend/src/Kernel.js

@@ -34,6 +34,7 @@ const uuid = require('uuid');
 const readline = require("node:readline/promises");
 const { RuntimeModuleRegistry } = require("./extension/RuntimeModuleRegistry");
 const { RuntimeModule } = require("./extension/RuntimeModule");
+const deep_proto_merge = require("./config/deep_proto_merge");
 
 const { quot } = libs.string;
 
@@ -381,6 +382,17 @@ class Kernel extends AdvancedBase {
                     mod_entry.jsons.puter = obj;
                 })());
             }
+
+            const config_json_path = path_.join(mod_path, 'config.json');
+            if ( fs.existsSync(config_json_path) ) {
+                promises.push((async () => {
+                    const buffer = await fs.promises.readFile(config_json_path);
+                    const json = buffer.toString();
+                    const obj = JSON.parse(json);
+                    mod_entry.priority = obj.priority ?? mod_entry.priority;
+                    mod_entry.jsons.config = obj;
+                })());
+            }
             
             // Copy mod contents to `/mod_packages`
             promises.push(fs.promises.cp(mod_path, mod_package_dir, {
@@ -445,6 +457,14 @@ class Kernel extends AdvancedBase {
         
         const packageJSON = mod_entry.jsons.package;
         
+        Object.defineProperty(mod.extension, 'config', {
+            get: () => {
+                const builtin_config = mod_entry.jsons.config ?? {};
+                const user_config = require('./config').extensions?.[packageJSON.name] ?? {};
+                return deep_proto_merge(user_config, builtin_config);
+            },
+        });
+        
         const maybe_promise = (typ => typ.trim().toLowerCase())(packageJSON.type ?? '') === 'module'
             ? await import(path_.join(require_dir, packageJSON.main ?? 'index.js'))
             : require(require_dir);
@@ -474,10 +494,6 @@ class Kernel extends AdvancedBase {
             mod.extension.on('init', exportObject.init);
         }
 
-        Object.defineProperty(mod.extension, 'config', {
-            get: () => require('./config').services?.[extension_name] ?? {},
-        });
-        
         // This is where the 'install' event gets triggered
         await mod.install(context);
     }