Validate plugin entrypoint

Check that Master/Instance/Control plugins export the correct class and
that this class is a subclass of the correct base class.  This gives a
better user experience when plugins are incorrectly set up and loads a
duplicated clusterio/lib, and when accidentally using the wrong name
when developing plugins.

packages/lib/plugin_loader.js

@@ -6,6 +6,7 @@
 "use strict";
 
 const libErrors = require("./errors");
+const libPlugin = require("./plugin");
 const path = require("path");
 
 
@@ -48,6 +49,22 @@ async function loadPluginInfos(pluginList) {
 	return plugins;
 }
 
+function loadPluginClass(entrypointName, className, pluginClass, pluginInfo) {
+	let resolvedPath = path.posix.join(pluginInfo.requirePath, pluginInfo[entrypointName]);
+	let entrypoint = require(resolvedPath);
+	if (!entrypoint[className]) {
+		throw new libErrors.PluginError(pluginInfo.name,
+			new Error(`Expected ${resolvedPath} to export a class named ${className}`)
+		);
+	}
+	if (!(entrypoint[className].prototype instanceof pluginClass)) {
+		throw new libErrors.PluginError(pluginInfo.name,
+			new Error(`Expected ${className} exported from ${resolvedPath} to be a subclass of ${pluginClass.name}`)
+		);
+	}
+	return entrypoint[className];
+}
+
 /**
  * Load master plugin class of a plugin
  *
@@ -58,8 +75,7 @@ async function loadPluginInfos(pluginList) {
  * @static
  */
 async function loadMasterPluginClass(pluginInfo) {
-	let entrypoint = require(path.posix.join(pluginInfo.requirePath, pluginInfo.masterEntrypoint));
-	return entrypoint.MasterPlugin;
+	return loadPluginClass("masterEntrypoint", "MasterPlugin", libPlugin.BaseMasterPlugin, pluginInfo);
 }
 
 /**
@@ -72,8 +88,7 @@ async function loadMasterPluginClass(pluginInfo) {
  * @static
  */
 async function loadInstancePluginClass(pluginInfo) {
-	let entrypoint = require(path.posix.join(pluginInfo.requirePath, pluginInfo.instanceEntrypoint));
-	return entrypoint.InstancePlugin;
+	return loadPluginClass("instanceEntrypoint", "InstancePlugin", libPlugin.BaseInstancePlugin, pluginInfo);
 }
 
 /**
@@ -86,8 +101,7 @@ async function loadInstancePluginClass(pluginInfo) {
  * @static
  */
 async function loadControlPluginClass(pluginInfo) {
-	let entrypoint = require(path.posix.join(pluginInfo.requirePath, pluginInfo.controlEntrypoint));
-	return entrypoint.ControlPlugin;
+	return loadPluginClass("controlEntrypoint", "ControlPlugin", libPlugin.BaseControlPlugin, pluginInfo);
 }
 
 module.exports = {

test/lib/plugin_loader.js

@@ -64,4 +64,45 @@ describe("lib/plugin_loader", function() {
 			);
 		});
 	});
+	describe("loadMasterPluginClass()", function() {
+		let baseDir = path.join("temp", "test", "plugin");
+		let missingClass = path.join(baseDir, "missing_class_plugin");
+		let wrongParentClass = path.join(baseDir, "wrong_parent_class_plugin");
+		before(async function() {
+			async function writeEntrypoint(pluginPath, content) {
+				await fs.outputFile(path.join(pluginPath, "master.js"), content);
+			}
+
+			await writeEntrypoint(missingClass, "");
+			await writeEntrypoint(wrongParentClass, "class MasterPlugin {}\n module.exports = { MasterPlugin };\n");
+		});
+		it("should throw if class is missing from entrypoint", async function() {
+			await assert.rejects(
+				libPluginLoader.loadMasterPluginClass({
+					name: "test",
+					masterEntrypoint: "master",
+					requirePath: path.resolve(missingClass),
+				}),
+				{
+					message:
+						`PluginError: Expected ${path.resolve(missingClass, "master")} ` +
+						"to export a class named MasterPlugin",
+				}
+			);
+		});
+		it("should throw if class is not a subclass of BaseMasterPlugin", async function() {
+			await assert.rejects(
+				libPluginLoader.loadMasterPluginClass({
+					name: "test",
+					masterEntrypoint: "master",
+					requirePath: path.resolve(wrongParentClass),
+				}),
+				{
+					message:
+						"PluginError: Expected MasterPlugin exported from " +
+						`${path.resolve(wrongParentClass, "master")} to be a subclass of BaseMasterPlugin`,
+				}
+			);
+		});
+	});
 });