Load plugins installed from npm by package name

Plugins installed from npm are listed by package name, but the plugin
loader joined that name onto the main and entrypoint paths and imported
the result as a file, which resolves relative to the working directory.
Every npm installed plugin then failed to load. Resolve these paths from
the directory of the plugin's package.json instead.

packages/lib/src/plugin_loader.ts

@@ -51,8 +51,9 @@ export async function loadPluginInfos(pluginList: Map<string, string>) {
 
 		try {
 			pluginPackage = (await import(pathToFileURL(packagePath).href, { with: { type: "json" }})).default;
+			// pluginPath is a package name for plugins installed from npm, so resolve from the package itself
 			pluginInfo = (await import(
-				pathToFileURL(path.posix.join(pluginPath, pluginPackage.main ?? "index.js")).href
+				pathToFileURL(path.join(path.dirname(packagePath), pluginPackage.main ?? "index.js")).href
 			)).plugin;
 		} catch (err: any) {
 			if (err.code === "InstallationError") {
@@ -106,7 +107,7 @@ export async function loadPlugin<
 		return;
 	}
 
-	const module = await import(pathToFileURL(path.posix.join(pluginInfo.requirePath, requirePath)).href);
+	const module = await import(pathToFileURL(path.join(path.dirname(pluginInfo.packagePath), requirePath)).href);
 	const pluginContext: libPlugin.PluginLoadContext<Context> = {
 		...context,
 		plugin: pluginInfo,

test/lib/plugin_loader.js

@@ -66,6 +66,43 @@ describe("lib/plugin_loader", function() {
 				{ message: `PluginError: ${brokenMessage}` }
 			);
 		});
+		describe("plugin installed from npm", function() {
+			// Plugins from npm are listed by package name, so they must be resolved like a dependency
+			const packageDir = path.resolve("node_modules", "@clusterio-test", "npm_plugin");
+			before(async function() {
+				await fs.mkdir(path.join(packageDir, "dist"), { recursive: true });
+				await fs.writeFile(path.join(packageDir, "package.json"), JSON.stringify({
+					name: "@clusterio-test/npm_plugin", version: "0.0.2", main: "dist/index.js", type: "module",
+				}));
+				await fs.writeFile(
+					path.join(packageDir, "dist", "index.js"),
+					'export const plugin = { name: "npm_plugin", controllerEntrypoint: "dist/controller.js" };',
+				);
+				await fs.writeFile(
+					path.join(packageDir, "dist", "controller.js"),
+					"export default async function(context) { context.controller.loadedNpmPlugin = true; }",
+				);
+			});
+			after(async function() {
+				await fs.rm(path.dirname(packageDir), { recursive: true, force: true });
+			});
+
+			it("should load the plugin and its entrypoints", async function() {
+				const [pluginInfo] = await lib.loadPluginInfos(
+					new Map([["npm_plugin", "@clusterio-test/npm_plugin"]])
+				);
+				assert.equal(pluginInfo.name, "npm_plugin");
+				assert.equal(pluginInfo.npmPackage, "@clusterio-test/npm_plugin");
+				assert.equal(pluginInfo.packagePath, path.join(packageDir, "package.json"));
+
+				const controller = new mock.MockController();
+				await lib.loadPlugin(
+					pluginInfo, "controller", { controller, metrics: {}, logger: new mock.MockLogger() },
+					"ControllerPlugin", BaseControllerPlugin,
+				);
+				assert.equal(controller.loadedNpmPlugin, true);
+			});
+		});
 		it("should reject on invalid plugin", async function() {
 			await assert.rejects(
 				lib.loadPluginInfos(new Map([["invalid", path.resolve(invalidPlugin)]])),
@@ -122,7 +159,12 @@ describe("lib/plugin_loader", function() {
 		});
 
 		function info(requirePath, entrypoint = "controller.js") {
-			return { name: "test", requirePath, controllerEntrypoint: entrypoint };
+			return {
+				name: "test",
+				requirePath,
+				packagePath: path.join(requirePath, "package.json"),
+				controllerEntrypoint: entrypoint,
+			};
 		}
 
 		it("should do nothing when the entrypoint is not set", async function() {