feat(plugin): implement close method for plugin mode handling

.changeset/plugin-mode-close.md

@@ -0,0 +1,5 @@
+---
+"webpack-dev-middleware": patch
+---
+
+Fixed a crash when calling `close()` in plugin mode (`isPlugin = true`). Since the host (webpack-cli, webpack-dev-server, etc.) owns `compiler.watch()`, the middleware has no `watching` of its own to close, so `close()` now just calls the callback instead of throwing.

src/index.js

@@ -596,10 +596,19 @@ function wdm(compiler, options = {}, isPlugin = false) {
   instance.invalidate = (callback = noop) => {
     middleware.ready(filledContext, callback);
 
+    // TODO for plugin usage (`isPlugin = true`) `watching` is `undefined` and this throws —
+    // invalidate the host's `compiler.watching` (each child's one for a `MultiCompiler`) instead
     filledContext.watching.invalidate();
   };
 
   instance.close = (callback = noop) => {
+    // For plugin usage the host (webpack-cli, webpack-dev-server, etc.) owns `compiler.watch()`,
+    // so there is no `watching` of our own to close (`compiler.close()` on the host handles it)
+    if (!filledContext.watching) {
+      callback(null);
+      return;
+    }
+
     filledContext.watching.close(callback);
   };
 

test/pluginMode.test.js

@@ -0,0 +1,39 @@
+import middleware from "../src";
+
+import webpackConfig from "./fixtures/webpack.config";
+import getCompiler from "./helpers/getCompiler";
+
+jest.spyOn(globalThis.console, "log").mockImplementation();
+
+// When used as a plugin (`isPlugin = true`) the host (webpack-cli,
+// webpack-dev-server, etc.) owns `compiler.watch()`, so the middleware has no
+// `watching` of its own
+describe("plugin mode", () => {
+  describe("close method", () => {
+    it("should not throw and call the callback when the host is not watching", (done) => {
+      const compiler = getCompiler(webpackConfig);
+      const instance = middleware(compiler, {}, true);
+
+      instance.close((error) => {
+        expect(error).toBeNull();
+
+        done();
+      });
+    });
+
+    it("should not close the watching owned by the host", (done) => {
+      const compiler = getCompiler(webpackConfig);
+      const watching = compiler.watch({}, () => {});
+      const instance = middleware(compiler, {}, true);
+
+      instance.waitUntilValid(() => {
+        instance.close((error) => {
+          expect(error).toBeNull();
+          expect(watching.closed).toBe(false);
+
+          watching.close(done);
+        });
+      });
+    });
+  });
+});