[dwds] Let the OS pick the port when starting HTTP servers

dwds/CHANGELOG.md

@@ -4,6 +4,9 @@
   is set, the expression compiler worker is started with
   `--[no-]deprecated-js-interop`. This requires a Dart SDK that supports the
   flag.
+- Fix the extension backend and debug service intermittently failing to start
+  with "Address already in use" by letting the OS pick the port instead of
+  probing for one on `[::1]` first.
 
 ## 27.1.4
 

dwds/lib/src/utilities/server.dart

@@ -33,28 +33,26 @@ Future<int> findUnusedPort() async {
   return port;
 }
 
-/// Finds unused port and binds a new http server to it.
+/// Binds a new http server to [hostname].
 ///
-/// Retries a few times to recover from errors due to
-/// another thread or process opening the same port.
-/// Starts by trying to bind to [port] if specified.
+/// Tries [port] first if specified, then falls back to port 0 so the OS picks a
+/// port that is free for this exact bind. A port from [findUnusedPort] is only
+/// known to be free on `[::1]`. It can still be held over IPv4, which makes the
+/// dual-stack `[::]` bind used for `'any'` fail with "Address already in use".
+///
+/// Retries a few times to recover from transient bind errors.
 Future<HttpServer> startHttpServer(String hostname, {int? port}) async {
-  HttpServer? httpServer;
-  final retries = 5;
-  var i = 0;
-  var foundPort = port ?? await findUnusedPort();
-  while (i < retries) {
-    i++;
+  const retries = 5;
+  var requestedPort = port ?? 0;
+  for (var attempt = 1; ; attempt++) {
     try {
-      httpServer = await HttpMultiServer.bind(hostname, foundPort);
+      return await HttpMultiServer.bind(hostname, requestedPort);
     } on SocketException {
-      if (i == retries) rethrow;
+      if (attempt == retries) rethrow;
     }
-    if (httpServer != null || i == retries) return httpServer!;
-    foundPort = await findUnusedPort();
+    requestedPort = 0;
     await Future<void>.delayed(const Duration(milliseconds: 100));
   }
-  return httpServer!;
 }
 
 /// Handles [requests] using [handler].

dwds/test/utilities/server_test.dart

@@ -0,0 +1,32 @@
+// Copyright (c) 2026, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+@TestOn('vm')
+library;
+
+import 'package:dwds/src/utilities/server.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('startHttpServer', () {
+    for (final hostname in ['any', 'localhost']) {
+      test('binds an OS-assigned port on $hostname', () async {
+        final server = await startHttpServer(hostname);
+        addTearDown(server.close);
+        expect(server.port, isNot(0));
+      });
+
+      test('falls back to another port on $hostname when the requested port is '
+          'taken', () async {
+        final holder = await startHttpServer(hostname);
+        addTearDown(holder.close);
+
+        final server = await startHttpServer(hostname, port: holder.port);
+        addTearDown(server.close);
+        expect(server.port, isNot(0));
+        expect(server.port, isNot(holder.port));
+      });
+    }
+  });
+}