[dds] Support using port=0 for DAP and print the bound host/port in JSON to stdout

Change-Id: Ib1e99f1004b59cba6fa6970dbddbf5cbe87a8b32
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/205160
Reviewed-by: Ben Konyi <bkonyi@google.com>
Commit-Queue: Ben Konyi <bkonyi@google.com>

pkg/dds/lib/src/dap/server.dart

@@ -15,8 +15,6 @@ import 'protocol_stream_transformers.dart';
 
 /// A DAP server that binds to a port and runs in multi-session mode.
 class DapServer {
-  static const defaultPort = 9200;
-
   final ServerSocket _socket;
   final bool enableDds;
   final bool enableAuthCodes;

pkg/dds/test/dap/integration/test_server.dart

@@ -6,15 +6,12 @@ import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 import 'dart:isolate';
-import 'dart:math' show Random;
 
 import 'package:dds/src/dap/logging.dart';
 import 'package:dds/src/dap/server.dart';
 import 'package:path/path.dart' as path;
 import 'package:pedantic/pedantic.dart';
 
-final _random = Random();
-
 abstract class DapTestServer {
   List<String> get errorLogs;
   String get host;
@@ -53,14 +50,6 @@ class InProcessDapTestServer extends DapTestServer {
 /// but will be a little more difficult to debug tests as the debugger will not
 /// be attached to the process.
 class OutOfProcessDapTestServer extends DapTestServer {
-  /// To avoid issues with port bindings if multiple test libraries are run
-  /// concurrently (in their own processes), start from a random port between
-  /// [DapServer.defaultPort] and [DapServer.defaultPort] + 5000.
-  ///
-  /// This number will then be increased should multiple libraries run within
-  /// this same process.
-  static var _nextPort = DapServer.defaultPort + _random.nextInt(5000);
-
   var _isShuttingDown = false;
   final Process _process;
   final int port;
@@ -75,10 +64,7 @@ class OutOfProcessDapTestServer extends DapTestServer {
     this.port,
     Logger? logger,
   ) {
-    // The DAP server should generally not write to stdout/stderr (unless -v is
-    // passed), but it may do if it fails to start or crashes. If this happens,
-    // and there's no logger, print to stdout.
-    _process.stdout.transform(utf8.decoder).listen(logger ?? print);
+    // Treat anything written to stderr as the DAP crashing and fail the test.
     _process.stderr.transform(utf8.decoder).listen((error) {
       logger?.call(error);
       _errors.add(error);
@@ -111,20 +97,37 @@ class OutOfProcessDapTestServer extends DapTestServer {
     final dapServerScript =
         path.join(ddsLibFolder, '../tool/dap/run_server.dart');
 
-    final port = OutOfProcessDapTestServer._nextPort++;
-    final host = 'localhost';
     final _process = await Process.start(
       Platform.resolvedExecutable,
       [
         dapServerScript,
         'dap',
-        '--host=$host',
-        '--port=$port',
         ...?additionalArgs,
         if (logger != null) '--verbose'
       ],
     );
 
+    final startedCompleter = Completer<void>();
+    late String host;
+    late int port;
+
+    // Scrape the `started` event to get the host/port. Any other output
+    // should be sent to the logger (as it may be verbose output for diagnostic
+    // purposes).
+    _process.stdout.transform(utf8.decoder).listen((text) {
+      if (!startedCompleter.isCompleted) {
+        final event = jsonDecode(text);
+        if (event['state'] == 'started') {
+          host = event['dapHost'];
+          port = event['dapPort'];
+          startedCompleter.complete();
+          return;
+        }
+      }
+      logger?.call(text);
+    });
+    await startedCompleter.future;
+
     return OutOfProcessDapTestServer._(_process, host, port, logger);
   }
 }

pkg/dds/tool/dap/run_server.dart

@@ -2,6 +2,7 @@
 // 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.
 
+import 'dart:convert';
 import 'dart:io';
 
 import 'package:args/command_runner.dart';
@@ -44,7 +45,7 @@ class DapCommand extends Command {
       ..addOption(
         argPort,
         abbr: 'p',
-        defaultsTo: DapServer.defaultPort.toString(),
+        defaultsTo: '0',
         help: 'The port to bind the server to',
       )
       ..addFlag(
@@ -69,12 +70,18 @@ class DapCommand extends Command {
     final port = int.parse(args[argPort]);
     final host = args[argHost];
 
-    await DapServer.create(
+    final server = await DapServer.create(
       host: host,
       port: port,
       enableDdds: args[argDds],
       enableAuthCodes: args[argAuthCodes],
       logger: args[argVerbose] ? print : null,
     );
+
+    stdout.write(jsonEncode({
+      'state': 'started',
+      'dapHost': server.host,
+      'dapPort': server.port,
+    }));
   }
 }