Expose content shell's remote debugging URL.

lib/src/runner/browser/content_shell.dart

Index: lib/src/runner/browser/content_shell.dart
diff --git a/lib/src/runner/browser/content_shell.dart b/lib/src/runner/browser/content_shell.dart
index 77ff14926ad9071be1f61ce398be922395b28043..acab425e7baa71cd494fb1be0ea90a58a57907a4 100644
--- a/lib/src/runner/browser/content_shell.dart
+++ b/lib/src/runner/browser/content_shell.dart
@@ -5,8 +5,10 @@
 library test.runner.browser.content_shell;
 
 import 'dart:async';
+import 'dart:convert';
 import 'dart:io';
 
+import '../../util/io.dart';
 import '../../utils.dart';
 import '../application_exception.dart';
 import 'browser.dart';
@@ -25,43 +27,97 @@ class ContentShell extends Browser {
 
   final Future<Uri> observatoryUrl;
 
+  final Future<Uri> remoteDebuggerUrl;
+
   factory ContentShell(url, {String executable, bool debug: false}) {
-    var completer = new Completer.sync();
-    return new ContentShell._(() async {
+    var observatoryCompleter = new Completer.sync();
+    var remoteDebuggerCompleter = new Completer.sync();
+    return new ContentShell._(() {
       if (executable == null) executable = _defaultExecutable();
 
-      var process = await Process.start(
-          executable, ["--dump-render-tree", url.toString()],
-          environment: {"DART_FLAGS": "--checked"});
-
-      if (debug) {
-        completer.complete(lineSplitter.bind(process.stdout).map((line) {
-          var match = _observatoryRegExp.firstMatch(line);
-          if (match == null) return null;
-          return Uri.parse(match[1]);
-        }).where((uri) => uri != null).first);
-      } else {
-        completer.complete(null);
+      tryPort([port]) async {
+        var args = ["--dump-render-tree", url.toString()];
+        if (port != null) args.add("--remote-debugging-port=$port");
+
+        var process = await Process.start(executable, args,
+            environment: {"DART_FLAGS": "--checked"});
+
+        if (debug) {
+          observatoryCompleter.complete(lineSplitter.bind(process.stdout)
+              .map((line) {
+            var match = _observatoryRegExp.firstMatch(line);
+            if (match == null) return null;
+            return Uri.parse(match[1]);
+          }).where((uri) => uri != null).first);
+        } else {
+          observatoryCompleter.complete(null);
+        }
+
+        var stderr = new StreamIterator(lineSplitter.bind(process.stderr));
+
+        // Before we can consider content_shell started successfully, we have to
+        // make sure it's not expired and that the remote debugging port worked.
+        // Any errors from this will always come before the "Running without
+        // renderer sanxbox" message.
+        while (await stderr.moveNext() &&
+            !stderr.current.endsWith("Running without renderer sandbox")) {
+          if (stderr.current == "[dartToStderr]: Dartium build has expired") {
+            stderr.cancel();
+            process.kill();
+            // TODO(nweiz): link to dartlang.org once it has download links for
+            // content shell
+            // (https://github.com/dart-lang/www.dartlang.org/issues/1164).
+            throw new ApplicationException(
+                "You're using an expired content_shell. Upgrade to the latest "
+                    "version:\n"
+                "http://gsdview.appspot.com/dart-archive/channels/stable/"
+                    "release/latest/dartium/");
+          } else if (stderr.current.contains("bind() returned an error")) {
+            // If we failed to bind to the port, return null to tell
+            // getUnusedPort to try another one.
+            stderr.cancel();
+            process.kill();
+            return null;
+          }
+        }
+
+        if (port != null) {
+          remoteDebuggerCompleter.complete(
+              _getRemoteDebuggerUrl(Uri.parse("http://localhost:$port")));
+        } else {
+          remoteDebuggerCompleter.complete(null);
+        }
+
+        stderr.cancel();
+        return process;
       }
 
-      lineSplitter.bind(process.stderr).listen((line) {
-        if (line != "[dartToStderr]: Dartium build has expired") return;
-
-        // TODO(nweiz): link to dartlang.org once it has download links for
-        // content shell
-        // (https://github.com/dart-lang/www.dartlang.org/issues/1164).
-        throw new ApplicationException(
-            "You're using an expired content_shell. Upgrade to the latest "
-                "version:\n"
-            "http://gsdview.appspot.com/dart-archive/channels/stable/release/"
-                "latest/dartium/");
-      });
-
-      return process;
-    }, completer.future);
+      if (!debug) return tryPort();
+      return getUnusedPort(tryPort);
+    }, observatoryCompleter.future, remoteDebuggerCompleter.future);
+  }
+
+  /// Returns the full URL of the remote debugger for the host page.
+  ///
+  /// This takes the base remote debugger URL (which points to a browser-wide
+  /// page) and uses its JSON API to find the resolved URL for debugging the
+  /// host page.
+  static Future<Uri> _getRemoteDebuggerUrl(Uri base) async {
+    try {
+      var client = new HttpClient();
+      var request = await client.getUrl(base.resolve("/json/list"));
+      var response = await request.close();
+      var json = await JSON.fuse(UTF8).decoder.bind(response).single;
+      return base.resolve(json.first["devtoolsFrontendUrl"]);
+    } catch (_) {
+      // If we fail to talk to the remote debugger protocol, give up and return
+      // the raw URL rather than crashing.
+      return base;
+    }
   }
 
-  ContentShell._(Future<Process> startBrowser(), this.observatoryUrl)
+  ContentShell._(Future<Process> startBrowser(), this.observatoryUrl,
+          this.remoteDebuggerUrl)
       : super(startBrowser);
 
   /// Return the default executable for the current operating system.