[dart:io] Remove collisions from IO Service port dispatch

runtime/bin/io_service_patch.dart

Index: runtime/bin/io_service_patch.dart
diff --git a/runtime/bin/io_service_patch.dart b/runtime/bin/io_service_patch.dart
index 20cc5aa7a10ee33ca60885d8d91c61345ea24a15..5d6b7bc1f3c909e9a820e89fd047013d740968fe 100644
--- a/runtime/bin/io_service_patch.dart
+++ b/runtime/bin/io_service_patch.dart
@@ -2,14 +2,35 @@
 // 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.
 
+class _IOServicePorts {
+  List<SendPort> _freeServicePorts = <SendPort>[];
+  HashMap<int, SendPort> _usedBy = new HashMap<int, SendPort>();
+
+  _IOServicePorts();
+
+  SendPort _getFreePort(int forRequestId) {
+    if (_freeServicePorts.isEmpty) {
+      _freeServicePorts.add(_newServicePort());
+    }
+    SendPort freePort = _freeServicePorts.removeLast();
+    assert(!_usedBy.containsKey(forRequestId));
+    _usedBy[forRequestId] = freePort;
+    return freePort;
+  }
+
+  void _returnPort(int forRequestId) {
+    _freeServicePorts.add(_usedBy.remove(forRequestId));
+  }
+
+  static SendPort _newServicePort() native "IOService_NewServicePort";
+}
+
 @patch
 class _IOService {
-  // Lazy initialize service ports, 32 per isolate.
-  static const int _SERVICE_PORT_COUNT = 32;
-  static List<SendPort> _servicePort = new List(_SERVICE_PORT_COUNT);
+  static _IOServicePorts _servicePorts = new _IOServicePorts();
   static RawReceivePort _receivePort;
   static SendPort _replyToPort;
-  static Map<int, Completer> _messageMap = {};
+  static HashMap<int, Completer> _messageMap = new HashMap<int, Completer>();
   static int _id = 0;
 
   @patch
@@ -18,12 +39,12 @@ class _IOService {
     do {
       id = _getNextId();
     } while (_messageMap.containsKey(id));
-    int index = id % _SERVICE_PORT_COUNT;
-    _initialize(index);
+    SendPort servicePort = _servicePorts._getFreePort(id);
+    _ensureInitialize();
     var completer = new Completer();
     _messageMap[id] = completer;
     try {
-      _servicePort[index].send([id, _replyToPort, request, data]);
+      servicePort.send([id, _replyToPort, request, data]);
     } catch (error) {
       _messageMap.remove(id).complete(error);
       if (_messageMap.length == 0) {
@@ -33,16 +54,14 @@ class _IOService {
     return completer.future;
   }
 
-  static void _initialize(int index) {
-    if (_servicePort[index] == null) {
-      _servicePort[index] = _newServicePort();
-    }
+  static void _ensureInitialize() {
     if (_receivePort == null) {
       _receivePort = new RawReceivePort();
       _replyToPort = _receivePort.sendPort;
       _receivePort.handler = (data) {
         assert(data is List && data.length == 2);
         _messageMap.remove(data[0]).complete(data[1]);
+        _servicePorts._returnPort(data[0]);
         if (_messageMap.length == 0) {
           _finalize();
         }
@@ -60,6 +79,4 @@ class _IOService {
     if (_id == 0x7FFFFFFF) _id = 0;
     return _id++;
   }
-
-  static SendPort _newServicePort() native "IOService_NewServicePort";
 }