Use the async package's new RestartableTimer class.

lib/pool.dart

 // Copyright (c) 2014, 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.
 
 library pool;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:async/async.dart';
@@ skipping 28 matching lines @@
   final _onReleaseCompleters = new Queue<Completer<PoolResource>>();
 
   /// The maximum number of resources that may be allocated at once.
   final int _maxAllocatedResources;
 
   /// The number of resources that are currently allocated.
   int _allocatedResources = 0;
 
   /// The timeout timer.
   ///
-  /// If [_timeout] isn't null, this timer is set as soon as the resource limit
-  /// is reached and is reset every time an resource is released or a new
-  /// resource is requested. If it fires, that indicates that the caller became
-  /// deadlocked, likely due to files waiting for additional files to be read
-  /// before they could be closed.
-  Timer _timer;
+  /// This timer is canceled as long as the pool is below the resource limit.
+  /// It's reset once the resource limit is reached and again every time an
+  /// resource is released or a new resource is requested. If it fires, that
+  /// indicates that the caller became deadlocked, likely due to files waiting
+  /// for additional files to be read before they could be closed.
+  ///
+  /// This is `null` if this pool shouldn't time out.
+  RestartableTimer _timer;
 
   /// The amount of time to wait before timing out the pending resources.
   final Duration _timeout;
 
   /// A [FutureGroup] that tracks all the `onRelease` callbacks for resources
   /// that have been marked releasable.
   ///
   /// This is `null` until [close] is called.
   FutureGroup _closeGroup;
 
   /// Whether [close] has been called.
   bool get isClosed => _closeGroup != null;
 
   /// Creates a new pool with the given limit on how many resources may be
   /// allocated at once.
   ///
   /// If [timeout] is passed, then if that much time passes without any activity
   /// all pending [request] futures will throw a [TimeoutException]. This is
   /// intended to avoid deadlocks.
   Pool(this._maxAllocatedResources, {Duration timeout})
-      : _timeout = timeout;
+      : _timeout = timeout {
+    if (timeout != null) {
+      // Start the timer canceled since we only want to start counting down once
+      // we've run out of available resources.
+      _timer = new RestartableTimer(timeout, _onTimeout)..cancel();

[Bob Nystrom, 2015/10/28 21:40:27]

"cancel" is a weird name for this. To me, it implies finality. Would "stop" make
more sense?

[nweiz, 2015/10/28 21:47:36]

It's an inherited name from Timer. I wanted to keep the APIs as similar as
possible for familiarity (and in fact RestartableTimer implements Timer).

+    }
+  }
 
   /// Request a [PoolResource].
   ///
   /// If the maximum number of resources is already allocated, this will delay
   /// until one of them is released.
   Future<PoolResource> request() {
     if (isClosed) {
       throw new StateError("request() may not be called on a closed Pool.");
     }
 
@@ skipping 97 matching lines @@
       _onReleaseCompleters.removeFirst().completeError(error, stackTrace);
     });
 
     var completer = new Completer.sync();
     _onReleaseCompleters.add(completer);
     return completer.future;
   }
 
   /// A resource has been requested, allocated, or released.
   void _resetTimer() {
-    if (_timer != null) _timer.cancel();
-    if (_timeout == null || _requestedResources.isEmpty) {
-      _timer = null;
+    if (_timer == null) return;
+
+    if (_requestedResources.isEmpty) {
+      _timer.cancel();
     } else {
-      _timer = new Timer(_timeout, _onTimeout);
+      _timer.reset();
     }
   }
 
   /// Handles [_timer] timing out by causing all pending resource completers to
   /// emit exceptions.
   void _onTimeout() {
     for (var completer in _requestedResources) {
       completer.completeError(
           new TimeoutException("Pool deadlock: all resources have been "
               "allocated for too long.",
@@ skipping 40 matching lines @@
   /// produce additional information later on. For example, an isolate's task
   /// may be complete, but it could still emit asynchronous errors.
   void allowRelease(onRelease()) {
     if (_released) {
       throw new StateError("A PoolResource may only be released once.");
     }
     _released = true;
     _pool._onResourceReleaseAllowed(onRelease);
   }
 }