Add Pool.close().

lib/pool.dart

Index: lib/pool.dart
diff --git a/lib/pool.dart b/lib/pool.dart
index e8ee99cab190783c9f0b9e97a89720c165fa5637..59b949e0eef0da58ef4d49844e205a46c67dc325 100644
--- a/lib/pool.dart
+++ b/lib/pool.dart
@@ -7,6 +7,7 @@ library pool;
 import 'dart:async';
 import 'dart:collection';
 
+import 'package:async/async.dart';
 import 'package:stack_trace/stack_trace.dart';
 
 /// Manages an abstract pool of resources with a limit on how many may be in use
@@ -55,6 +56,15 @@ class Pool {
   /// 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.
   ///
@@ -69,6 +79,10 @@ class Pool {
   /// 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.");
+    }
+
     if (_allocatedResources < _maxAllocatedResources) {
       _allocatedResources++;
       return new Future.value(new PoolResource._(this));
@@ -87,20 +101,56 @@ class Pool {
   ///
   /// The return value of [callback] is piped to the returned Future.
   Future withResource(callback()) {
-    return request().then((resource) => new Future.sync(callback).whenComplete(resource.release));
+    if (isClosed) {
+      throw new StateError(
+          "withResource() may not be called on a closed Pool.");
+    }
+
+    // TODO(nweiz): Use async/await when sdk#23497 is fixed.
+    return request().then((resource) {
+      return new Future.sync(callback).whenComplete(resource.release);
+    });
+  }
+
+  /// Closes the pool so that no more resources are requested.
+  ///
+  /// Existing resource requests remain unchanged.
+  ///
+  /// Any resources that are marked as releasable using
+  /// [PoolResource.allowRelease] are released immediately. Once all resources
+  /// have been released and any `onRelease` callbacks have completed, the
+  /// returned future completes successfully. If any `onRelease` callback throws
+  /// an error, the returned future completes with that error.
+  ///
+  /// This may be called more than once; it returns the same [Future] each time.
+  Future close() {
+    if (_closeGroup != null) return _closeGroup.future;
+
+    _resetTimer();
+
+    _closeGroup = new FutureGroup();
+    for (var callback in _onReleaseCallbacks) {
+      _closeGroup.add(new Future.sync(callback));
+    }
+
+    _allocatedResources -= _onReleaseCallbacks.length;
+    _onReleaseCallbacks.clear();
+
+    if (_allocatedResources == 0) _closeGroup.close();
+    return _closeGroup.future;
   }
 
   /// If there are any pending requests, this will fire the oldest one.
   void _onResourceReleased() {
     _resetTimer();
 
-    if (_requestedResources.isEmpty) {
+    if (_requestedResources.isNotEmpty) {
+      var pending = _requestedResources.removeFirst();
+      pending.complete(new PoolResource._(this));
+    } else {
       _allocatedResources--;
-      return;
+      if (isClosed && _allocatedResources == 0) _closeGroup.close();
     }
-
-    var pending = _requestedResources.removeFirst();
-    pending.complete(new PoolResource._(this));
   }
 
   /// If there are any pending requests, this will fire the oldest one after
@@ -108,14 +158,17 @@ class Pool {
   void _onResourceReleaseAllowed(onRelease()) {
     _resetTimer();
 
-    if (_requestedResources.isEmpty) {
+    if (_requestedResources.isNotEmpty) {
+      var pending = _requestedResources.removeFirst();
+      pending.complete(_runOnRelease(onRelease));
+    } else if (isClosed) {
+      _closeGroup.add(new Future.sync(onRelease));
+      _allocatedResources--;
+      if (_allocatedResources == 0) _closeGroup.close();
+    } else {
       _onReleaseCallbacks.add(
           Zone.current.bindCallback(onRelease, runGuarded: false));
-      return;
     }
-
-    var pending = _requestedResources.removeFirst();
-    pending.complete(_runOnRelease(onRelease));
   }
 
   /// Runs [onRelease] and returns a Future that completes to a resource once an
@@ -202,4 +255,3 @@ class PoolResource {
     _pool._onResourceReleaseAllowed(onRelease);
   }
 }
-