Add observatory_pub_packages snapshot to third_party

observatory_pub_packages/pool/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:stack_trace/stack_trace.dart';
+
+/// Manages an abstract pool of resources with a limit on how many may be in use
+/// at once.
+///
+/// When a resource is needed, the user should call [request]. When the returned
+/// future completes with a [PoolResource], the resource may be allocated. Once
+/// the resource has been released, the user should call [PoolResource.release].
+/// The pool will ensure that only a certain number of [PoolResource]s may be
+/// allocated at once.
+class Pool {
+  /// Completers for requests beyond the first [_maxAllocatedResources].
+  ///
+  /// When an item is released, the next element of [_requestedResources] will
+  /// be completed.
+  final _requestedResources = 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;
+
+  /// The amount of time to wait before timing out the pending resources.
+  final Duration _timeout;
+
+  /// 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;
+
+  /// 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 (_allocatedResources < _maxAllocatedResources) {
+      _allocatedResources++;
+      return new Future.value(new PoolResource._(this));
+    } else {
+      var completer = new Completer<PoolResource>();
+      _requestedResources.add(completer);
+      _resetTimer();
+      return completer.future;
+    }
+  }
+
+  /// Requests a resource for the duration of [callback], which may return a
+  /// Future.
+  ///
+  /// The return value of [callback] is piped to the returned Future.
+  Future withResource(callback()) {
+    return request().then((resource) =>
+        Chain.track(new Future.sync(callback)).whenComplete(resource.release));
+  }
+
+  /// If there are any pending requests, this will fire the oldest one.
+  void _onResourceReleased() {
+    if (_requestedResources.isEmpty) {
+      _allocatedResources--;
+      if (_timer != null) {
+        _timer.cancel();
+        _timer = null;
+      }
+      return;
+    }
+
+    _resetTimer();
+    var pending = _requestedResources.removeFirst();
+    pending.complete(new PoolResource._(this));
+  }
+
+  /// A resource has been requested, allocated, or released.
+  void _resetTimer() {
+    if (_timer != null) _timer.cancel();
+    if (_timeout == null) {
+      _timer = null;
+    } else {
+      _timer = new Timer(_timeout, _onTimeout);
+    }
+  }
+
+  /// 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.",
+              _timeout),
+          new Chain.current());
+    }
+    _requestedResources.clear();
+    _timer = null;
+  }
+}
+
+/// A member of a [Pool].
+///
+/// A [PoolResource] is a token that indicates that a resource is allocated.
+/// When the associated resource is released, the user should call [release].
+class PoolResource {
+  final Pool _pool;
+
+  /// Whether [this] has been released yet.
+  bool _released = false;
+
+  PoolResource._(this._pool);
+
+  /// Tells the parent [Pool] that the resource associated with this resource is
+  /// no longer allocated, and that a new [PoolResource] may be allocated.
+  void release() {
+    if (_released) {
+      throw new StateError("A PoolResource may only be released once.");
+    }
+    _released = true;
+    _pool._onResourceReleased();
+  }
+}
+