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: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>>();

  /// Callbacks that must be called before additional resources can be
  /// allocated.
  ///
  /// See [PoolResource.allowRelease].
  final _onReleaseCallbacks = new Queue<Function>();

  /// Completers that will be completed once `onRelease` callbacks are done
  /// running.
  ///
  /// These are kept in a queue to ensure that the earliest request completes
  /// first regardless of what order the `onRelease` callbacks complete in.
  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;

  /// 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 if (_onReleaseCallbacks.isNotEmpty) {
      return _runOnRelease(_onReleaseCallbacks.removeFirst());
    } 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() {
    _resetTimer();

    if (_requestedResources.isEmpty) {
      _allocatedResources--;
      return;
    }

    var pending = _requestedResources.removeFirst();
    pending.complete(new PoolResource._(this));
  }

  /// If there are any pending requests, this will fire the oldest one after
  /// running [onRelease].
  void _onResourceReleaseAllowed(onRelease()) {
    _resetTimer();

    if (_requestedResources.isEmpty) {
      _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
  /// [onRelease] callback completes.
  ///
  /// Futures returned by [_runOnRelease] always complete in the order they were
  /// created, even if earlier [onRelease] callbacks take longer to run.
  Future<PoolResource> _runOnRelease(onRelease()) {
    new Future.sync(onRelease).then((value) {
      _onReleaseCompleters.removeFirst().complete(new PoolResource._(this));
    }).catchError((error, stackTrace) {
      _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;
    } 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();
  }

  /// Tells the parent [Pool] that the resource associated with this resource is
  /// no longer necessary, but should remain allocated until more resources are
  /// needed.
  ///
  /// When [Pool.request] is called and there are no remaining available
  /// resources, the [onRelease] callback is called. It should free the
  /// resource, and it may return a Future or `null`. Once that completes, the
  /// [Pool.request] call will complete to a new [PoolResource].
  ///
  /// This is useful when a resource's main function is complete, but it may
  /// 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);
  }
}