Synnefo » Pithos MS Client

// -----------------------------------------------------------------------

// <copyright file="NetworkAgent.cs" company="GRNET">

// Copyright 2011 GRNET S.A. All rights reserved.

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// -----------------------------------------------------------------------

using System;

using System.Collections.Concurrent;

using System.Collections.Generic;

using System.ComponentModel.Composition;

using System.Diagnostics;

using System.Diagnostics.Contracts;

using System.IO;

using System.Linq;

using System.Net;

using System.Threading;

using System.Threading.Tasks;

using System.Threading.Tasks.Dataflow;

using Castle.ActiveRecord;

using Pithos.Interfaces;

using Pithos.Network;

using log4net;

namespace Pithos.Core.Agents

{

    //TODO: Ensure all network operations use exact casing. Pithos is case sensitive

    [Export]

    public class NetworkAgent

    {

        private Agent<CloudAction> _agent;

        //A separate agent is used to execute delete actions immediatelly;

        private ActionBlock<CloudDeleteAction> _deleteAgent;

        readonly ConcurrentDictionary<string,DateTime> _deletedFiles=new ConcurrentDictionary<string, DateTime>();

        private readonly ManualResetEventSlim _pauseAgent = new ManualResetEventSlim(true);

        [System.ComponentModel.Composition.Import]

        public IStatusKeeper StatusKeeper { get; set; }

        public IStatusNotification StatusNotification { get; set; }

        private static readonly ILog Log = LogManager.GetLogger("NetworkAgent");

        private readonly ConcurrentBag<AccountInfo> _accounts = new ConcurrentBag<AccountInfo>();

        [System.ComponentModel.Composition.Import]

        public IPithosSettings Settings { get; set; }

        private bool _firstPoll = true;

        private TaskCompletionSource<bool> _tcs;

        public void Start()

        {

            _firstPoll = true;

            _agent = Agent<CloudAction>.Start(inbox =>

            {

                Action loop = null;

                loop = () =>

                {

                    _pauseAgent.Wait();

                    var message = inbox.Receive();

                    var process=message.Then(Process,inbox.CancellationToken);

                    inbox.LoopAsync(process, loop);

                };

                loop();

            });

            _deleteAgent = new ActionBlock<CloudDeleteAction>(message =>ProcessDelete(message),new ExecutionDataflowBlockOptions{MaxDegreeOfParallelism=4});

            /*

                Action loop = null;

                loop = () =>

                            {

                                var message = inbox.Receive();

                                var process = message.Then(ProcessDelete,inbox.CancellationToken);

                                inbox.LoopAsync(process, loop);

                            };

                loop();

*/

        }

        private async Task Process(CloudAction action)

        {

            if (action == null)

                throw new ArgumentNullException("action");

            if (action.AccountInfo==null)

                throw new ArgumentException("The action.AccountInfo is empty","action");

            Contract.EndContractBlock();

            var accountInfo = action.AccountInfo;

            using (log4net.ThreadContext.Stacks["NETWORK"].Push("PROCESS"))

            {                

                Log.InfoFormat("[ACTION] Start Processing {0}", action);

                var cloudFile = action.CloudFile;

                var downloadPath = action.GetDownloadPath();

                try

                {

                    if (action.Action == CloudActionType.DeleteCloud)

                    {

                        //Redirect deletes to the delete agent 

                        _deleteAgent.Post((CloudDeleteAction)action);                        

                    }

                    if (IsDeletedFile(action))

                    {

                        //Clear the status of already deleted files to avoid reprocessing

                        if (action.LocalFile != null)

                            this.StatusKeeper.ClearFileStatus(action.LocalFile.FullName);

                    }

                    else

                    {

                        switch (action.Action)

                        {

                            case CloudActionType.UploadUnconditional:

                                //Abort if the file was deleted before we reached this point

                                await UploadCloudFile(action);

                                break;

                            case CloudActionType.DownloadUnconditional:

                                await DownloadCloudFile(accountInfo, cloudFile, downloadPath);

                                break;

                            case CloudActionType.RenameCloud:

                                var moveAction = (CloudMoveAction) action;

                                RenameCloudFile(accountInfo, moveAction);

                                break;

                            case CloudActionType.MustSynch:

                                if (!File.Exists(downloadPath) && !Directory.Exists(downloadPath))

                                {

                                    await DownloadCloudFile(accountInfo, cloudFile, downloadPath);

                                }

                                else

                                {

                                    await SyncFiles(accountInfo, action);

                                }

                                break;

                        }

                    }

                    Log.InfoFormat("[ACTION] End Processing {0}:{1}->{2}", action.Action, action.LocalFile,

                                           action.CloudFile.Name);

                }

                catch (WebException exc)

                {

                    Log.ErrorFormat("[WEB ERROR] {0} : {1} -> {2} due to exception\r\n{3}", action.Action, action.LocalFile, action.CloudFile, exc);

                }

                catch (OperationCanceledException)

                {

                    throw;

                }

                catch (DirectoryNotFoundException)

                {

                    Log.ErrorFormat("{0} : {1} -> {2}  failed because the directory was not found.\n Rescheduling a delete",

                        action.Action, action.LocalFile, action.CloudFile);

                    //Post a delete action for the missing file

                    Post(new CloudDeleteAction(action));                    

                }

                catch (FileNotFoundException)

                {

                    Log.ErrorFormat("{0} : {1} -> {2}  failed because the file was not found.\n Rescheduling a delete",

                        action.Action, action.LocalFile, action.CloudFile);

                    //Post a delete action for the missing file

                    Post(new CloudDeleteAction(action));

                }

                catch (Exception exc)

                {

                    Log.ErrorFormat("[REQUEUE] {0} : {1} -> {2} due to exception\r\n{3}",

                                     action.Action, action.LocalFile, action.CloudFile, exc);

                    _agent.Post(action);

                }                

            }

        }

        /// <summary>

        /// Processes cloud delete actions

        /// </summary>

        /// <param name="action">The delete action to execute</param>

        /// <returns></returns>

        /// <remarks>

        /// When a file/folder is deleted locally, we must delete it ASAP from the server and block any download

        /// operations that may be in progress.

        /// <para>

        /// A separate agent is used to process deletes because the main agent may be busy with a long operation.

        /// </para>

        /// </remarks>

        private async Task ProcessDelete(CloudDeleteAction action)

        {

            if (action == null)

                throw new ArgumentNullException("action");

            if (action.AccountInfo==null)

                throw new ArgumentException("The action.AccountInfo is empty","action");

            Contract.EndContractBlock();

            var accountInfo = action.AccountInfo;

            using (log4net.ThreadContext.Stacks["NETWORK"].Push("PROCESS"))

            {                

                Log.InfoFormat("[ACTION] Start Processing {0}", action);

                var cloudFile = action.CloudFile;

                try

                {

                    //Acquire a lock on the deleted file to prevent uploading/downloading operations from the normal

                    //agent

                    using (var gate = NetworkGate.Acquire(action.LocalFile.FullName, NetworkOperation.Deleting))

                    {

                        //Add the file URL to the deleted files list

                        var key = GetFileKey(action.CloudFile);

                        _deletedFiles[key] = DateTime.Now;

                        _pauseAgent.Reset();

                        // and then delete the file from the server

                        DeleteCloudFile(accountInfo, cloudFile);

                        Log.InfoFormat("[ACTION] End Delete {0}:{1}->{2}", action.Action, action.LocalFile,

                                       action.CloudFile.Name);

                    }

                }

                catch (WebException exc)

                {

                    Log.ErrorFormat("[WEB ERROR] {0} : {1} -> {2} due to exception\r\n{3}", action.Action, action.LocalFile, action.CloudFile, exc);

                }

                catch (OperationCanceledException)

                {

                    throw;

                }

                catch (DirectoryNotFoundException)

                {

                    Log.ErrorFormat("{0} : {1} -> {2}  failed because the directory was not found.\n Rescheduling a delete",

                        action.Action, action.LocalFile, action.CloudFile);

                    //Repost a delete action for the missing file

                    _deleteAgent.Post(action);

                }

                catch (FileNotFoundException)

                {

                    Log.ErrorFormat("{0} : {1} -> {2}  failed because the file was not found.\n Rescheduling a delete",

                        action.Action, action.LocalFile, action.CloudFile);

                    //Post a delete action for the missing file

                    _deleteAgent.Post(action);

                }

                catch (Exception exc)

                {

                    Log.ErrorFormat("[REQUEUE] {0} : {1} -> {2} due to exception\r\n{3}",

                                     action.Action, action.LocalFile, action.CloudFile, exc);

                    _deleteAgent.Post(action);

                }

                finally

                {

                    if (_deleteAgent.InputCount == 0)

                        _pauseAgent.Set();

                }

            }

        }

        private static string GetFileKey(ObjectInfo info)

        {

            var key = String.Format("{0}/{1}/{2}", info.Account, info.Container,info.Name);

            return key;

        }

        private async Task SyncFiles(AccountInfo accountInfo,CloudAction action)

        {

            if (accountInfo == null)

                throw new ArgumentNullException("accountInfo");

            if (action==null)

                throw new ArgumentNullException("action");

            if (action.LocalFile==null)

                throw new ArgumentException("The action's local file is not specified","action");

            if (!Path.IsPathRooted(action.LocalFile.FullName))

                throw new ArgumentException("The action's local file path must be absolute","action");

            if (action.CloudFile== null)

                throw new ArgumentException("The action's cloud file is not specified", "action");

            Contract.EndContractBlock();

            var localFile = action.LocalFile;

            var cloudFile = action.CloudFile;

            var downloadPath=action.LocalFile.GetProperCapitalization();

            var cloudHash = cloudFile.Hash.ToLower();

            var localHash = action.LocalHash.Value.ToLower();

            var topHash = action.TopHash.Value.ToLower();

            //Not enough to compare only the local hashes, also have to compare the tophashes

            //If any of the hashes match, we are done

            if ((cloudHash == localHash || cloudHash == topHash))

            {

                Log.InfoFormat("Skipping {0}, hashes match",downloadPath);

                return;

            }

            //The hashes DON'T match. We need to sync

            var lastLocalTime = localFile.LastWriteTime;

            var lastUpTime = cloudFile.Last_Modified;

            //If the local file is newer upload it

            if (lastUpTime <= lastLocalTime)

            {

                //It probably means it was changed while the app was down                        

                UploadCloudFile(action);

            }

            else

            {

                //It the cloud file has a later date, it was modified by another user or computer.

                //We need to check the local file's status                

                var status = StatusKeeper.GetFileStatus(downloadPath);

                switch (status)

                {

                    case FileStatus.Unchanged:                        

                        //If the local file's status is Unchanged, we can go on and download the newer cloud file

                        await DownloadCloudFile(accountInfo,cloudFile,downloadPath);

                        break;

                    case FileStatus.Modified:

                        //If the local file is Modified, we may have a conflict. In this case we should mark the file as Conflict

                        //We can't ensure that a file modified online since the last time will appear as Modified, unless we 

                        //index all files before we start listening.                       

                    case FileStatus.Created:

                        //If the local file is Created, it means that the local and cloud files aren't related,

                        // yet they have the same name.

                        //In both cases we must mark the file as in conflict

                        ReportConflict(downloadPath);

                        break;

                    default:

                        //Other cases should never occur. Mark them as Conflict as well but log a warning

                        ReportConflict(downloadPath);

                        Log.WarnFormat("Unexcepted status {0} for file {1}->{2}", status,

                                       downloadPath, action.CloudFile.Name);

                        break;

                }

            }

        }

        private void ReportConflict(string downloadPath)

        {

            if (String.IsNullOrWhiteSpace(downloadPath))

                throw new ArgumentNullException("downloadPath");

            Contract.EndContractBlock();

            StatusKeeper.SetFileOverlayStatus(downloadPath, FileOverlayStatus.Conflict);

            var message = String.Format("Conflict detected for file {0}", downloadPath);

            Log.Warn(message);

            StatusNotification.NotifyChange(message, TraceLevel.Warning);

        }

        public void Post(CloudAction cloudAction)

        {

            if (cloudAction == null)

                throw new ArgumentNullException("cloudAction");

            if (cloudAction.AccountInfo==null)

                throw new ArgumentException("The CloudAction.AccountInfo is empty","cloudAction");

            Contract.EndContractBlock();

            _pauseAgent.Wait();

            //If the action targets a local file, add a treehash calculation

            if (!(cloudAction is CloudDeleteAction) && cloudAction.LocalFile as FileInfo != null)

            {

                var accountInfo = cloudAction.AccountInfo;

                var localFile = (FileInfo) cloudAction.LocalFile;

                if (localFile.Length > accountInfo.BlockSize)

                    cloudAction.TopHash =

                        new Lazy<string>(() => Signature.CalculateTreeHashAsync(localFile,

                                                                                accountInfo.BlockSize,

                                                                                accountInfo.BlockHash).Result

                                                    .TopHash.ToHashString());

                else

                {

                    cloudAction.TopHash = new Lazy<string>(() => cloudAction.LocalHash.Value);

                }

            }

            else

            {

                //The hash for a directory is the empty string

                cloudAction.TopHash = new Lazy<string>(() => String.Empty);

            }

            if (cloudAction is CloudDeleteAction)

                _deleteAgent.Post((CloudDeleteAction)cloudAction);

            else

                _agent.Post(cloudAction);

        }

       /* class ObjectInfoByNameComparer:IEqualityComparer<ObjectInfo>

        {

            public bool Equals(ObjectInfo x, ObjectInfo y)

            {

                return x.Name.Equals(y.Name,StringComparison.InvariantCultureIgnoreCase);

            }

            public int GetHashCode(ObjectInfo obj)

            {

                return obj.Name.ToLower().GetHashCode();

            }

        }*/

        public void SynchNow()

        {             

            if (_tcs!=null)

                _tcs.SetResult(true);

            else

            {

                //TODO: This may be OK for testing purposes, but we have no guarantee that it will

                //work properly in production

                PollRemoteFiles(repeat:false);

            }

        }

        //Remote files are polled periodically. Any changes are processed

        public async Task PollRemoteFiles(DateTime? since = null,bool repeat=true)

        {

            using (log4net.ThreadContext.Stacks["Retrieve Remote"].Push("All accounts"))

            {

                //If this poll fails, we will retry with the same since value

                var nextSince = since;

                try

                {

                    //Next time we will check for all changes since the current check minus 1 second

                    //This is done to ensure there are no discrepancies due to clock differences

                    DateTime current = DateTime.Now.AddSeconds(-1);

                    var tasks = from accountInfo in _accounts

                                select ProcessAccountFiles(accountInfo, since);

                    await TaskEx.WhenAll(tasks.ToList());

                    _firstPoll = false;

                    //Reschedule the poll with the current timestamp as a "since" value

                    if (repeat)

                        nextSince = current;

                    else

                        return;

                }

                catch (Exception ex)

                {

                    Log.ErrorFormat("Error while processing accounts\r\n{0}",ex);

                    //In case of failure retry with the same "since" value

                }

                //Wait for the polling interval to pass or the Manual flat to be toggled

                nextSince = await WaitForScheduledOrManualPoll(nextSince);

                PollRemoteFiles(nextSince);

            }

        }

        private async Task<DateTime?> WaitForScheduledOrManualPoll(DateTime? since)

        {            

            _tcs = new TaskCompletionSource<bool>();

            var wait = TaskEx.Delay(TimeSpan.FromSeconds(Settings.PollingInterval), _agent.CancellationToken);

            var signaledTask = await TaskEx.WhenAny(_tcs.Task, wait);

            //If polling is signalled by SynchNow, ignore the since tag

            if (signaledTask is Task<bool>)

                return null;

            return since;

        }

        public async Task ProcessAccountFiles(AccountInfo accountInfo,DateTime? since=null)

        {   

            if (accountInfo==null)

                throw new ArgumentNullException("accountInfo");

            if (String.IsNullOrWhiteSpace(accountInfo.AccountPath))

                throw new ArgumentException("The AccountInfo.AccountPath is empty","accountInfo");

            Contract.EndContractBlock();

            using (log4net.ThreadContext.Stacks["Retrieve Remote"].Push(accountInfo.UserName))

            {

                Log.Info("Scheduled");

                var client=new CloudFilesClient(accountInfo);

                var containers = client.ListContainers(accountInfo.UserName);

                CreateContainerFolders(accountInfo, containers);

                try

                {

                    _pauseAgent.Wait();

                    //Get the poll time now. We may miss some deletions but it's better to keep a file that was deleted

                    //than delete a file that was created while we were executing the poll

                    var pollTime = DateTime.Now;

                    //Get the list of server objects changed since the last check

                    //The name of the container is passed as state in order to create a dictionary of tasks in a subsequent step

                    var listObjects = from container in containers

                                      select  Task<IList<ObjectInfo>>.Factory.StartNew(_ =>

                                            client.ListObjects(accountInfo.UserName,container.Name, since),container.Name);

                    var listTasks = await Task.Factory.WhenAll(listObjects.ToArray());

                    using (log4net.ThreadContext.Stacks["SCHEDULE"].Push("Process Results"))

                    {

                        var dict = listTasks.ToDictionary(t => t.AsyncState);

                        //Get all non-trash objects. Remember, the container name is stored in AsyncState

                        var remoteObjects = from objectList in listTasks

                                            where (string) objectList.AsyncState != "trash"

                                            from obj in objectList.Result

                                            select obj;

                        //TODO: Change the way deleted objects are detected.

                        //The list operation returns all existing objects so we could detect deleted remote objects

                        //by detecting objects that exist only locally. There are several cases where this is NOT the case:

                        //1.    The first time the application runs, as there may be files that were added while 

                        //      the application was down.

                        //2.    An object that is currently being uploaded will not appear in the remote list

                        //      until the upload finishes.

                        //      SOLUTION 1: Check the upload/download queue for the file

                        //      SOLUTION 2: Check the SQLite states for the file's entry. If it is being uploaded, 

                        //          or its last modification was after the current poll, don't delete it. This way we don't

                        //          delete objects whose upload finished too late to be included in the list.

                        //We need to detect and protect against such situations

                        //TODO: Does FileState have a LastModification field?

                        //TODO: How do we update the LastModification field? Do we need to add SQLite triggers?

                        //      Do we need to use a proper SQLite schema?

                        //      We can create a trigger with 

                        // CREATE TRIGGER IF NOT EXISTS update_last_modified UPDATE ON FileState FOR EACH ROW

                        //  BEGIN

                        //      UPDATE FileState SET LastModification=datetime('now')  WHERE Id=old.Id;

                        //  END;

                        //

                        //NOTE: Some files may have been deleted remotely while the application was down. 

                        //  We DO have to delete those files. Checking the trash makes it easy to detect them,

                        //  Otherwise, we can't be really sure whether we need to upload or delete 

                        //  the local-only files.

                        //  SOLUTION 1: Ask the user when such a local-only file is detected during the first poll.

                        //  SOLUTION 2: Mark conflict and ask the user as in #1

                        var trashObjects = dict["trash"].Result;

                        //var sharedObjects = ((Task<IList<ObjectInfo>>) task.Result[2]).Result;

                        //Items with the same name, hash may be both in the container and the trash

                        //Don't delete items that exist in the container

                        var realTrash = from trash in trashObjects

                                        where

                                            !remoteObjects.Any(

                                                info => info.Name == trash.Name && info.Hash == trash.Hash)

                                        select trash;

                        ProcessTrashedFiles(accountInfo, realTrash);

                        var cleanRemotes = (from info in remoteObjects

                                     //.Union(sharedObjects)

                                     let name = info.Name

                                     where !name.EndsWith(".ignore", StringComparison.InvariantCultureIgnoreCase) &&

                                           !name.StartsWith(FolderConstants.CacheFolder + "/",

                                                            StringComparison.InvariantCultureIgnoreCase)

                                     select info).ToList();

                        ProcessDeletedFiles(accountInfo, cleanRemotes, pollTime);

                        //Create a list of actions from the remote files

                        var allActions = ObjectsToActions(accountInfo, cleanRemotes);

                        //var relativePath = objectInfo.RelativeUrlToFilePath(accountInfo.UserName);

                        //And remove those that are already being processed by the agent

                        var distinctActions = allActions

                            .Except(_agent.GetEnumerable(), new PithosMonitor.LocalFileComparer())

                            .ToList();

                        //Queue all the actions

                        foreach (var message in distinctActions)

                        {

                            Post(message);

                        }

                        Log.Info("[LISTENER] End Processing");

                    }

                }

                catch (Exception ex)

                {

                    Log.ErrorFormat("[FAIL] ListObjects for{0} in ProcessRemoteFiles with {1}", accountInfo.UserName, ex);

                    return;

                }

                Log.Info("[LISTENER] Finished");

            }

        }

        /// <summary>

        /// Deletes local files that are not found in the list of cloud files

        /// </summary>

        /// <param name="accountInfo"></param>

        /// <param name="cloudFiles"></param>

        /// <param name="pollTime"></param>

        private void ProcessDeletedFiles(AccountInfo accountInfo, IEnumerable<ObjectInfo> cloudFiles, DateTime pollTime)

        {

            if (accountInfo == null)

                throw new ArgumentNullException("accountInfo");

            if (String.IsNullOrWhiteSpace(accountInfo.AccountPath))

                throw new ArgumentException("The AccountInfo.AccountPath is empty", "accountInfo");

            if (cloudFiles == null)

                throw new ArgumentNullException("cloudFiles");

            Contract.EndContractBlock();

            //Check the Modified date to ensure that were just created and haven't been uploaded yet

            //NOTE: The NHibernate LINQ provider doesn't support custom functions so we need to break the query 

            //in two steps

            //NOTE: DON'T return files that are already in conflict. The first poll would mark them as 

            //"In Conflict" but subsequent polls would delete them

/*            var t=FileState.Find(new Guid("{cd664c9a-5f17-47c9-b27f-3bcbcb0595ff}"));

            var d0 = FileState.Queryable

                .Where(state => 

                            state.FilePath.StartsWith(accountInfo.AccountPath)).ToList();

            var d1 = FileState.Queryable

                .Where(state => state.Modified <= pollTime).ToList();

            var d2= FileState.Queryable

                .Where(state => state.Modified <= pollTime

                            &&

                            state.FilePath.StartsWith(accountInfo.AccountPath)).ToList();*/

            var deleteCandidates = FileState.Queryable

                .Where(state => state.Modified <= pollTime

                            &&

                            state.FilePath.StartsWith(accountInfo.AccountPath)

                            && state.FileStatus != FileStatus.Conflict).ToList();

/*

            var deleteCandidates = (from state in FileState.Queryable

                                   where 

                                        state.Modified <= pollTime 

                                        && state.FilePath.StartsWith(accountInfo.AccountPath)

                                        && state.FileStatus != FileStatus.Conflict

                                   select state).ToList();

*/

            var filesToDelete = (from deleteCandidate in deleteCandidates 

                         let localFile = FileInfoExtensions.FromPath(deleteCandidate.FilePath) 

                         let relativeFilePath = localFile.AsRelativeTo(accountInfo.AccountPath) 

                         where !cloudFiles.Any(r => Path.Combine(r.Container, r.Name) == relativeFilePath) 

                         select localFile).ToList();

            //On the first run

            if (_firstPoll)

            {

                //Set the status of missing files to Conflict

                foreach (var item in filesToDelete)

                {

                    StatusKeeper.SetFileState(item.FullName, FileStatus.Conflict, FileOverlayStatus.Deleted);

                }

                StatusNotification.NotifyConflicts(filesToDelete, String.Format("{0} local files are missing from Pithos, possibly because they were deleted",filesToDelete.Count));

            }

            else

            {

                foreach (var item in filesToDelete)

                {

                    item.Delete();

                    StatusKeeper.ClearFileStatus(item.FullName);

                }

                StatusNotification.NotifyForFiles(filesToDelete, String.Format("{0} files were deleted",filesToDelete.Count),TraceLevel.Info);

            }

        }

        private static void CreateContainerFolders(AccountInfo accountInfo, IEnumerable<ContainerInfo> containers)

        {

            var containerPaths = from container in containers

                                 let containerPath = Path.Combine(accountInfo.AccountPath, container.Name)

                                 where container.Name != FolderConstants.TrashContainer && !Directory.Exists(containerPath)

                                 select containerPath;

            foreach (var path in containerPaths)

            {

                Directory.CreateDirectory(path);

            }

        }

        //Creates an appropriate action for each server file

        private IEnumerable<CloudAction> ObjectsToActions(AccountInfo accountInfo,IEnumerable<ObjectInfo> remote)

        {

            if (remote==null)

                throw new ArgumentNullException();

            Contract.EndContractBlock();

            var fileAgent = GetFileAgent(accountInfo);

            //In order to avoid multiple iterations over the files, we iterate only once

            //over the remote files

            foreach (var objectInfo in remote)

            {

                var relativePath = objectInfo.RelativeUrlToFilePath(accountInfo.UserName);

                //and remove any matching objects from the list, adding them to the commonObjects list

                if (fileAgent.Exists(relativePath))

                {

                    //If a directory object already exists, we don't need to perform any other action                    

                    var localFile = fileAgent.GetFileSystemInfo(relativePath);

                    if (objectInfo.Content_Type == @"application/directory" && localFile is DirectoryInfo)

                        continue;

                    using (new SessionScope(FlushAction.Never))

                    {

                        var state =  StatusKeeper.GetStateByFilePath(localFile.FullName);

                        //FileState.FindByFilePath(localFile.FullName);

                        //Common files should be checked on a per-case basis to detect differences, which is newer

                        yield return new CloudAction(accountInfo, CloudActionType.MustSynch,

                                                     localFile, objectInfo, state, accountInfo.BlockSize,

                                                     accountInfo.BlockHash);

                    }

                }

                else

                {

                    //If there is no match we add them to the localFiles list

                    //but only if the file is not marked for deletion

                    var targetFile = Path.Combine(accountInfo.AccountPath, relativePath);

                    var fileStatus = StatusKeeper.GetFileStatus(targetFile);

                    if (fileStatus != FileStatus.Deleted)

                    {

                        //Remote files should be downloaded

                        yield return new CloudDownloadAction(accountInfo,objectInfo);

                    }

                }

            }            

        }

        private static FileAgent GetFileAgent(AccountInfo accountInfo)

        {

            return AgentLocator<FileAgent>.Get(accountInfo.AccountPath);

        }

        private void ProcessTrashedFiles(AccountInfo accountInfo,IEnumerable<ObjectInfo> trashObjects)

        {

            var fileAgent = GetFileAgent(accountInfo);

            foreach (var trashObject in trashObjects)

            {

                var barePath = trashObject.RelativeUrlToFilePath(accountInfo.UserName);

                //HACK: Assume only the "pithos" container is used. Must find out what happens when

                //deleting a file from a different container

                var relativePath = Path.Combine("pithos", barePath);

                fileAgent.Delete(relativePath);                                

            }

        }

        private void RenameCloudFile(AccountInfo accountInfo,CloudMoveAction action)

        {

            if (accountInfo==null)

                throw new ArgumentNullException("accountInfo");

            if (action==null)

                throw new ArgumentNullException("action");

            if (action.CloudFile==null)

                throw new ArgumentException("CloudFile","action");

            if (action.LocalFile==null)

                throw new ArgumentException("LocalFile","action");

            if (action.OldLocalFile==null)

                throw new ArgumentException("OldLocalFile","action");

            if (action.OldCloudFile==null)

                throw new ArgumentException("OldCloudFile","action");

            Contract.EndContractBlock();

            var newFilePath = action.LocalFile.FullName;

            //How do we handle concurrent renames and deletes/uploads/downloads?

            //* A conflicting upload means that a file was renamed before it had a chance to finish uploading

            //  This should never happen as the network agent executes only one action at a time

            //* A conflicting download means that the file was modified on the cloud. While we can go on and complete

            //  the rename, there may be a problem if the file is downloaded in blocks, as subsequent block requests for the 

            //  same name will fail.

            //  This should never happen as the network agent executes only one action at a time.

            //* A conflicting delete can happen if the rename was followed by a delete action that didn't have the chance

            //  to remove the rename from the queue.

            //  We can probably ignore this case. It will result in an error which should be ignored            

            //The local file is already renamed

            StatusKeeper.SetFileOverlayStatus(newFilePath, FileOverlayStatus.Modified);

            var account = action.CloudFile.Account ?? accountInfo.UserName;

            var container = action.CloudFile.Container;

            var client = new CloudFilesClient(accountInfo);

            //TODO: What code is returned when the source file doesn't exist?

            client.MoveObject(account, container, action.OldCloudFile.Name, container, action.CloudFile.Name);

            StatusKeeper.SetFileStatus(newFilePath, FileStatus.Unchanged);

            StatusKeeper.SetFileOverlayStatus(newFilePath, FileOverlayStatus.Normal);

            NativeMethods.RaiseChangeNotification(newFilePath);

        }

        private void DeleteCloudFile(AccountInfo accountInfo, ObjectInfo cloudFile)

        {

            if (accountInfo == null)

                throw new ArgumentNullException("accountInfo");

            if (cloudFile==null)

                throw new ArgumentNullException("cloudFile");

            if (String.IsNullOrWhiteSpace(cloudFile.Container))

                throw new ArgumentException("Invalid container", "cloudFile");

            Contract.EndContractBlock();

            var fileAgent = GetFileAgent(accountInfo);

            using ( log4net.ThreadContext.Stacks["DeleteCloudFile"].Push("Delete"))

            {

                var fileName= cloudFile.RelativeUrlToFilePath(accountInfo.UserName);

                var info = fileAgent.GetFileSystemInfo(fileName);                

                var fullPath = info.FullName.ToLower();

                StatusKeeper.SetFileOverlayStatus(fullPath, FileOverlayStatus.Modified);

                var account = cloudFile.Account ?? accountInfo.UserName;

                var container = cloudFile.Container ;//?? FolderConstants.PithosContainer;

                var client = new CloudFilesClient(accountInfo);

                client.DeleteObject(account, container, cloudFile.Name);

                StatusKeeper.ClearFileStatus(fullPath);

            }

        }

        //Download a file.

        private async Task DownloadCloudFile(AccountInfo accountInfo, ObjectInfo cloudFile , string filePath)

        {

            if (accountInfo == null)

                throw new ArgumentNullException("accountInfo");

            if (cloudFile == null)

                throw new ArgumentNullException("cloudFile");

            if (String.IsNullOrWhiteSpace(cloudFile.Account))

                throw new ArgumentNullException("cloudFile");

            if (String.IsNullOrWhiteSpace(cloudFile.Container))

                throw new ArgumentNullException("cloudFile");

            if (String.IsNullOrWhiteSpace(filePath))

                throw new ArgumentNullException("filePath");

            if (!Path.IsPathRooted(filePath))

                throw new ArgumentException("The filePath must be rooted", "filePath");

            Contract.EndContractBlock();

            var localPath = Interfaces.FileInfoExtensions.GetProperFilePathCapitalization(filePath);

            var relativeUrl = new Uri(cloudFile.Name, UriKind.Relative);

            var url = relativeUrl.ToString();

            if (cloudFile.Name.EndsWith(".ignore", StringComparison.InvariantCultureIgnoreCase))

                return;

            //Are we already downloading or uploading the file? 

            using (var gate=NetworkGate.Acquire(localPath, NetworkOperation.Downloading))

            {

                if (gate.Failed)

                    return;

                //The file's hashmap will be stored in the same location with the extension .hashmap

                //var hashPath = Path.Combine(FileAgent.CachePath, relativePath + ".hashmap");

                var client = new CloudFilesClient(accountInfo);

                var account = cloudFile.Account;

                var container = cloudFile.Container;

                if (cloudFile.Content_Type == @"application/directory")

                {

                    if (!Directory.Exists(localPath))

                        Directory.CreateDirectory(localPath);

                }

                else

                {

                    //Retrieve the hashmap from the server

                    var serverHash = await client.GetHashMap(account, container, url);

                    //If it's a small file

                    if (serverHash.Hashes.Count == 1)

                        //Download it in one go

                        await

                            DownloadEntireFileAsync(accountInfo, client, cloudFile, relativeUrl, localPath, serverHash);

                        //Otherwise download it block by block

                    else

                        await DownloadWithBlocks(accountInfo, client, cloudFile, relativeUrl, localPath, serverHash);

                    if (cloudFile.AllowedTo == "read")

                    {

                        var attributes = File.GetAttributes(localPath);

                        File.SetAttributes(localPath, attributes | FileAttributes.ReadOnly);                        

                    }

                }

                //Now we can store the object's metadata without worrying about ghost status entries

                StatusKeeper.StoreInfo(localPath, cloudFile);

            }

        }

        //Download a small file with a single GET operation

        private async Task DownloadEntireFileAsync(AccountInfo accountInfo, CloudFilesClient client, ObjectInfo cloudFile, Uri relativeUrl, string filePath,TreeHash serverHash)

        {

            if (client == null)

                throw new ArgumentNullException("client");

            if (cloudFile==null)

                throw new ArgumentNullException("cloudFile");

            if (relativeUrl == null)

                throw new ArgumentNullException("relativeUrl");

            if (String.IsNullOrWhiteSpace(filePath))

                throw new ArgumentNullException("filePath");

            if (!Path.IsPathRooted(filePath))

                throw new ArgumentException("The localPath must be rooted", "filePath");

            Contract.EndContractBlock();

            var localPath = Pithos.Interfaces.FileInfoExtensions.GetProperFilePathCapitalization(filePath);

            //If the file already exists

            if (File.Exists(localPath))

            {

                //First check with MD5 as this is a small file

                var localMD5 = Signature.CalculateMD5(localPath);

                var cloudHash=serverHash.TopHash.ToHashString();

                if (localMD5==cloudHash)

                    return;

                //Then check with a treehash

                var localTreeHash = Signature.CalculateTreeHash(localPath, serverHash.BlockSize, serverHash.BlockHash);

                var localHash = localTreeHash.TopHash.ToHashString();

                if (localHash==cloudHash)

                    return;

            }

            var fileAgent = GetFileAgent(accountInfo);

            //Calculate the relative file path for the new file

            var relativePath = relativeUrl.RelativeUriToFilePath();

            //The file will be stored in a temporary location while downloading with an extension .download

            var tempPath = Path.Combine(fileAgent.CachePath, relativePath + ".download");

            //Make sure the target folder exists. DownloadFileTask will not create the folder

            var tempFolder = Path.GetDirectoryName(tempPath);

            if (!Directory.Exists(tempFolder))

                Directory.CreateDirectory(tempFolder);

            //Download the object to the temporary location

            await client.GetObject(cloudFile.Account, cloudFile.Container, relativeUrl.ToString(), tempPath);

            //Create the local folder if it doesn't exist (necessary for shared objects)

            var localFolder = Path.GetDirectoryName(localPath);

            if (!Directory.Exists(localFolder))

                Directory.CreateDirectory(localFolder);            

            //And move it to its actual location once downloading is finished

            if (File.Exists(localPath))

                File.Replace(tempPath,localPath,null,true);

            else

                File.Move(tempPath,localPath);

            //Notify listeners that a local file has changed

            StatusNotification.NotifyChangedFile(localPath);

        }

        //Download a file asynchronously using blocks

        public async Task DownloadWithBlocks(AccountInfo accountInfo, CloudFilesClient client, ObjectInfo cloudFile, Uri relativeUrl, string filePath, TreeHash serverHash)

        {

            if (client == null)

                throw new ArgumentNullException("client");