// **************************************************************************
// * This file is part of the FreeFileSync project. It is distributed under *
// * GNU General Public License: http://www.gnu.org/licenses/gpl-3.0        *
// * Copyright (C) Zenju (zenju AT gmx DOT de) - All Rights Reserved        *
// **************************************************************************

#include "dir_watcher.h"
#include <algorithm>
#include <set>
#include "thread.h"
#include "scope_guard.h"

#ifdef ZEN_WIN
    #include "device_notify.h"
    #include "win.h" //includes "windows.h"
    #include "long_path_prefix.h"
    #include "optional.h"

#elif defined ZEN_LINUX
    #include <map>
    #include <sys/inotify.h>
    #include <fcntl.h> //fcntl
    #include <unistd.h> //close
    #include <limits.h> //NAME_MAX
    #include "file_traverser.h"

#elif defined ZEN_MAC
    #include <CoreServices/CoreServices.h>
    #include "osx_string.h"
#endif

using namespace zen;


#ifdef ZEN_WIN
namespace
{
class SharedData
{
public:
    //context of worker thread
    bool tryAddChanges(const char* buffer, DWORD bytesWritten, const Zstring& dirPathPf) //noexcept; return false on failure (already reported!)
    {
        std::lock_guard<std::mutex> dummy(lockAccess);

        if (bytesWritten == 0) //according to docu this may happen in case of internal buffer overflow: report some "dummy" change
            changedFiles.emplace_back(DirWatcher::ACTION_CREATE, L"Overflow.");
        else
        {
            size_t offset = 0;
            for (;;)
            {
                const FILE_NOTIFY_INFORMATION& notifyInfo = *reinterpret_cast<const FILE_NOTIFY_INFORMATION*>(buffer + offset);

                //access-violation crash dumps suggest that the buffer can be corrupt, so let's validate:
                if (offset + sizeof(FILE_NOTIFY_INFORMATION) > bytesWritten ||
                    offset + offsetof(FILE_NOTIFY_INFORMATION, FileName) + notifyInfo.FileNameLength > bytesWritten)
                {
                    reportError(replaceCpy(_("Cannot monitor directory %x."), L"%x", fmtPath(dirPathPf)), L"ReadDirectoryChangesW: corrupt data returned");
                    return false;
                }

                const Zstring fullpath = dirPathPf + Zstring(notifyInfo.FileName, notifyInfo.FileNameLength / sizeof(WCHAR));

                [&]
                {
                    //skip modifications sent by changed directories: reason for change, child element creation/deletion, will notify separately!
                    //and if this child element is a .ffs_lock file we'll want to ignore all associated events!
                    if (notifyInfo.Action == FILE_ACTION_MODIFIED)
                    {
                        //note: this check will not work if top watched directory has been renamed
                        const DWORD ret = ::GetFileAttributes(applyLongPathPrefix(fullpath).c_str());
                        if (ret != INVALID_FILE_ATTRIBUTES && (ret & FILE_ATTRIBUTE_DIRECTORY)) //returns true for (dir-)symlinks also
                            return;
                    }

                    //note: a move across directories will show up as FILE_ACTION_ADDED/FILE_ACTION_REMOVED!
                    switch (notifyInfo.Action)
                    {
                        case FILE_ACTION_ADDED:
                        case FILE_ACTION_RENAMED_NEW_NAME: //harmonize with "move" which is notified as "create + delete"
                            changedFiles.emplace_back(DirWatcher::ACTION_CREATE, fullpath);
                            break;
                        case FILE_ACTION_REMOVED:
                        case FILE_ACTION_RENAMED_OLD_NAME:
                            changedFiles.emplace_back(DirWatcher::ACTION_DELETE, fullpath);
                            break;
                        case FILE_ACTION_MODIFIED:
                            changedFiles.emplace_back(DirWatcher::ACTION_UPDATE, fullpath);
                            break;
                    }
                }();

                if (notifyInfo.NextEntryOffset == 0)
                    break;
                offset += notifyInfo.NextEntryOffset;
            }
        }
        return true;
    }

    ////context of main thread
    //void addChange(const Zstring& dirPath) //throw ()
    //{
    //    std::lock_guard<std::mutex> dummy(lockAccess);
    //    changedFiles.insert(dirPath);
    //}


    //context of main thread
    void fetchChanges(std::vector<DirWatcher::Entry>& output) //throw FileError
    {
        std::lock_guard<std::mutex> dummy(lockAccess);

        //first check whether errors occurred in thread
        if (errorInfo)
            throw FileError(copyStringTo<std::wstring>(errorInfo->msg),
                            copyStringTo<std::wstring>(errorInfo->descr));

        output.swap(changedFiles);
        changedFiles.clear();
    }


    //context of worker thread
    void reportError(const std::wstring& msg, const std::wstring& description) //throw()
    {
        std::lock_guard<std::mutex> dummy(lockAccess);
        errorInfo = ErrorInfo({ copyStringTo<BasicWString>(msg), copyStringTo<BasicWString>(description) });
    }

private:
    typedef Zbase<wchar_t> BasicWString; //thread safe string class for UI texts

    std::mutex lockAccess;
    std::vector<DirWatcher::Entry> changedFiles;

    struct ErrorInfo
    {
        BasicWString msg;
        BasicWString descr;
    };
    Opt<ErrorInfo> errorInfo; //non-empty if errors occurred in thread
};


class ReadChangesAsync
{
public:
    //constructed in main thread!
    ReadChangesAsync(const Zstring& directory, //make sure to not leak-in thread-unsafe types!
                     const std::shared_ptr<SharedData>& shared) :
        shared_(shared),
        dirPathPf(appendSeparator(directory))
    {
        hDir = ::CreateFile(applyLongPathPrefix(dirPathPf).c_str(),                 //_In_      LPCTSTR lpFileName,
                            FILE_LIST_DIRECTORY,                                    //_In_      DWORD dwDesiredAccess,
                            FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE, //_In_      DWORD dwShareMode,
                            nullptr,              //_In_opt_  LPSECURITY_ATTRIBUTES lpSecurityAttributes,
                            OPEN_EXISTING,        //_In_      DWORD dwCreationDisposition,
                            FILE_FLAG_BACKUP_SEMANTICS |
                            FILE_FLAG_OVERLAPPED, //_In_      DWORD dwFlagsAndAttributes,
                            nullptr);             //_In_opt_  HANDLE hTemplateFile
        if (hDir == INVALID_HANDLE_VALUE)
            THROW_LAST_FILE_ERROR(replaceCpy(_("Cannot monitor directory %x."), L"%x", fmtPath(directory)), L"CreateFile");

        //end of constructor, no need to start managing "hDir"
    }

    ReadChangesAsync(ReadChangesAsync&& other) : shared_(std::move(other.shared_)),
        dirPathPf(std::move(other.dirPathPf)),
        hDir(other.hDir)
    {
        other.hDir = INVALID_HANDLE_VALUE;
    }

    ~ReadChangesAsync()
    {
        if (hDir != INVALID_HANDLE_VALUE) //valid hDir is NOT an invariant, see move constructor!
            ::CloseHandle(hDir);
    }

    void operator()() const //thread entry
    {
        std::vector<char> buffer(64 * 1024); //needs to be aligned on a DWORD boundary; maximum buffer size restricted by some networks protocols (according to docu)

        for (;;)
        {
            interruptionPoint(); //throw ThreadInterruption

            //actual work
            OVERLAPPED overlapped = {};
            overlapped.hEvent = ::CreateEvent(nullptr,  //__in_opt  LPSECURITY_ATTRIBUTES lpEventAttributes,
                                              true,     //__in      BOOL bManualReset,
                                              false,    //__in      BOOL bInitialState,
                                              nullptr); //__in_opt  LPCTSTR lpName
            if (overlapped.hEvent == nullptr)
            {
                const DWORD ec = ::GetLastError(); //copy before directly/indirectly making other system calls!
                return shared_->reportError(replaceCpy(_("Cannot monitor directory %x."), L"%x", fmtPath(dirPathPf)), formatSystemError(L"CreateEvent", ec));
            }
            ZEN_ON_SCOPE_EXIT(::CloseHandle(overlapped.hEvent));

            DWORD bytesReturned = 0; //should not be needed for async calls, still pass it to help broken drivers

            //asynchronous variant: runs on this thread's APC queue!
            if (!::ReadDirectoryChangesW(hDir,                              //  __in   HANDLE hDirectory,
                                         &buffer[0],                        //  __out  LPVOID lpBuffer,
                                         static_cast<DWORD>(buffer.size()), //  __in   DWORD nBufferLength,
                                         true,                              //  __in   BOOL bWatchSubtree,
                                         FILE_NOTIFY_CHANGE_FILE_NAME |
                                         FILE_NOTIFY_CHANGE_DIR_NAME  |
                                         FILE_NOTIFY_CHANGE_SIZE      |
                                         FILE_NOTIFY_CHANGE_LAST_WRITE, //  __in         DWORD dwNotifyFilter,
                                         &bytesReturned,                //  __out_opt    LPDWORD lpBytesReturned,
                                         &overlapped,                   //  __inout_opt  LPOVERLAPPED lpOverlapped,
                                         nullptr))                      //  __in_opt     LPOVERLAPPED_COMPLETION_ROUTINE lpCompletionRoutine
            {
                const DWORD ec = ::GetLastError(); //copy before directly/indirectly making other system calls!
                return shared_->reportError(replaceCpy(_("Cannot monitor directory %x."), L"%x", fmtPath(dirPathPf)), formatSystemError(L"ReadDirectoryChangesW", ec));
            }

            //async I/O is a resource that needs to be guarded since it will write to local variable "buffer"!
            auto guardAio = zen::makeGuard<ScopeGuardRunMode::ON_EXIT>([&]
            {
                //Canceling Pending I/O Operations: http://msdn.microsoft.com/en-us/library/aa363789(v=vs.85).aspx
#ifdef ZEN_WIN_VISTA_AND_LATER
                if (::CancelIoEx(hDir, &overlapped) /*!= FALSE*/ || ::GetLastError() != ERROR_NOT_FOUND)
#else
                if (::CancelIo(hDir) /*!= FALSE*/ || ::GetLastError() != ERROR_NOT_FOUND)
#endif
                {
                    DWORD bytesWritten = 0;
                    ::GetOverlappedResult(hDir, &overlapped, &bytesWritten, true); //must wait until cancellation is complete!
                }
            });

            //wait for results
            DWORD bytesWritten = 0;
            while (!::GetOverlappedResult(hDir,          //__in   HANDLE hFile,
                                          &overlapped,   //__in   LPOVERLAPPED lpOverlapped,
                                          &bytesWritten, //__out  LPDWORD lpNumberOfBytesTransferred,
                                          false))        //__in   BOOL bWait
            {
                const DWORD ec = ::GetLastError(); //copy before directly/indirectly making other system calls!
                if (ec != ERROR_IO_INCOMPLETE)
                    return shared_->reportError(replaceCpy(_("Cannot monitor directory %x."), L"%x", fmtPath(dirPathPf)), formatSystemError(L"GetOverlappedResult", ec));

                //execute asynchronous procedure calls (APC) queued on this thread
                ::SleepEx(50,    // __in  DWORD dwMilliseconds,
                          true); // __in  BOOL bAlertable

                interruptionPoint(); //throw ThreadInterruption
            }
            guardAio.dismiss();

            if (!shared_->tryAddChanges(&buffer[0], bytesWritten, dirPathPf)) //noexcept
                return;
        }
    }

    HANDLE getDirHandle() const { return hDir; } //for reading/monitoring purposes only, don't abuse (e.g. close handle)!

private:
    ReadChangesAsync           (const ReadChangesAsync&) = delete;
    ReadChangesAsync& operator=(const ReadChangesAsync&) = delete;

    //shared between main and worker:
    std::shared_ptr<SharedData> shared_;
    //worker thread only:
    Zstring dirPathPf; //thread safe!
    HANDLE hDir = INVALID_HANDLE_VALUE;
};


class HandleVolumeRemoval
{
public:
    HandleVolumeRemoval(HANDLE hDir,
                        const Zstring& displayPath,
                        InterruptibleThread& worker) :
        notificationHandle(registerFolderRemovalNotification(hDir, //throw FileError
                                                             displayPath,
                                                             [this] { this->onRequestRemoval (); }, //noexcept!
    [this](bool successful) { this->onRemovalFinished(); })), //
    worker_(worker) {}

    ~HandleVolumeRemoval()
    {
        unregisterDeviceNotification(notificationHandle);
    }

    //all functions are called by main thread!

    bool requestReceived() const { return removalRequested; }
    bool finished() const { return operationComplete; }

private:
    void onRequestRemoval() //noexcept!
    {
        //must release hDir immediately => stop monitoring!
        if (worker_.joinable()) //= join() precondition: play safe; can't trust Windows to only call-back once
        {
            worker_.interrupt();
            worker_.join(); //we assume precondition "worker.joinable()"!!!
            //now hDir should have been released
        }

        removalRequested = true;
    } //don't throw!

    void onRemovalFinished() { operationComplete = true; } //noexcept!

    DeviceNotificationHandle* notificationHandle;
    InterruptibleThread& worker_;
    bool removalRequested  = false;
    bool operationComplete = false;
};
}


struct DirWatcher::Pimpl
{
    InterruptibleThread worker;
    std::shared_ptr<SharedData> shared;
    std::unique_ptr<HandleVolumeRemoval> volRemoval;
};


DirWatcher::DirWatcher(const Zstring& dirPath) : //throw FileError
    baseDirPath(dirPath),
    pimpl_(std::make_unique<Pimpl>())
{
    pimpl_->shared = std::make_shared<SharedData>();

    ReadChangesAsync reader(dirPath, pimpl_->shared); //throw FileError
    pimpl_->volRemoval = std::make_unique<HandleVolumeRemoval>(reader.getDirHandle(), dirPath, pimpl_->worker); //throw FileError
    pimpl_->worker = InterruptibleThread(std::move(reader));
}


DirWatcher::~DirWatcher()
{
    if (pimpl_->worker.joinable()) //= thread::detach() precondition! -> may already be joined by HandleVolumeRemoval::onRequestRemoval()
    {
        pimpl_->worker.interrupt();
        pimpl_->worker.detach(); //we don't have time to wait... will take ~50ms anyway:
    }
    //caveat: exitting the app may simply kill this thread!
}


std::vector<DirWatcher::Entry> DirWatcher::getChanges(const std::function<void()>& processGuiMessages) //throw FileError
{
    std::vector<Entry> output;
    pimpl_->shared->fetchChanges(output); //throw FileError

    //wait until device removal is confirmed, to prevent locking hDir again by some new watch!
    if (pimpl_->volRemoval->requestReceived())
    {
        const std::chrono::steady_clock::time_point endTime = std::chrono::steady_clock::now() + std::chrono::seconds(15);
        //HandleVolumeRemoval::finished() not guaranteed! note: Windows gives unresponsive applications ca. 10 seconds until unmounting the usb stick in worst case

        while (!pimpl_->volRemoval->finished() && std::chrono::steady_clock::now() < endTime)
        {
            processGuiMessages(); //DBT_DEVICEREMOVECOMPLETE message is sent here!
            std::this_thread::sleep_for(std::chrono::milliseconds(50));
        }

        output.emplace_back(ACTION_DELETE, baseDirPath); //report removal as change to main directory
    }

    return output;
}


#elif defined ZEN_LINUX
struct DirWatcher::Pimpl
{
    int notifDescr = 0;
    std::map<int, Zstring> watchDescrs; //watch descriptor and (sub-)directory name (postfixed with separator) -> owned by "notifDescr"
};


DirWatcher::DirWatcher(const Zstring& dirPath) : //throw FileError
    baseDirPath(dirPath),
    pimpl_(std::make_unique<Pimpl>())
{
    //get all subdirectories
    std::vector<Zstring> fullFolderList { baseDirPath };
    {
        std::function<void (const Zstring& path)> traverse;

        traverse = [&traverse, &fullFolderList](const Zstring& path)
        {
            traverseFolder(path, nullptr,
            [&](const DirInfo& di ) { fullFolderList.push_back(di.fullPath); traverse(di.fullPath); },
            nullptr, //don't traverse into symlinks (analog to windows build)
            [&](const std::wstring& errorMsg) { throw FileError(errorMsg); });
        };

        traverse(baseDirPath);
    }

    //init
    pimpl_->notifDescr  = ::inotify_init();
    if (pimpl_->notifDescr == -1)
        THROW_LAST_FILE_ERROR(replaceCpy(_("Cannot monitor directory %x."), L"%x", fmtPath(baseDirPath)), L"inotify_init");

    ZEN_ON_SCOPE_FAIL( ::close(pimpl_->notifDescr); );

    //set non-blocking mode
    bool initSuccess = false;
    {
        int flags = ::fcntl(pimpl_->notifDescr, F_GETFL);
        if (flags != -1)
            initSuccess = ::fcntl(pimpl_->notifDescr, F_SETFL, flags | O_NONBLOCK) != -1;
    }
    if (!initSuccess)
        THROW_LAST_FILE_ERROR(replaceCpy(_("Cannot monitor directory %x."), L"%x", fmtPath(baseDirPath)), L"fcntl");

    //add watches
    for (const Zstring& subDirPath : fullFolderList)
    {
        int wd = ::inotify_add_watch(pimpl_->notifDescr, subDirPath.c_str(),
                                     IN_ONLYDIR     | //"Only watch pathname if it is a directory."
                                     IN_DONT_FOLLOW | //don't follow symbolic links
                                     IN_CREATE      |
                                     IN_MODIFY      |
                                     IN_CLOSE_WRITE |
                                     IN_DELETE      |
                                     IN_DELETE_SELF |
                                     IN_MOVED_FROM  |
                                     IN_MOVED_TO    |
                                     IN_MOVE_SELF);
        if (wd == -1)
        {
            const ErrorCode ec = getLastError(); //copy before directly/indirectly making other system calls!
            if (ec == ENOSPC) //fix misleading system message "No space left on device"
                throw FileError(replaceCpy(_("Cannot monitor directory %x."), L"%x", fmtPath(subDirPath)),
                                formatSystemError(L"inotify_add_watch", ec, L"The user limit on the total number of inotify watches was reached or the kernel failed to allocate a needed resource."));

            throw FileError(replaceCpy(_("Cannot monitor directory %x."), L"%x", fmtPath(subDirPath)), formatSystemError(L"inotify_add_watch", ec));
        }

        pimpl_->watchDescrs.emplace(wd, appendSeparator(subDirPath));
    }
}


DirWatcher::~DirWatcher()
{
    ::close(pimpl_->notifDescr); //associated watches are removed automatically!
}


std::vector<DirWatcher::Entry> DirWatcher::getChanges(const std::function<void()>&) //throw FileError
{
    std::vector<char> buffer(512 * (sizeof(struct ::inotify_event) + NAME_MAX + 1));

    ssize_t bytesRead = 0;
    do
    {
        //non-blocking call, see O_NONBLOCK
        bytesRead = ::read(pimpl_->notifDescr, &buffer[0], buffer.size());
    }
    while (bytesRead < 0 && errno == EINTR); //"Interrupted function call; When this happens, you should try the call again."

    if (bytesRead < 0)
    {
        if (errno == EAGAIN)  //this error is ignored in all inotify wrappers I found
            return std::vector<Entry>();

        THROW_LAST_FILE_ERROR(replaceCpy(_("Cannot monitor directory %x."), L"%x", fmtPath(baseDirPath)), L"read");
    }

    std::vector<Entry> output;

    ssize_t bytePos = 0;
    while (bytePos < bytesRead)
    {
        struct ::inotify_event& evt = reinterpret_cast<struct ::inotify_event&>(buffer[bytePos]);

        if (evt.len != 0) //exclude case: deletion of "self", already reported by parent directory watch
        {
            auto it = pimpl_->watchDescrs.find(evt.wd);
            if (it != pimpl_->watchDescrs.end())
            {
                //Note: evt.len is NOT the size of the evt.name c-string, but the array size including all padding 0 characters!
                //It may be even 0 in which case evt.name must not be used!
                const Zstring fullname = it->second + evt.name;

                if ((evt.mask & IN_CREATE) ||
                    (evt.mask & IN_MOVED_TO))
                    output.emplace_back(ACTION_CREATE, fullname);
                else if ((evt.mask & IN_MODIFY) ||
                         (evt.mask & IN_CLOSE_WRITE))
                    output.emplace_back(ACTION_UPDATE, fullname);
                else if ((evt.mask & IN_DELETE     ) ||
                         (evt.mask & IN_DELETE_SELF) ||
                         (evt.mask & IN_MOVE_SELF  ) ||
                         (evt.mask & IN_MOVED_FROM))
                    output.emplace_back(ACTION_DELETE, fullname);
            }
        }
        bytePos += sizeof(struct ::inotify_event) + evt.len;
    }

    return output;
}

#elif defined ZEN_MAC
namespace
{
void eventCallback(ConstFSEventStreamRef streamRef,
                   void* clientCallBackInfo,
                   size_t numEvents,
                   void* eventPaths,
                   const FSEventStreamEventFlags eventFlags[],
                   const FSEventStreamEventId eventIds[])
{
    std::vector<DirWatcher::Entry>& changedFiles = *static_cast<std::vector<DirWatcher::Entry>*>(clientCallBackInfo);

    auto paths = static_cast<const char**>(eventPaths);
    for (size_t i = 0; i < numEvents; ++i)
    {
        //::printf("0x%08x\t%s\n", static_cast<unsigned int>(eventFlags[i]), paths[i]);

        //events are aggregated => it's possible to see a single event with flags
        //kFSEventStreamEventFlagItemCreated | kFSEventStreamEventFlagItemModified | kFSEventStreamEventFlagItemRemoved

        //https://developer.apple.com/library/mac/documentation/Darwin/Reference/FSEvents_Ref/index.html#//apple_ref/doc/constant_group/FSEventStreamEventFlags
        if (eventFlags[i] & kFSEventStreamEventFlagItemCreated ||
            eventFlags[i] & kFSEventStreamEventFlagMount)
            changedFiles.emplace_back(DirWatcher::ACTION_CREATE, paths[i]);
        if (eventFlags[i] & kFSEventStreamEventFlagItemModified      || //
            eventFlags[i] & kFSEventStreamEventFlagItemXattrMod      || //
            eventFlags[i] & kFSEventStreamEventFlagItemChangeOwner   || //aggregate these into a single event
            eventFlags[i] & kFSEventStreamEventFlagItemInodeMetaMod  || //
            eventFlags[i] & kFSEventStreamEventFlagItemFinderInfoMod || //
            eventFlags[i] & kFSEventStreamEventFlagItemRenamed       || //OS X sends the same event flag for both old and new names!!!
            eventFlags[i] & kFSEventStreamEventFlagMustScanSubDirs)  //something changed in one of the subdirs: NOT expected due to kFSEventStreamCreateFlagFileEvents
            changedFiles.emplace_back(DirWatcher::ACTION_UPDATE, paths[i]);
        if (eventFlags[i] & kFSEventStreamEventFlagItemRemoved ||
            eventFlags[i] & kFSEventStreamEventFlagRootChanged || //root is (indirectly) deleted or renamed
            eventFlags[i] & kFSEventStreamEventFlagUnmount)
            changedFiles.emplace_back(DirWatcher::ACTION_DELETE, paths[i]);

        //kFSEventStreamEventFlagEventIdsWrapped -> irrelevant!
        //kFSEventStreamEventFlagHistoryDone -> not expected due to kFSEventStreamEventIdSinceNow below
    }
}
}


struct DirWatcher::Pimpl
{
    FSEventStreamRef eventStream = nullptr;
    std::vector<DirWatcher::Entry> changedFiles;
};


DirWatcher::DirWatcher(const Zstring& dirPath) :
    baseDirPath(dirPath),
    pimpl_(std::make_unique<Pimpl>())
{
    CFStringRef dirpathCf = osx::createCFString(baseDirPath.c_str()); //returns nullptr on error
    if (!dirpathCf)
        throw FileError(replaceCpy(_("Cannot monitor directory %x."), L"%x", fmtPath(baseDirPath)), L"Function call failed: createCFString"); //no error code documented!
    ZEN_ON_SCOPE_EXIT(::CFRelease(dirpathCf));

    CFArrayRef dirpathCfArray = ::CFArrayCreate(nullptr,    //CFAllocatorRef allocator,
                                                reinterpret_cast<const void**>(&dirpathCf), //const void** values,
                                                1,          //CFIndex numValues,
                                                nullptr);   //const CFArrayCallBacks* callBacks
    if (!dirpathCfArray)
        throw FileError(replaceCpy(_("Cannot monitor directory %x."), L"%x", fmtPath(baseDirPath)), L"Function call failed: CFArrayCreate"); //no error code documented!
    ZEN_ON_SCOPE_EXIT(::CFRelease(dirpathCfArray));

    FSEventStreamContext context = {};
    context.info = &pimpl_->changedFiles;

    //can this fail?? not documented!
    pimpl_->eventStream = ::FSEventStreamCreate(nullptr,        //CFAllocatorRef allocator,
                                                &eventCallback, //FSEventStreamCallback callback,
                                                &context,       //FSEventStreamContext* context,
                                                dirpathCfArray, //CFArrayRef pathsToWatch,
                                                kFSEventStreamEventIdSinceNow, //FSEventStreamEventId sinceWhen,
                                                0,              //CFTimeInterval latency, in seconds
                                                kFSEventStreamCreateFlagWatchRoot |
                                                kFSEventStreamCreateFlagFileEvents); //FSEventStreamCreateFlags flags
    ZEN_ON_SCOPE_FAIL( ::FSEventStreamRelease(pimpl_->eventStream); );

    //no-fail:
    ::FSEventStreamScheduleWithRunLoop(pimpl_->eventStream,     //FSEventStreamRef streamRef,
                                       ::CFRunLoopGetCurrent(), //CFRunLoopRef runLoop;  CFRunLoopGetCurrent(): failure not documented!
                                       kCFRunLoopDefaultMode);  //CFStringRef runLoopMode
    ZEN_ON_SCOPE_FAIL( ::FSEventStreamInvalidate(pimpl_->eventStream); );

    if (!::FSEventStreamStart(pimpl_->eventStream))
        throw FileError(replaceCpy(_("Cannot monitor directory %x."), L"%x", fmtPath(baseDirPath)), L"Function call failed: FSEventStreamStart"); //no error code documented!
}


DirWatcher::~DirWatcher()
{
    ::FSEventStreamStop      (pimpl_->eventStream);
    ::FSEventStreamInvalidate(pimpl_->eventStream);
    ::FSEventStreamRelease   (pimpl_->eventStream);
}


std::vector<DirWatcher::Entry> DirWatcher::getChanges(const std::function<void()>&)
{
    ::FSEventStreamFlushSync(pimpl_->eventStream); //flushes pending events + execs runloop

    std::vector<DirWatcher::Entry> changes;
    changes.swap(pimpl_->changedFiles);
    return changes;
}
#endif