How to clear QFuture results in QFutureWatcher - qt

Question

I tried to block multiple file at once, then copy them to another location.
Both source and destination files should be blocked simultaneously. Therefore I can't use static QFile::copy() function.
To hold and move files I use QSharedPointer< QFile > due to QFile is neither copyable nor moveable.
To perform whole the operation entirely I use QtConcurrent framework. Namely: QtConcurrent::mappedReduced and QFutureWatcher.
To open all pairs of files I use map functor, then to sequentially copy them I use reduce functor.
using PFile = QSharedPointer< QFile >;
using PFileList = QList< PFile >;
template< typename Result, typename Functor >
struct FunctorWithResultType
: std::decay_t< Functor >
{
using result_type = Result;
FunctorWithResultType(Functor & functor)
: std::decay_t< Functor >{std::forward< Functor >(functor)}
{ ; }
};
template< typename Result, typename Functor >
FunctorWithResultType< Result, Functor >
addResultType(Functor && functor)
{
return {functor};
}
class Test
{
public :
QDir currentDirectory;
QFutureWatcher< PFileList > fileCopyFutureWatcher;
// ...
};
// ...
Test::Test()
{
auto onFilesCopied = [&]
{
PFileList copiedFiles = fileCopyFutureWatcher.result();
qCInfo(usbDevice) << "Files copy operation from drive finished.";
qCInfo(usbDevice) << copiedFiles.size();
for (PFile const & file : copiedFiles) {
//file->close(); // I want copiedFiles to be closed automatically at the end of current scope
}
};
connect(&fileCopyFutureWatcher, &fileCopyFutureWatcher.finished, onFilesCopied);
}
// ...
void Test::onDeviceAdded(QString deviceName)
{
qCInfo(usbDevice) << "USB device" << deviceName << "is added.";
if (!fileCopyFutureWatcher.isFinished()) {
return;
}
QDir sourceDirectory{deviceName};
if (!sourceDirectory.cd("dir")) {
qCInfo(usbDevice) << "Drive" << deviceName << "does not contain dir subdirectory.";
return;
}
auto destinationDirectory = currentDirectory;
if (!destinationDirectory.mkpath("fileCache")) {
qCCritical(usbDevice) << "Can't create fileCache subdirectory in"
<< destinationDirectory.absolutePath()
<< "directory";
return;
}
if (!destinationDirectory.cd("fileCache")) {
qCCritical(usbDevice) << "Can't change directory to fileCache subdirectory in"
<< destinationDirectory.absolutePath()
<< "directory";
return;
}
struct PFilePair
{
PFile source, destination;
};
auto openSourceAndDestinationFiles = [&, destinationDirectory] (QFileInfo const & fileInfo) -> PFilePair
{
auto source = PFile::create(fileInfo.absoluteFilePath());
QFile & sourceFile = *source;
if (!sourceFile.open(QFile::ReadOnly)) {
qCCritical(usbDevice) << "Can't open file" << sourceFile.fileName()
<< "to read:" << sourceFile.errorString();
return {};
}
auto destination = PFile::create(destinationDirectory.absoluteFilePath(fileInfo.fileName()));
QFile & destinationFile = *destination;;
if (!destinationFile.open(QFile::ReadWrite | QFile::Truncate)) {
qCCritical(usbDevice) << "Can't open file" << destinationFile.fileName()
<< "to write:" << destinationFile.errorString();
return {};
}
return {qMove(source), qMove(destination)};
};
auto copyFiles = [&] (PFileList & files, PFilePair const & filePair)
{
if (filePair.source.isNull() || filePair.destination.isNull()) {
return;
}
QFile & sourceFile = *filePair.source;
QFile & destinationFile = *filePair.destination;
qCInfo(usbDevice) << sourceFile.fileName() << "->" << destinationFile.fileName();
constexpr int size = (1 << 20); // 1MiB
QByteArray buffer{size, 0};
char * const data = buffer.data();
while (!sourceFile.atEnd()) {
auto bytesRead = sourceFile.read(data, size);
if (bytesRead < 0) {
qCCritical(usbDevice) << "Can't read file" << sourceFile.fileName()
<< ":" << sourceFile.errorString();
return;
}
auto bytesWritten = destinationFile.write(data, bytesRead);
while (bytesWritten < bytesRead) {
auto sizeWritten = destinationFile.write(data + bytesWritten, bytesRead - bytesWritten);
if (sizeWritten < 0) {
qCCritical(usbDevice) << "Can't write file" << destinationFile.fileName()
<< ":" << destinationFile.errorString();
return;
}
bytesWritten += sizeWritten;
}
Q_ASSERT(bytesWritten == bytesRead);
}
Q_ASSERT(sourceFile.size() == destinationFile.size());
destinationFile.flush();
files.append(filePair.destination);
};
QStringList nameFilters;
nameFilters << "file.dat";
// many other entries
auto entryInfoList = sourceDirectory.entryInfoList(nameFilters, (QDir::Readable | QDir::Files));
fileCopyFutureWatcher.setFuture(QtConcurrent::mappedReduced< PFileList >(qMove(entryInfoList), addResultType< PFilePair >(openSourceAndDestinationFiles), copyFiles));
}
// ...
After reading result in QFutureWatcher::finished event all the destination files are still opened and blocked by my application. Therefore I can conclude that copies of QSharedPointer< QFile > are still exists. I suspect that they leave in QFuture inside QFutureWatcher. How can I clear all of them (i.e. how to cause QFile::~QFile() to close all files) without calling QFutureWatcher::setFuture with fake QFuture instance?
To achieve this I need to steal result from QFuture, not to copy.

Answer 1

My workaround looks like following:
using PFile = QSharedPointer< QFile >;
using PFileList = QList< PFile >;
template< typename Result, typename Functor >
struct FunctorWithResultType
: std::decay_t< Functor >
{
using result_type = Result;
FunctorWithResultType(Functor & functor)
: std::decay_t< Functor >{std::forward< Functor >(functor)}
{ ; }
};
template< typename Result, typename Functor >
FunctorWithResultType< Result, Functor >
addResultType(Functor && functor)
{
return {functor};
}
class Test
{
public :
QDir currentDirectory;
// ...
};
// ...
void Test::onDeviceAdded(QString deviceName)
{
qCInfo(usbDevice) << "USB device" << deviceName << "is added.";
QDir sourceDirectory{deviceName};
if (!sourceDirectory.cd("dir")) {
qCInfo(usbDevice) << "Drive" << deviceName << "does not contain dir subdirectory.";
return;
}
auto destinationDirectory = currentDirectory;
if (!destinationDirectory.mkpath("fileCache")) {
qCCritical(usbDevice) << "Can't create fileCache subdirectory in"
<< destinationDirectory.absolutePath()
<< "directory";
return;
}
if (!destinationDirectory.cd("fileCache")) {
qCCritical(usbDevice) << "Can't change directory to fileCache subdirectory in"
<< destinationDirectory.absolutePath()
<< "directory";
return;
}
struct PFilePair
{
PFile source, destination;
};
auto openSourceAndDestinationFiles = [&, destinationDirectory] (QFileInfo const & fileInfo) -> PFilePair
{
auto source = PFile::create(fileInfo.absoluteFilePath());
QFile & sourceFile = *source;
if (!sourceFile.open(QFile::ReadOnly)) {
qCCritical(usbDevice) << "Can't open file" << sourceFile.fileName()
<< "to read:" << sourceFile.errorString();
return {};
}
auto destination = PFile::create(destinationDirectory.absoluteFilePath(fileInfo.fileName()));
QFile & destinationFile = *destination;;
if (!destinationFile.open(QFile::ReadWrite | QFile::Truncate)) {
qCCritical(usbDevice) << "Can't open file" << destinationFile.fileName()
<< "to write:" << destinationFile.errorString();
return {};
}
return {qMove(source), qMove(destination)};
};
auto copyFiles = [&] (PFileList & files, PFilePair const & filePair)
{
if (filePair.source.isNull() || filePair.destination.isNull()) {
return;
}
QFile & sourceFile = *filePair.source;
QFile & destinationFile = *filePair.destination;
qCInfo(usbDevice) << sourceFile.fileName() << "->" << destinationFile.fileName();
constexpr int size = (1 << 20); // 1MiB
QByteArray buffer{size, 0};
char * const data = buffer.data();
while (!sourceFile.atEnd()) {
auto bytesRead = sourceFile.read(data, size);
if (bytesRead < 0) {
qCCritical(usbDevice) << "Can't read file" << sourceFile.fileName()
<< ":" << sourceFile.errorString();
return;
}
auto bytesWritten = destinationFile.write(data, bytesRead);
while (bytesWritten < bytesRead) {
auto sizeWritten = destinationFile.write(data + bytesWritten, bytesRead - bytesWritten);
if (sizeWritten < 0) {
qCCritical(usbDevice) << "Can't write file" << destinationFile.fileName()
<< ":" << destinationFile.errorString();
return;
}
bytesWritten += sizeWritten;
}
Q_ASSERT(bytesWritten == bytesRead);
}
Q_ASSERT(sourceFile.size() == destinationFile.size());
destinationFile.flush();
files.append(filePair.destination);
};
auto & fileCopyFutureWatcher = *new QFutureWatcher< PFileList >{this};
auto onFilesCopied = [&]
{
fileCopyFutureWatcher.deleteLater();
PFileList copiedFiles = fileCopyFutureWatcher.result();
qCInfo(usbDevice) << "Files copy operation from drive finished.";
qCInfo(usbDevice) << copiedFiles.size();
for (PFile const & file : copiedFiles) {
// ...
}
};
connect(&fileCopyFutureWatcher, &fileCopyFutureWatcher.finished, onFilesCopied);
QStringList nameFilters;
nameFilters << "*.dat";
auto entryInfoList = sourceDirectory.entryInfoList(nameFilters, (QDir::Readable | QDir::Files));
fileCopyFutureWatcher.setFuture(QtConcurrent::mappedReduced< PFileList >(qMove(entryInfoList), addResultType< PFilePair >(openSourceAndDestinationFiles), copyFiles));
}
// ...
I almost sure, that fileCopyFutureWatcher instance will be deleted reliably.