在处理内存泄漏问题上，我们有个bug就是循环播放14天视频，会出现内存上升大概20M的样子，也就是这个问题，发现了QThread的隐藏问题：如果QThread完成之后，不显式的连接finished信号并执行deleteLater，那么QThread就会造成内存泄漏。这个内存泄漏用任意的内存泄漏检测工具都可以轻易的检测出来。关于本问题，连续播放视频一周，那么意味着创建了无数个QThread结构实例，但是没有被回收。
本文基于asan获取的堆栈如下
Indirect leak of 272000 byte(s) in 1700 object(s) allocated from: #0 0x7f83cc0d5c in operator new(unsigned long) (/usr/lib/aarch64-linux-gnu/libasan.so.5.0.0+0xeed5c) #1 0x7f7fb9e89c in QThread::QThread(QObject*) (/lib/aarch64-linux-gnu/libQt5Core.so.5+0xa189c) #2 0x7f7fb9ea60 in QThread::createThreadImpl(std::future<void>&&) (/lib/aarch64-linux-gnu/libQt5Core.so.5+0xa1a60) #3 0x5564d9e098 in create<MpvCore::LoadFileInfo()::<lambda()> > /usr/include/aarch64-linux-gnu/qt5/QtCore/qthread.h:235 #4 0x5564d9e098 in MpvCore::LoadFileInfo() core/mpvcore.cpp:1163 #5 0x5564da679c in MpvCore::event(QEvent*) core/mpvcore.cpp:1670 #6 0x7f809088e8 in QApplicationPrivate::notify_helper(QObject*, QEvent*) (/lib/aarch64-linux-gnu/libQt5Widgets.so.5+0x15e8e8) #7 0x7f80911eec in QApplication::notify(QObject*, QEvent*) (/lib/aarch64-linux-gnu/libQt5Widgets.so.5+0x167eec) #8 0x7f7fd6a9c0 in QCoreApplication::notifyInternal2(QObject*, QEvent*) (/lib/aarch64-linux-gnu/libQt5Core.so.5+0x26d9c0) #9 0x7f7fd6d8d4 in QCoreApplicationPrivate::sendPostedEvents(QObject*, int, QThreadData*) (/lib/aarch64-linux-gnu/libQt5Core.so.5+0x2708d4) #10 0x7f7fdc89c4 (/lib/aarch64-linux-gnu/libQt5Core.so.5+0x2cb9c4) #11 0x7f80f44708 in g_main_context_dispatch (/lib/aarch64-linux-gnu/libglib-2.0.so.0+0x51708) #12 0x7f80f44974 (/lib/aarch64-linux-gnu/libglib-2.0.so.0+0x51974) #13 0x7f80f44a18 in g_main_context_iteration (/lib/aarch64-linux-gnu/libglib-2.0.so.0+0x51a18) #14 0x7f7fdc7e70 in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) (/lib/aarch64-linux-gnu/libQt5Core.so.5+0x2cae70) #15 0x7f7fd6916c in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) (/lib/aarch64-linux-gnu/libQt5Core.so.5+0x26c16c) #16 0x7f7fd71770 in QCoreApplication::exec() (/lib/aarch64-linux-gnu/libQt5Core.so.5+0x274770) #17 0x5564d7ed60 in main src/main.cpp:68 #18 0x7f7f690d8c in __libc_start_main (/lib/aarch64-linux-gnu/libc.so.6+0x20d8c) #19 0x5564d886a4 (/home/kylin/kylin-video+0x686a4)
根据上面的堆栈，我们看到泄漏存在于QThread的构造函数中。我们翻一下qtbase的源码
class QThreadCreateThread : public QThread { public: explicit QThreadCreateThread(std::future<void> &&future) : m_future(std::move(future)) { } private: void run() override { m_future.get(); } std::future<void> m_future; }; QThread *QThread::createThreadImpl(std::future<void> &&future) { return new QThreadCreateThread(std::move(future)); } QThread::QThread(QObject *parent) : QObject(*(new QThreadPrivate), parent) { Q_D(QThread); // fprintf(stderr, "QThreadData %p created for thread %p\n", d->data, this); d->data->thread = this; }
可以很明显的看到QThread执行了new操作，和asan给出的报告相符合。
我们参照qt的文档如下
When any QObject in the tree is deleted, if the object has a parent, the destructor automatically removes the object from its parent
如果QObject对象有父对象，那么会在父对象中析构所有的成员
但是我们要知道的是QThread继承于QObject，但是没有父对象
qt的文档隐晦的提示你要调用QObject::deleteLater()。文章链接如下
void QThread::finished() This signal is emitted from the associated thread right before it finishes executing. When this signal is emitted, the event loop has already stopped running. No more events will be processed in the thread, except for deferred deletion events. This signal can be connected to QObject::deleteLater(), to free objects in that thread.
根据上面的信息，我们可以总结出来，QThread创建的线程，如果没有父对象，那么需要显式的连接finished信号来执行deleteLater，否则就需要将其绑定到一个对象上。所以解决问题的方法有两种：
QThread* thread = new QThread; Worker* worker = new Worker; worker->setParent(thread); worker->moveToThread(thread);
这种情况下，其对象是否析构取决于worker的管理。
QThread::connect(thread, &QThread::finished, thread, &QThread::deleteLater);
一般QThread不会频繁的绑定对象父子关系，这样不容易管理，所以对于父子关系不清晰的代码，建议直接连接finished即可
对于bug而言，我解决此问题的方法是连接finished，为了测试，下面提供了复现这个问题的测试代码
#include <QThread> #include <QDebug> int main(int argc, char *argv[]) { Q_UNUSED(argc); Q_UNUSED(argv); for (int i = 0; i < 100; ++i) { QThread* thread = QThread::create([i]{ qDebug() << "Thread Started:" << i; }); thread->start(); // QThread::connect(thread, &QThread::finished, thread, &QThread::deleteLater); } QThread::sleep(3); return 0; }
我创建100个线程，如果不连接finished信号调用deleteLater，那么可以很明显的报告内存泄漏，如果添加信号则代码正常。
================================================================= ==52273==ERROR: LeakSanitizer: detected memory leaks Indirect leak of 16000 byte(s) in 100 object(s) allocated from: #0 0x7fbcb04d5c in operator new(unsigned long) (/usr/lib/aarch64-linux-gnu/libasan.so.5.0.0+0xeed5c) #1 0x7fbbfce89c in QThread::QThread(QObject*) (/lib/aarch64-linux-gnu/libQt5Core.so.5+0xa189c) #2 0x7fbbfcea60 in QThread::createThreadImpl(std::future<void>&&) (/lib/aarch64-linux-gnu/libQt5Core.so.5+0xa1a60) #3 0x55855c38b4 in create<main(int, char**)::<lambda()> > /usr/include/aarch64-linux-gnu/qt5/QtCore/qthread.h:199 #4 0x55855c38b4 in main /tmp/qthread/test.cpp:9 #5 0x7fbbac2d8c in __libc_start_main (/lib/aarch64-linux-gnu/libc.so.6+0x20d8c) #6 0x55855c41e0 (/root/test+0x41e0) Indirect leak of 12000 byte(s) in 100 object(s) allocated from: #0 0x7fbcb04d5c in operator new(unsigned long) (/usr/lib/aarch64-linux-gnu/libasan.so.5.0.0+0xeed5c) #1 0x7fbbfce8fc in QThread::QThread(QObject*) (/lib/aarch64-linux-gnu/libQt5Core.so.5+0xa18fc) #2 0x7fbbfcea60 in QThread::createThreadImpl(std::future<void>&&) (/lib/aarch64-linux-gnu/libQt5Core.so.5+0xa1a60) #3 0x55855c38b4 in create<main(int, char**)::<lambda()> > /usr/include/aarch64-linux-gnu/qt5/QtCore/qthread.h:199 #4 0x55855c38b4 in main /tmp/qthread/test.cpp:9 #5 0x7fbbac2d8c in __libc_start_main (/lib/aarch64-linux-gnu/libc.so.6+0x20d8c) #6 0x55855c41e0 (/root/test+0x41e0) Indirect leak of 10400 byte(s) in 100 object(s) allocated from: #0 0x7fbcb04d5c in operator new(unsigned long) (/usr/lib/aarch64-linux-gnu/libasan.so.5.0.0+0xeed5c) #1 0x7fbbfd6e40 in QWaitCondition::QWaitCondition() (/lib/aarch64-linux-gnu/libQt5Core.so.5+0xa9e40) #2 0x7fbbfce8f0 in QThread::QThread(QObject*) (/lib/aarch64-linux-gnu/libQt5Core.so.5+0xa18f0) #3 0x7fbbfcea60 in QThread::createThreadImpl(std::future<void>&&) (/lib/aarch64-linux-gnu/libQt5Core.so.5+0xa1a60) #4 0x55855c38b4 in create<main(int, char**)::<lambda()> > /usr/include/aarch64-linux-gnu/qt5/QtCore/qthread.h:199 #5 0x55855c38b4 in main /tmp/qthread/test.cpp:9 #6 0x7fbbac2d8c in __libc_start_main (/lib/aarch64-linux-gnu/libc.so.6+0x20d8c) #7 0x55855c41e0 (/root/test+0x41e0) Indirect leak of 3200 byte(s) in 100 object(s) allocated from: #0 0x7fbcb04d5c in operator new(unsigned long) (/usr/lib/aarch64-linux-gnu/libasan.so.5.0.0+0xeed5c) #1 0x7fbbfcea54 in QThread::createThreadImpl(std::future<void>&&) (/lib/aarch64-linux-gnu/libQt5Core.so.5+0xa1a54) #2 0x55855c38b4 in create<main(int, char**)::<lambda()> > /usr/include/aarch64-linux-gnu/qt5/QtCore/qthread.h:199 #3 0x55855c38b4 in main /tmp/qthread/test.cpp:9 #4 0x7fbbac2d8c in __libc_start_main (/lib/aarch64-linux-gnu/libc.so.6+0x20d8c) #5 0x55855c41e0 (/root/test+0x41e0) SUMMARY: AddressSanitizer: 41600 byte(s) leaked in 400 allocation(s).