Fixed possible deadlock if we get a debug message in the wrong moment

This was the problematic backtrace (the qsettigns lockfile resulted in a
message that tried to acquire the lock again.

Thread 1 (Thread 0x7f00417aee00 (LWP 530)):
0  0x00007f003eb9cae9 in syscall () from /lib64/libc.so.6
1  0x00007f003f85d22d in QBasicMutex::lockInternal() () from /lib64/libQt5Core.so.5
2  0x00007f0040ebce51 in QMutexLocker::QMutexLocker (this=0x7ffdb83e4678, m=0x7f004175ff40 <(anonymous namespace)::Q_QGS_sDebugAreaCollector::innerFunction()::holder>) at /usr/include/qt5/QtCore/qmutex.h:206
3  0x00007f00410ab681 in DebugAreaCollector::add (this=0x7f004175ff40 <(anonymous namespace)::Q_QGS_sDebugAreaCollector::innerFunction()::holder>, area="caldavresource.default") at /src/sink/common/log.cpp:269
4  0x00007f00410a6f4b in collectDebugArea (debugArea="caldavresource.default") at /src/sink/common/log.cpp:296
5  0x00007f00410a5b44 in Sink::Log::debugStream (debugLevel=Sink::Log::Warning, line=0, file=0x0, function=0x0, debugArea=0x7f003fabdfa0 "default", debugComponent=0x0) at /src/sink/common/log.cpp:379
6  0x0000000000406d8d in qtMessageHandler (type=QtWarningMsg, context=..., msg="Could not remove our own lock file \"/home/developer/.qttest/share/sink/debugAreas.ini.lock\" maybe permissions changed meanwhile?") at /src/sink/synchronizer/main.cpp:162
7  0x00007f003f8509be in qt_message_print(QtMsgType, QMessageLogContext const&, QString const&) () from /lib64/libQt5Core.so.5
8  0x00007f003f8529ec in qt_message_output(QtMsgType, QMessageLogContext const&, QString const&) () from /lib64/libQt5Core.so.5
9  0x00007f003f93eb40 in QDebug::~QDebug() () from /lib64/libQt5Core.so.5
10 0x00007f003f9b55ba in QLockFile::unlock() () from /lib64/libQt5Core.so.5
11 0x00007f003f959ffd in QLockFile::~QLockFile() () from /lib64/libQt5Core.so.5
12 0x00007f003f98f4e3 in QConfFileSettingsPrivate::syncConfFile(QConfFile*) () from /lib64/libQt5Core.so.5
13 0x00007f003f98fcee in QConfFileSettingsPrivate::sync() () from /lib64/libQt5Core.so.5
14 0x00007f003f985df9 in QSettings::~QSettings() () from /lib64/libQt5Core.so.5
15 0x00007f00410a8cd1 in QtSharedPointer::ExternalRefCountWithContiguousData<QSettings>::deleter (self=0x17e2890) at /usr/include/qt5/QtCore/qsharedpointer_impl.h:255
16 0x00007f0040e7092b in QtSharedPointer::ExternalRefCountData::destroy (this=0x17e2890) at /usr/include/qt5/QtCore/qsharedpointer_impl.h:157
17 0x00007f00410a8e2d in QSharedPointer<QSettings>::deref (dd=0x17e2890) at /usr/include/qt5/QtCore/qsharedpointer_impl.h:461
18 0x00007f00410a8de9 in QSharedPointer<QSettings>::deref (this=0x7ffdb83e4ea8) at /usr/include/qt5/QtCore/qsharedpointer_impl.h:456
19 0x00007f00410a8b75 in QSharedPointer<QSettings>::~QSharedPointer (this=0x7ffdb83e4ea8) at /usr/include/qt5/QtCore/qsharedpointer_impl.h:313
20 0x00007f00410a9f23 in DebugAreaCollector::~DebugAreaCollector (this=0x7f004175ff40 <(anonymous namespace)::Q_QGS_sDebugAreaCollector::innerFunction()::holder>) at /src/sink/common/log.cpp:264
21 0x00007f00410a76b9 in (anonymous namespace)::Q_QGS_sDebugAreaCollector::innerFunction()::Holder::~Holder() (this=0x7f004175ff40 <(anonymous namespace)::Q_QGS_sDebugAreaCollector::innerFunction()::holder>) at /src/sink/common/log.cpp:283
22 0x00007f003eae172c in __run_exit_handlers () from /lib64/libc.so.6
23 0x00007f003eae185c in exit () from /lib64/libc.so.6
24 0x00007f003eacb252 in __libc_start_main () from /lib64/libc.so.6
25 0x00000000004064da in _start ()

Closes https://phabricator.kde.org/T9435

1 files changed, 12 insertions, 4 deletions

diff --git a/common/log.cpp b/common/log.cpp
index 5156a23..e5ed29b 100644
--- a/common/log.cpp
+++ b/common/log.cpp
@@ -253,14 +253,22 @@ class DebugAreaCollector {
 public:
     DebugAreaCollector()
     {
-        QMutexLocker locker(&mutex);
-        mDebugAreas = debugAreasConfig()->value("areas").value<QString>().split(';').toSet();
+        //This call can potentially print a log message (if we fail to remove the qsettings lockfile), which would result in a deadlock if we locked over all of it.
+        const auto areas = debugAreasConfig()->value("areas").value<QString>().split(';').toSet();
+        {
+            QMutexLocker locker(&mutex);
+            mDebugAreas = areas;
+        }
     }
 
     ~DebugAreaCollector()
     {
-        QMutexLocker locker(&mutex);
-        mDebugAreas += debugAreasConfig()->value("areas").value<QString>().split(';').toSet();
+        //This call can potentially print a log message (if we fail to remove the qsettings lockfile), which would result in a deadlock if we locked over all of it.
+        const auto areas = debugAreasConfig()->value("areas").value<QString>().split(';').toSet();
+        {
+            QMutexLocker locker(&mutex);
+            mDebugAreas += areas;
+        }
         debugAreasConfig()->setValue("areas", QVariant::fromValue(mDebugAreas.toList().join(';')));
     }