Review Request 119110: Release Blocker - KProtocolManager: Fix double mutex locking on a non recursive mutex

[Vishesh Handa]

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https://git.reviewboard.kde.org/r/119110/
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Review request for KDE Frameworks and David Faure.


Repository: kio


Description
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    KProtocolManager: Fix double mutex locking on a non recursive mutex

    Currently called reparseConfiguration results in an infinite block
    because the code tries to lock the mutex twice even though it is not a
    recursive mutex.

    Stack trace -
     KProtocolManager::entryMap -> Tries to lock it
     KIO::SlaveConfigPrivate::readGlobalConfig
     KIO::SlaveConfig::reset
     KProtocolManager::reparseConfiguration -> Holds the lock

    This can easily be solved by making the mutex recursive, but that breaks
    the asserts as they are using QMutex::tryLock to check if the mutex is
    locked. With the mutex being recursive tryLock just results in them
    getting locked again.

    I'm not sure if this is the correct way of fixing it

Here is the full backtrace if anyone is interested -

#0  0x00007ffff328a359 in syscall () from /usr/lib/libc.so.6
#1  0x00007ffff3e21830 in _q_futex (addr=0x7ffff31a4b00 <(anonymous namespace)::Q_QGS_kProtocolManagerPrivate::innerFunction()::holder>, op=0, val=3, timeout=0x0)
    at /home/vishesh/kde5/qt5/qtbase/src/corelib/thread/qmutex_linux.cpp:154
#2  0x00007ffff3e21a04 in lockInternal_helper<false> (d_ptr=..., timeout=-1, elapsedTimer=0x0) at /home/vishesh/kde5/qt5/qtbase/src/corelib/thread/qmutex_linux.cpp:195
#3  0x00007ffff3e21891 in QBasicMutex::lockInternal (this=0x7ffff31a4b00 <(anonymous namespace)::Q_QGS_kProtocolManagerPrivate::innerFunction()::holder>)
    at /home/vishesh/kde5/qt5/qtbase/src/corelib/thread/qmutex_linux.cpp:211
#4  0x00007ffff3e2164c in QMutex::lock (this=0x7ffff31a4b00 <(anonymous namespace)::Q_QGS_kProtocolManagerPrivate::innerFunction()::holder>)
    at /home/vishesh/kde5/qt5/qtbase/src/corelib/thread/qmutex.cpp:225
#5  0x00007ffff2eed989 in QMutexLocker::QMutexLocker (this=0x7fffffffc6f0, m=0x7ffff31a4b00 <(anonymous namespace)::Q_QGS_kProtocolManagerPrivate::innerFunction()::holder>)
    at /opt/qt5/include/QtCore/qmutex.h:136        
#6  0x00007ffff2ee7f27 in KProtocolManager::entryMap (group=...) at /home/vishesh/kde5/src/frameworks/kio/src/core/kprotocolmanager.cpp:294
#7  0x00007ffff2ee5748 in KIO::SlaveConfigPrivate::readGlobalConfig (this=0x7b5d10) at /home/vishesh/kde5/src/frameworks/kio/src/core/slaveconfig.cpp:73
#8  0x00007ffff2ee5fa6 in KIO::SlaveConfig::reset (this=0x7b6b20) at /home/vishesh/kde5/src/frameworks/kio/src/core/slaveconfig.cpp:227
#9  0x00007ffff2ee7dfc in KProtocolManager::reparseConfiguration () at /home/vishesh/kde5/src/frameworks/kio/src/core/kprotocolmanager.cpp:278
#10 0x00007ffff2ea37ee in KIO::SessionData::reset (this=0x7b52f0) at /home/vishesh/kde5/src/frameworks/kio/src/core/sessiondata.cpp:136
#11 0x00007ffff2ea3289 in KIO::SessionData::configDataFor (this=0x7b52f0, configData=..., proto=...) at /home/vishesh/kde5/src/frameworks/kio/src/core/sessiondata.cpp:102
#12 0x00007ffff2edb7f9 in KIO::SchedulerPrivate::metaDataFor (this=0x7b52d0, protocol=..., proxyList=..., url=...)
    at /home/vishesh/kde5/src/frameworks/kio/src/core/scheduler.cpp:1049
#13 0x00007ffff2edc18b in KIO::SchedulerPrivate::setupSlave (this=0x7b52d0, slave=0x82d3d0, url=..., protocol=..., proxyList=..., newSlave=true, config=0x0)
    at /home/vishesh/kde5/src/frameworks/kio/src/core/scheduler.cpp:1094
#14 0x00007ffff2edb737 in setupSlave (slave=0x82d3d0, url=..., protocol=..., proxyList=..., newSlave=true, config=0x0)
    at /home/vishesh/kde5/src/frameworks/kio/src/core/scheduler.cpp:1042
#15 0x00007ffff2eda759 in KIO::ProtoQueue::startAJob (this=0x7c11b0) at /home/vishesh/kde5/src/frameworks/kio/src/core/scheduler.cpp:621
#16 0x00007ffff2edd795 in KIO::ProtoQueue::qt_static_metacall (_o=0x7c11b0, _c=QMetaObject::InvokeMetaMethod, _id=0, _a=0x7fffffffcfa0)
    at /home/vishesh/kde5/build/frameworks/kio/src/core/moc_scheduler_p.cpp:244
#17 0x00007ffff40879bf in QMetaObject::activate (sender=0x7c1208, signalOffset=3, local_signal_index=0, argv=0x0)
    at /home/vishesh/kde5/qt5/qtbase/src/corelib/kernel/qobject.cpp:3680


Diffs
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  src/core/kprotocolmanager.cpp 63baa6d 

Diff: https://git.reviewboard.kde.org/r/119110/diff/


Testing
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This can be tested by running `khotnewstuff "plasmoids.knsrc"` in knewstuff/tests/. Without this patch it just blocks.


Thanks,

Vishesh Handa

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