Random crash in MythSystemLegacy

[kevin@…]

I'm getting a crash in mythbackend about once or twice a day, usually after the backend runs my external channel change script. The backtrace is:

#0  0xb39a0ede in QSemaphore::release(int) ()
   from /usr/lib/i386-linux-gnu/libQtCore.so.4
#1  0xb5d78534 in MythSystemLegacy::Unlock (this=0x0) at mythsystemlegacy.h:160
#2  0xb5d786aa in MythSystemLegacyPrivate::Unlock (this=0x9cdd17b8)
    at mythsystemprivate.h:72
#3  0xb5d74e5a in MythSystemLegacySignalManager::run (this=0xac122930)
    at mythsystemunix.cpp:559
#4  0xb5c784fc in MThreadInternal::run (this=0xac13bd30) at mthread.cpp:79
#5  0xb39a4f00 in ?? () from /usr/lib/i386-linux-gnu/libQtCore.so.4
#6  0xb4bb3c39 in start_thread ()
   from /lib/i386-linux-gnu/i686/cmov/libpthread.so.0
#7  0xb377a78e in clone () from /lib/i386-linux-gnu/i686/cmov/libc.so.6

As you can see on line #1, "this" is null. Here's the section of code in question (actually it's from line #2 above):

    void Unlock(void)                { m_parent->Unlock(); }

Here "m_parent" is null. If we go up one call frame, we have:

            if (ms->m_parent)
            {
                if (ms->GetStatus() == GENERIC_EXIT_OK)
                    emit ms->finished();
                else
                    emit ms->error(ms->GetStatus());

                ms->disconnect();
                ms->Unlock();
            }

So sometime between the check for m_parent being null, and calling "ms->Unlock", m_parent does become null. Since m_parent is a QPointer, if code on another thread somewhere deletes what m_parent is pointing to, then m_parent will become a null pointer, rather than pointing into deleted memory.

Since this is running in a separate thread, I'm thinking another thread is deleting the parent MythSystemLegacy object before the signal manager thread is done with it.

I haven't had time to further track this down, but I wanted to file this in case the resident expert (whoever that may be) of MythSystem might know exactly where the problem is.

Thanks! -- Kevin

[Stuart Morgan <smorgan@…>]

In 9059c42eab34bcdaa160afa988ed99c4ae30dbaa/mythtv:

Prevent a segfault in MythSystemLegacyPrivate?. This is just a hack to avoid the crash until we can figure out the root cause. Refs #11795

[Stuart Morgan <smorgan@…>]

In 3787c76146fc08d24b8a0a896a7de8dcebbf3634/mythtv:

Prevent a segfault in MythSystemLegacyPrivate?. This is just a hack to avoid the crash until we can figure out the root cause. Refs #11795
(cherry picked from commit 9059c42eab34bcdaa160afa988ed99c4ae30dbaa)

[Karl Egly]

Reschedule all tickets planned for, but not solved in time for, 0.27.6 to 0.27.7.

[Stuart Auchterlonie]

Closing any remaining open tickets for 0.27

If the issue still persists, feel free to reopen and align to a current release (v29 or master)