Boost scoped lock assert fails - c++

Question

I'm using Boost 1.41 in a linux app that receives data on one thread and sticks it in a queue, another thread pops it off the queue and processes it. To make it thread safe I'm using scoped locks.
My problem is that very infrequently the lock function fails in the read function with the message:
void boost::mutex::lock() Assertion '!pthread_mutext_lock(&m)' failed
It is very infrequent, on last run, it took 36 hours (~425M transactions) before it failed. The read and write functions are listed below, its always in the read function that the Assert arises
Write to queue
void PacketForwarder::Enqueue(const byte_string& newPacket, long sequenceId)
{
try
{
boost::mutex::scoped_lock theScopedLock(pktQueueLock);
queueItem itm(newPacket,sequenceId);
packetQueue.push(itm);
if (IsConnecting() && packetQueue.size() > MaximumQueueSize)
{
// Reached maximum queue size while client unavailable; popping.
packetQueue.pop();
}
}
catch(...)
{
std::cout << name << " Exception was caught:" << std::endl;
}
}
Read from queue
while ( shouldRun )
{
try
{
if (clientSetsHaveChanged)
{
tryConnect();
}
size_t size = packetQueue.size();
if (size > 0)
{
byte_string packet;
boost::mutex::scoped_lock theQLock(pktQueueLock);
queueItem itm = packetQueue.front();
packet = itm.data;
packetQueue.pop();
BytesSent += packet.size();
trySend(packet);
}
else
{
boost::this_thread::sleep(boost::posix_time::milliseconds(50));
}
}
catch (...)
{
cout << name << " Other exception in send packet" << endl;
}
I've googled and found a few problems when destroying scoped_locks but nothing on failing to get a lock. I have also had a search through boost release notes and Trac logs to see if this has been identified as an issue by anyone else. I thought my code was about as simple as it gets but obviously something is up. Any thoughts?
TIA
Paul

Answer 1

There is one thread-safety issue in your program, in this piece of code:
size_t size = packetQueue.size();
if (size > 0)
{
byte_string packet;
boost::mutex::scoped_lock theQLock(pktQueueLock);
queueItem itm = packetQueue.front();
packet = itm.data;
packetQueue.pop();
// ...
}
The issue here is that between the time you checked the queue size and the time you got the lock some other reader thread might take the last item out of the queue, which will cause front() and pop() to fail. Unless you have only one reader thread, you need the size check to be under the lock as well.
I do not know if this is the reason of the assertion failure though. The assertion means the call to pthread_mutex_lock returned a non-zero value signaling an error. Unfortunately, Boost does not show which exactly of possible pthread_mutex_lock errors has happened.