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From "Francois Godin (JIRA)" <j...@apache.org>
Subject [jira] [Created] (AMQCPP-617) Segfault when using a QueueBrowser.
Date Wed, 12 Jul 2017 18:06:00 GMT
Francois Godin created AMQCPP-617:
-------------------------------------

             Summary: Segfault when using a QueueBrowser.
                 Key: AMQCPP-617
                 URL: https://issues.apache.org/jira/browse/AMQCPP-617
             Project: ActiveMQ C++ Client
          Issue Type: Bug
            Reporter: Francois Godin
            Assignee: Timothy Bish
            Priority: Critical


I encountered a rare (but that still happened a couple of time) crash due to the ActiveMQ
C++ library.

h2. Information
I've done some digging and the crash happen when the function activemq::core::Browser::dispatch
call activemq::core::ActiveMQQueueBrowser::notifyMessageAvailable. The segfault happen at
the first member access in ActiveMQQueueBrowser.

h3. Core dump
#0  0x00007f38f7736758 in decaf::util::concurrent::Lock::lock() ()
   from /home/Fasttracctest/daemon/FastTraccTestDaemon/lib/libactivemq-cpp.so.19
#1  0x00007f38f7736875 in decaf::util::concurrent::Lock::Lock(decaf::util::concurrent::Synchronizable*,
bool) ()
   from /home/Fasttracctest/daemon/FastTraccTestDaemon/lib/libactivemq-cpp.so.19
#2  0x00007f38f73f0f5d in activemq::core::ActiveMQQueueBrowser::notifyMessageAvailable() ()
   from /home/Fasttracctest/daemon/FastTraccTestDaemon/lib/libactivemq-cpp.so.19
#3  0x00007f38f73f4cd1 in activemq::core::Browser::dispatch(decaf::lang::Pointer<activemq::commands::MessageDispatch,
decaf::util::concurrent::atomic::AtomicRefCounter> const&) ()
   from /home/Fasttracctest/daemon/FastTraccTestDaemon/lib/libactivemq-cpp.so.19
#4  0x00007f38f7406c24 in activemq::core::ActiveMQSessionExecutor::dispatch(decaf::lang::Pointer<activemq::commands::MessageDispatch,
decaf::util::concurrent::atomic::AtomicRefCounter> const&) ()
   from /home/Fasttracctest/daemon/FastTraccTestDaemon/lib/libactivemq-cpp.so.19
#5  0x00007f38f7407ce5 in activemq::core::ActiveMQSessionExecutor::iterate() ()
   from /home/Fasttracctest/daemon/FastTraccTestDaemon/lib/libactivemq-cpp.so.19
#6  0x00007f38f7543565 in activemq::threads::DedicatedTaskRunner::run() ()
   from /home/Fasttracctest/daemon/FastTraccTestDaemon/lib/libactivemq-cpp.so.19
#7  0x00007f38f76acbef in ?? ()
   from /home/Fasttracctest/daemon/FastTraccTestDaemon/lib/libactivemq-cpp.so.19
#8  0x00007f38f76ac964 in ?? ()
   from /home/Fasttracctest/daemon/FastTraccTestDaemon/lib/libactivemq-cpp.so.19
#9  0x00007f38f9120aa1 in start_thread () from /lib64/libpthread.so.0
#10 0x00007f38f941eaad in ioperm () from /lib64/libc.so.6
#11 0x0000000000000000 in ?? ()

h3. Usage of the library
int getMessageCountInQueue(cms::Session* session, cms::Queue* queue) {
	std::unique_ptr<cms::QueueBrowser> browser(session->createBrowser(queue));
	cms::MessageEnumeration* enumeration = browser->getEnumeration();
	int count = 0;
	while(enumeration->hasMoreMessages()) {
		count++;
		cms::Message* msg = enumeration->nextMessage();
		delete msg;
	}
	return count;
}

h2. After some analysis, I have the following hypothesis:
It seems that when I destroy the ActiveMQQueueBrowser (and thus call destroyConsumer), the
Browser instance that refer to it may be kept alive by activemq::core::ActiveMQSessionExecutor::dispatch
(as it is stored in a shared_ptr). 
Often it is no problem as Browser::dispatch is not called but I guess it may happen rarely
and cause the segfault.

h2. Proposed solution
The basic ActiveMQConsumerKernel::dispatch should be protected against that problem because
it lock and check if the consumer is closed (which it is). It just happen that the Browser::dispatch
did not have those check and thus will always execute the code.
As such, I propose splitting ActiveMQConsumerKernel::dispatch into two:
# The cleanup, locking and check into the new ActiveMQConsumerKernel::dispatch
# The operation itself into ActiveMQConsummerKernel::dispatchImpl.

Then, Browser only need to override dispatchImpl and will have the same protection that ActiveMQConsumerKernel
had.




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