Questions about MamaStartCallback


Yury Batrakov
 

Classification: Public

Hi guys,

As everybody knows there is possibility to start MAMA bridge in background thread in C++ API: function Mama::startBackground (mamaBridge bridgeImpl, MamaStartCallback* cb)
There are a couple of problems with this function though:
1. We can't ignore "MAMA startup finished" event setting the value for the second argument to nullptr - this can be fixed as a simple nullptr check in void MAMACALLTYPE stopCb (mama_status status, mamaBridge, void* closure) in mamacpp.cpp
2. There is race condition between thread calling Mama::stop and internal MAMA thread spawned by startBackground function. This race may lead to program crash.

Let me describe #2 - consider the following scenario:

We have a simple class MyMamaConnection

class MyMamaStartCallback : public MamaStartCallback {
void onStartComplete (Wombat::MamaStatus status) override {
// .... Some bridge specific callback code
}
};

class MyMamaConnection {
mamaBridge m_bridge;
MyMamaStartCallback m_callback;

MyMamaConnection() {
Mama::startBackground (m_bridge, &m_callback)
}

~MyMamaConnection() {
Mama::stop(m_bridge);
}
};

The crash occur in destructor because Mama::stop doesn't guarantee that no threads are still using m_callback pointer after this function exited. In our case MyMamaConnection's destructor destroys m_callback object while it is still in use by the thread created by Mama::startBackground.

Here are more details, file mama.c:

static void* mamaStartThread (void* closure)
{
...
rval = mama_start (cb->mBridgeImpl); // Imagine user thread Thread_1 calling ~MyMamaConnection() from our example.

// Current thread may be suspended by OS at this point while Thread_1 continues its work and deletes the object pointed by closure
...

if (cb->mStopCallbackEx)

// cb->mClosure points to destroyed object.

cb->mStopCallbackEx (rval, cb->mBridgeImpl, cb->mClosure);
...
}

One of the possible solutions is to move this code from mama_close() to mama_stop():
// Get name of start background thread and destroy it
const char* threadName = wombatThread_getThreadName(middlewareLib->bridge->mStartBackgroundThread);
wombatThread_destroy (threadName);

Let me know if it's possible to have this fixed in the next release.


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