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COM threading issue with overlap windows


Problem:  The application sometimes hangs during normal operation. The hang is more prominent in case of more overlap window usage.
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Analysis: The problem occurred rarely and in some operating systems. There were no clear steps on how and why the hang happens. On debugging the hang dumps from customers site, it pointed to fact that the hang happens while releasing a com object of the process. The application used smart class auto pointers like CComPtr and CComQIPtr for creating and deleting the com objects. On more digging the dump files we could notice that the com object thread where it was created is not available in the dump files. Com objects are created on the thread where we call co-create instance. In case of overlaps opened from the application, each overlap is a different thread. And it is possible to open another overlap from the already opened overlap via context menu. Hence the parent of the new overlap is the first overlap that was opened from the main thread of the application. And this was the trigger for the issue. Sometimes the user may close the parent overlap before closing the child overlap. In that case if the child overlap or parent overlap is being referred by some other thread, for example main thread then it would throw an exception if the com framework tries to destroy an null com object. And this was the case in our hang also, the overlap information is maintained by the main thread for some other functionality. When main thread tries to release the com objects owned by overlap threads, then the application would hang if it tries to release NULL com objects (when the overlap thread dies, it releases all the objects it owns).

Solution: The solution of this issue was to follow the factory design pattern. All the overlap thread creation was passed on to the main thread of the application. Hence the creation and destruction of the threads are fully maintained under the control of the application. Instead of passing the com objects directly between the threads, unique Id of the com objects were passed. The user of this com objects then can use the ID’s of the com object and request the main thread for the com object. Reference handling being managed by the main thread of the process, the life time handling of the com objects thus been taken care. This fixed the issue without breaking any of the existing functionality of the application.

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