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I tried the following workaround which is working (causes no 
handle leaks)

Workaround is to change threading.semaphore to Windows 
Extension module APIs win32event.CreateMutex(),  
win32event.WaitForSingleObject and win32event.ReleaseMutex
()

After this change, there are no handle leaks. So, my 
questions are:
1) Is this workaround OK or are there any other issues 
regarding the win32api usage ?
2) you suggested to create minimal python wrappers for C++ 
code and call C++ from python (instead of C++ thread 
callbacks). So I would like to know,  in general, whether it is a 
bad idea for c++ threads to callback into Python. If yes, what 
are the issues (apart from the handle leak mentioned before). 
If no, I would like to live with the above workaround.

Thanks in advance