Repeated shared_mutex.unlock_shared() can underflow

[Zack Moratto <zmoratto@…>]

Boost 1.53's shared_mutex can underflow if the user calls unlock_shared twice or right after construction.

shared_mutex m();
m.unlock_shared();

The problem is specifically this line which can be found inside boost/thread/pthread/shared_mutex.hpp. (should be line 148)

            bool const last_reader=!--state.shared_count;

It doesn't check that state.shared_count is nonzero before decrementing.

[viboes]

This is not a bug. It is a precondition that the mutex must be locked when you use unlock.