In some situation, _handle_mem_exceed_limit will alloc a large memory block, more than 5G. After add some log, we found that:
alloc memory was made in vector::insert_realloc
writers_to_reduce_mem's size is more than 8 million.
which indicated that an infinite loop was met in while (!tablets_mem_heap.empty()).
By reviewing codes, """ if (std::get<0>(tablet_mem_item)++ != std::get<1>(tablet_mem_item)) """ is wrong,
which must be """ if (++std::get<0>(tablet_mem_item) != std::get<1>(tablet_mem_item)) """.
In the original code, we will made ++ on end iterator, and then compare to end iterator, the behavior is undefined.
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