Fixed various threadpool errors

[sergio-nsk]

• double decremented threadpool->num_threads
• data race while writing threadpool->stop and reading in threads
• attempt to unlock of unlocked mutex

[nmoinvaz]

I think for the double-decrement of num_threads, that the decrement should be removed from threadpool_delete_threads instead of inside the thread. The other changes look good.

[sergio-nsk]

I don't think the decrement in a thread function is a good idea by the several reasons:

• threadpool_do_work can early exit if thread is NULL and num_threads will not be decremented.
• pthread_create() can fail and num_threads will be kept incremented.
• num_threads is a list length from the code logic, thus the decrement should be at the list manipulations.

[nmoinvaz]

threadpool_do_work can early exit if thread is NULL and num_threads will not be decremented.

We can remove the check for if (!thread) since this will never be the case.

pthread_create() can fail and num_threads will be kept incremented.

We can just do:

    if (pthread_create(&thread->handle, NULL, threadpool_do_work, thread))
        threadpool->num_threads--;

num_threads is a list length from the code logic, thus the decrement should be at the list manipulations.

The problem with decrementing num_threads from threadpool_delete_threads instead of threadpool_do_work is that threadpool_delete_threads is just there to clean up all the threads. So it doesn't even make sense that I am decrementing there.

[nmoinvaz]

Looks like does not pass build.