Decrement thread count on thread exit.

[jothan]

I hit a problem that after 10 seconds idle, no blocking tasks would
run anymore as all blocking threads exited and the Pool was convinced
that there were still runners available.

Nowhere in the code is num_th decremented. Please make sure the num_idle decrement is correct at that point in the code.

Motivation

Solution

[jothan]

It looks like I duped #1560, but that patch does not decrement the total thread count.

[jothan]

I don't know what happened, but I managed to hit a decrement that underflowed num_idle to 2^32-1 with this patch applied.

[jothan]

@carllerche Thanks for the review !

[AnickaBurova]

I just noticed another issue:
shared.num_idle += 1; is incrementing,
but when the task is executed, we continue 'outer'; and the increment will happen again. And we have one thread but 2 idles!
I think num_idle should be decremented there as well. Please check.

[jothan]

@AnickaBurova for the case you are mentionning, if I am not mistaken, the idle count should be decremented by 1 for every wakeup signaled. Have you hit a case in practice where num_idle > num_th ?

edit: Have you hit a case in practice where num_idle > num_th with this patch applied ?

[carllerche]

Merging master should fix the clippy failure (#1569).

[carllerche]

I took the liberty of merging master to get this fix in sooner 👍