Invalid overflow handling in CancelWait of the notifier

[pbrunet]

Describe the bug
When using the notifier, deadlock may happen when a cancel is called with the max value of epoch.

To Reproduce
I don't have repro step. I guess it is a timing issue :-(

Desktop (please complete the following information):

• OS: Windows

Additional context
When setting the new state, we inc epoch and decr waiter but with a single waiter and epoch at its max value and no "mask", new state will be :
epoch = 0
waiter = 0
mask = 0
while mask should stay the same. Certainly something like this:

uint64_t newstate = state - kWaiterInc + kEpochInc;
newstate = static_cast<uint64_t>( ( newstate & ~kStackMask ) | static_cast<uint64_t>( state & kStackMask ) );

The same issue looks to be present when notify is called while there is a waiter in prepare_wait

[kaushal-malpure]

Hey @tsung-wei-huang ,
I want to contribute to this issue, can you guide me for this? Thanks in advance.

[tsung-wei-huang]

@kaushal-malpure this in fact is under the file taskflow/core/notifier.hpp - it's a standalone hpp file to implement worker notification.

[kaushal-malpure]

@tsung-wei-huang
Attempted to address the problem.
PR id - #560

[kaushal-malpure]

@tsung-wei-huang Can you please review the changes in the above pull request?