[ruby-core:116146] [Ruby master Feature#19717] `ConditionVariable#signal` is not fair when the wakeup is consistently spurious.

Issue #19717 has been updated by kjtsanaktsidis (KJ Tsanaktsidis).


@ioquatix  do we want to do anything about this?

This really did stem from a real bug in somebody's code it seems (https://github.com/socketry/async/issues/99) so I guess it would be good to do something. The "something"s we cooked up so far seem to be...

* Add a ConditionVariable#wait_for method which takes a block and returns whether or not the wakeup was actually not spurious (from the application's perspective - i.e. can the thread make forward progress now, or does it need to wait on the condvar again?). Pros: admits a fairly simple implementation, and the API kind of feels more ruby-like anyway; Cons: requires code to actually use the new API
* Make Mutex#unlock cause the calling thread  to give up the GVL and schedule the thread waiting for the mutex instead. Pros: sort of simple, solves the problem for users of the current API Cons: probably not great for performance (the calling thread might have been about to do some IO anyway, so if we'd just given it a bit more time it would be sleeping doing something productive (waiting for IO) instead of just being blocked on the GVL). 
* Make Mutex be "reserved" for a thread if it's blocked on Mutex#lock, so that if the thread which currently has the mutex calls #unlock and then #lock again, it will find the mutex is reserved for someone else and transfer control. Pros: solves the problem for users of the current API; Cons: Pretty complex (https://github.com/ruby/ruby/pull/8331)

What is the next step for this? Should I put this on the agenda for a dev meeting to discuss? Something else?

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Feature #19717: `ConditionVariable#signal` is not fair when the wakeup is consistently spurious.
https://bugs.ruby-lang.org/issues/19717#change-106151

* Author: ioquatix (Samuel Williams)
* Status: Open
* Priority: Normal
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For background, see this issue <https://github.com/socketry/async/issues/99>.

It looks like `ConditionVariable#signal` is not fair, if the calling thread immediately reacquires the resource.

I've given a detailed reproduction here as it's non-trivial: <https://github.com/ioquatix/ruby-condition-variable-timeout>.

Because the spurious wakeup occurs, the thread is pushed to the back of the waitq, which means any other waiting thread will acquire the resource, and that thread will perpetually be at the back of the queue.

I believe the solution is to change `ConditionVarialbe#signal` should only remove the thread from the waitq if it's possible to acquire the lock. Otherwise it should be left in place, so that the order is retained, this should result in fair scheduling.



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