Is there any way to wait for a `Mutex` to unlock w...
# coroutinesk
kevin.cianfarini
11/16/2023, 4:40 PMIs there any way to wait for a
Mutexkevin.cianfarini
11/16/2023, 4:41 PMI am suspicious that what I’m reaching for is a reader-writer lock https://en.wikipedia.org/wiki/Readers%E2%80%93writer_lock
kevin.cianfarini
11/16/2023, 4:41 PMWhich appears to be something that’s long been desired. https://github.com/Kotlin/kotlinx.coroutines/issues/94
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Toby
11/20/2023, 1:38 AMI suspect there is a problem with simply waiting for a Mutex to unlock.
It might unlock, but immediately re-lock. So even if you suspended until it unlocked, there is no guarantee it stays unlocked while your code is executing.
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gildor
11/20/2023, 4:32 AMCurious about your use case
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kevin.cianfarini
11/20/2023, 1:51 PMMy use case is I want to allow concurrent reads, but exclusive writes to an authorization token that's appended to every single one of my http requests.
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gildor
11/20/2023, 1:55 PMYeah, makes sense. Is it for MPP?
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kevin.cianfarini
11/20/2023, 1:56 PMYup. It’s done in an apollo interceptor but could conceptually be done from any http interceptor like thing
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gildor
11/20/2023, 1:57 PMBecause for JVM I would probably just use ReadWriteLock, yes it will block, but if there is no write pressure (which is probably the case), it would be negligible
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kevin.cianfarini
11/20/2023, 1:57 PMThis is common code and consumed in K/N
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gildor
11/20/2023, 1:58 PMYeah, JVM and JS is easy, Native could be a problem
gildor
11/20/2023, 1:59 PMIn general probably using MutableStateFlow be enough for this, it's safe and doesn't block on reading
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kevin.cianfarini
11/20/2023, 2:01 PMI don’t think it would be for my cause, because when an auth token is expired we try to reauthenticate. Upon doing so we only want to issue one “winner” reauth call, all of the other concurrent ones end up being no-ops. I don’t see an easy way to do that with MSF
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gildor
11/20/2023, 2:01 PMAh, I see, makes awnae
gildor
11/20/2023, 2:01 PMForgot about need for suspend
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kevin.cianfarini
11/20/2023, 2:01 PMI suppose maybe I could have a writer lock around the MSF but that seems silly
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gildor
11/20/2023, 2:01 PMYeah, it doesn't look necessary
gildor
11/20/2023, 2:03 PMI would probably just do with mutex and conditional logic (so to check if it lcoked), it should be sufficient, even if not absolutely correct
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kevin.cianfarini
11/20/2023, 2:04 PMOur auth stack is fairly sensitive and has been the source of lots of prod issues. I’m not very comfortable with mostly correct at this point
kevin.cianfarini
11/20/2023, 2:05 PMLast week it caused a DDOS of our backend service because of an promotional event which drove lots of traffic. Improper locking in part caused unnecessary reauth calls and made our BE fall over
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gildor
11/20/2023, 2:08 PMYes, I understand, but isn't it in the worst case just sending one additional non-authorized request? It's anyway a matter of milliseconds
gildor
11/20/2023, 2:09 PMDon't get me wrong, not claiming that know the best answer, more like trying to brainstorm it
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kevin.cianfarini
11/20/2023, 2:21 PMAh I see
kevin.cianfarini
11/20/2023, 2:21 PMI appreciate your brainstorming here
kevin.cianfarini
11/20/2023, 2:21 PMI might take their suggestion for a RW mutex built atop semaphores and use that until coroutines has a first party one
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uli
11/27/2023, 3:45 PMOne more hint here. Why not implement your own? There are two algorithms found in wikipedia
Both look pretty straight forward.
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kevin.cianfarini
11/27/2023, 3:47 PMYup that’s the plan!
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