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Prevent Hotswap#get from acquiring a lock when there is no Resource present #3922

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merged 8 commits into from
Jan 9, 2024
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Prevent Hotswap#get from acquiring a lock when there is no Resource present #3922

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3b12e3e
fix: acquire semaphore only when there is a resource present
josgarmar28 Dec 28, 2023
3fe4f8c
fix: release permit condition and scope of the poll
josgarmar28 Dec 29, 2023
8e1ff54
fix: race condition
josgarmar28 Dec 29, 2023
281b9db
fix: run state.get only when semaphore is aquired
josgarmar28 Dec 30, 2023
3f43315
fix: acquire semaphore before state.get
josgarmar28 Dec 31, 2023
139d3f2
chore: merge shared and get methods and add code comment
josgarmar28 Dec 31, 2023
bd3472c
Update std/shared/src/main/scala/cats/effect/std/Hotswap.scala
josgarmar28 Jan 2, 2024
c04e123
fix: test and revert description changes
josgarmar28 Jan 9, 2024
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12 changes: 10 additions & 2 deletions std/shared/src/main/scala/cats/effect/std/Hotswap.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -121,7 +121,15 @@ object Hotswap {
def raise(message: String): F[Unit] =
F.raiseError[Unit](new RuntimeException(message))

def shared: Resource[F, Unit] = semaphore.permit
def shared(state: Ref[F, State]): Resource[F, Unit] =
Resource.makeFull[F, Unit] { poll =>
poll(
state.get.flatMap {
case Acquired(_, _) => semaphore.acquire
case _ => F.unit
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}
)
} { _ => semaphore.release }
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def exclusive: Resource[F, Unit] =
Resource.makeFull[F, Unit](poll => poll(semaphore.acquireN(Long.MaxValue)))(_ =>
Expand All @@ -141,7 +149,7 @@ object Hotswap {
}

override def get: Resource[F, Option[R]] =
shared.evalMap { _ =>
shared(state).evalMap { _ =>
state.get.map {
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case Acquired(r, _) => Some(r)
case _ => None
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12 changes: 11 additions & 1 deletion tests/shared/src/test/scala/cats/effect/std/HotswapSpec.scala
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Expand Up @@ -165,6 +165,16 @@ class HotswapSpec extends BaseSpec { outer =>

TestControl.executeEmbed(go, IORuntimeConfig(1, 2)).replicateA_(1000) must completeAs(())
}
}

"get should not acquire a lock when there is no resource present" in ticked {
implicit ticker =>
val go = Hotswap.create[IO, Unit].use { hs =>
hs.get.useForever.start *>
hs.swap(IO.sleep(1.second).toResource) *>
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@armanbilge armanbilge Jan 8, 2024 •
edited
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We should insert a sleep between these two steps to be sure that the fiber starts. Also I think it's okay to just swap in a Resource.unit, basically we are checking that the swap succeeds and does not hang. If the get above had acquired a shared lock, then the swap would hang due to be unable to obtain an exclusive lock.

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Agreed!

IO.sleep(2.seconds) *>
hs.get.use_.timeout(1.second).void
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I'm not sure if the rest tests anything meaningful.

}
go must completeAs(())
}
}
}
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