typelevel / typelevel/fs2

Resource finalization out of order when using with concurrent stream.

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bug
Dominant language
Scala
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Merged PRs (30d)
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Description

It appears that resource finalizer is not being called correctly in order. In this simple example, almost all runs I did, the finalizer gets called before the last .use statement

Version: 3.2.8

import cats.effect.syntax.all.*
import cats.syntax.all.*
import cats.effect.*
import fs2.Stream

object Main extends IOApp.Simple:
  val run = Stream
    .resource(
      Resource.make(IO.println("hello").as(100))(n =>
        IO.println(s"goodbye: $n")
      )
    )
    .concurrently(Stream.emits[IO, Int](1 to 10).evalMap(IO.println))
    .compile
    .resource
    .lastOrError
    .use(x => IO.println(s"From use: $x"))

Output:

1
hello
2
goodbye: 100
3
4
From use: 100
5

https://scastie.scala-lang.org/NmxxGQjXREyyA3RflV9nqA

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start by running the Scala reproducer from the issue and tracing the interaction between Stream.concurrently, compile.resource, and use. Investigate the resource-finalization ordering around concurrent streams; done means the finalizer runs only after the stream and the outer use callback have completed.

Written by the indexing model from the issue text.

Assessment

Tech stack
scala
Domain
stream-processing
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

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