A borrowed resource can postpone its termination

Start by locating the pool finalization and borrowed-resource lifecycle code in the keypool repository. Reproduce the Scala example from the issue, including the IO.never case, and inspect how borrowed resources are tracked after the pool closes. Done means the borrowed connection is destroyed and closed before the pool finalization completes, matching the expected output.

keypool version: 0.4.7

When a resource is borrowed from the pool, it's not a part of the PoolMap anymore. Hence it will not be destroyed during the finalization of the pool.
The resource will be open until it's not completed itself. Thus IO.never as a part of the operation with the resource can hang the whole system.

Example:

sealed trait Key
case object Key extends Key

class Connection(val isOpen: Ref[IO, Boolean])

val pool = KeyPool
  .Builder(
    (_: Key) => IO.ref(true).map(new Connection(_)),
    (conn: Connection) => conn.isOpen.set(false)
  )
  .doOnCreate(conn => IO.println(s"created $conn"))
  .doOnDestroy(conn => IO.println(s"destroyed $conn"))
  .build
  .onFinalize(IO.println("pool was closed"))
  
def example(outerAwait: Deferred[IO, Unit]): IO[FiberIO[(Boolean, Boolean)]] =
  pool.use { p =>
    def job(innerAwait: Deferred[IO, Unit]): IO[(Boolean, Boolean)] =
      p.take(Key).use { managed =>
        val connection = managed.value
        
        for {
          status1 <- connection.isOpen.get
          _       <- IO.println("awaiting inner")
          _       <- innerAwait.complete(())
          _       <- IO.println("awaiting outer")
          _       <- outerAwait.get          // wait for the signal from the outside
          _       <- IO.println("after awaits")
          status2 <- connection.isOpen.get   // isOpen should be 'false' there
          // _ <- IO.never[Unit] // once uncommented the program will hang
        } yield (status1, status2)
      }
      
    for {
      await <- IO.deferred[Unit]
      fiber <- job(await).start
      _     <- await.get // make sure first part happens inside of the open pool
    } yield fiber
  }
  
for {
  await   <- IO.deferred[Unit]
  fiber   <- example(await)
  _       <- await.complete(()) // pool is closed and we continue execution of the job
  outcome <- fiber.join
  (status1, status2) <- outcome.embedNever
  _                  <- IO.println(s"(pool open): is connection open $status1")
  _                  <- IO.println(s"(pool closed): is connection open $status2")
} yield ()

Output:

created Connection@76426537
awaiting inner
awaiting outer
pool was closed
after awaits
destroyed Connection@76426537
(pool open): is connection open true
(pool closed): is connection open true

Expected output:

created Connection@76426537
awaiting inner
awaiting outer
destroyed Connection@76426537
pool was closed
after awaits
(pool open): is connection open true
(pool closed): is connection open false