async-exfiltration-prevention.scala

//> using scala 3.4.0

import language.experimental.captureChecking

class DbCon:
  def runUpdate(): Unit = println(s"runUpdate on ${Thread.currentThread()}")
  def close(): Unit = println("closing DbCon")

def transact[A](f: DbCon^ => A): A =
  val dbCon = DbCon()
  val res = f(dbCon)
  dbCon.close()
  res

trait Fork[A]:
  def join: A

def fork[A](block: -> A): Fork[A] = new Fork[A]:
  @volatile var out: Option[A] = None
  val thread = Thread(() => {
    out = Some(block)
  })
  thread.start()

  def join: A =
    thread.join()
    out.get

@main def main(): Unit =
  transact { dbCon =>
    Thread(() => dbCon.runUpdate()).start()
  }

  transact { dbCon =>
    val frk = fork {
      dbCon.runUpdate()
      23
    }
    println(frk.join)
  }