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mergePreferred
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‎core/shared/src/main/scala/fs2/Stream.scala‎

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Original file line numberDiff line numberDiff line change
@@ -2015,6 +2015,224 @@ final class Stream[+F[_], +O] private[fs2] (private[fs2] val underlying: Pull[F,
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): Stream[F2, O2] =
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that.mergeHaltL(this)
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/** Merges two streams with priority given to the first stream.
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*
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* Internally, this uses two bounded queues (of one element).
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* Queue has tryTake() which allows to try a non-blocking read.
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* This is used to check for elements on the prioritized queue,
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* before a blocking read through racePair() is tried on both
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* queues, if no data is available on the prioritized queue.
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*/
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def mergePreferred[F2[x] >: F[x], O2 >: O](
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that: Stream[F2, O2]
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)(implicit F: Concurrent[F2]): Stream[F2, O2] = {
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val fstream: F2[Stream[F2, O2]] =
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for {
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interrupt <- F.deferred[Unit]
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resultL <- F.deferred[Either[Throwable, Unit]]
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resultR <- F.deferred[Either[Throwable, Unit]]
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resultQL <- Queue.bounded[F2, Option[Stream[F2, O2]]](1)
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resultQR <- Queue.bounded[F2, Option[Stream[F2, O2]]](1)
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} yield {
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def watchInterrupted(str: Stream[F2, O2]): Stream[F2, O2] =
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str.interruptWhen(interrupt.get.attempt)
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// action to signal that one stream is finished (by putting a None in it)
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def doneAndClose(q: Queue[F2, Option[Stream[F2, O2]]]): F2[Unit] = q.offer(None).void
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// action to interrupt the processing of both streams by completing interrupt
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val signalInterruption: F2[Unit] = interrupt.complete(()).void
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// Read from a stream and (possibly blocking) write to the bounded queue for that stream
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def go(s: Stream[F2, O2], q: Queue[F2, Option[Stream[F2, O2]]]): Pull[F2, Nothing, Unit] =
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s.pull.uncons
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.flatMap {
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case Some((hd, tl)) =>
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val send = q.offer(Some(Stream.chunk(hd)))
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Pull.eval(send) >> go(tl, q)
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case None =>
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Pull.done
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}
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def runStream(
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s: Stream[F2, O2],
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whenDone: Deferred[F2, Either[Throwable, Unit]],
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q: Queue[F2, Option[Stream[F2, O2]]]
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): F2[Unit] = {
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val str = watchInterrupted(go(s, q).stream)
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str.compile.drain.attempt
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.flatMap {
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// signal completion of our side before we will signal interruption,
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// to make sure our result is always available to others
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case r @ Left(_) =>
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whenDone.complete(r) >> signalInterruption
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case r @ Right(_) =>
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whenDone.complete(r) >> doneAndClose(q)
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}
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}
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// Typedef for the fibres that read from the queues.
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// That's contained in the Either returned by racePair()
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type FBR = Fiber[F2, Throwable, Option[Stream[F2, O2]]]
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// An ADT for tracking state of the two queues.
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// The types describe the state, starting with BothActive.
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// Next state is either LeftDone or RightDone.
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// Final state is BothDone.
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// The members of those states store the loosing fibre
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// of a racePair()-call, which will be reused during the
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// next read.
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sealed trait QueuesState
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final case class BothActive(v: Option[Either[FBR, FBR]]) extends QueuesState
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final case class LeftDone(rFbr: Option[FBR]) extends QueuesState
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final case class RightDone(lFbr: Option[FBR]) extends QueuesState
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case object BothDone extends QueuesState
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// Race the given effects, returning the result of the winner
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// plus the still active fibre of the looser
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def raceQueues(
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lq: F2[Option[Stream[F2, O2]]],
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rq: F2[Option[Stream[F2, O2]]]
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): F2[(Option[Stream[F2, O2]], Either[FBR, FBR])] =
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F.racePair(lq, rq)
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.flatMap {
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case Left((result, fiber)) =>
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result.embedError.map(_ -> fiber.asRight[FBR])
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case Right((fiber, result)) =>
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result.embedError.map(_ -> fiber.asLeft[FBR])
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}
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// stream that is generated from pumping out the elements of the queue.
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val pumpFromQueue: Stream[F2, O2] =
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Stream
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.unfoldEval[F2, QueuesState, Stream[F2, O2]](BothActive(None)) { s =>
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// Returning None from unfoldEval will stop the stream. If we read a None
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// from any queue, we cannot return that but must continue reading on the
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// other queue. Thus, we need a method which can be called recursively to
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// continue reading in case of None.
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def readNext(s: QueuesState): F2[(Option[Stream[F2, O2]], QueuesState)] =
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s match {
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// The initial state, both queues are active and there are no fibres left over
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case BothActive(None) =>
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// check available data on left, which would be prioritized
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resultQL.tryTake
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.flatMap {
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_.fold(
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// no data available on prioritized queue, race both queues
2123+
raceQueues(resultQL.take, resultQR.take)
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.flatMap[(Option[Stream[F2, O2]], QueuesState)] {
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case (None, Left(fbr)) =>
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readNext(RightDone(fbr.some))
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case (None, Right(fbr)) =>
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readNext(LeftDone(fbr.some))
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case (Some(s), fbr) =>
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F.pure(s.some -> BothActive(fbr.some))
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}
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)(os =>
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// we read data from the prioritized queue, however, this sill could be a None,
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// signalling that queue is done. Handle that:
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os.fold(readNext(LeftDone(None)))(ls =>
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F.pure(ls.some -> BothActive(None))
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)
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)
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}
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// right was looser during the last run
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case BothActive(Some(Right(fbr))) =>
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// anyway, check for available data on left first, ignoring the incoming fibre for right
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resultQL.tryTake
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.flatMap(
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_.fold(
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// use the incoming fibre to read from right queue
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raceQueues(resultQL.take, fbr.joinWithNever)
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.flatMap[(Option[Stream[F2, O2]], QueuesState)] {
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case (None, Left(fbr)) =>
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readNext(RightDone(fbr.some))
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case (None, Right(fbr)) =>
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readNext(LeftDone(fbr.some))
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case (Some(s), fbr) =>
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F.pure(s.some -> BothActive(fbr.some))
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}
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)(os =>
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// important to reuse the incoming fibre here!
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os.fold(readNext(LeftDone(fbr.some)))(ls =>
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F.pure(ls.some -> BothActive(fbr.asRight[FBR].some))
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)
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)
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)
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// left was looser during the last run
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case BothActive(Some(Left(fbr))) =>
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// Can't check for available data on left this time,
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// because there's an active fibre reading from the left queue.
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// Start a race and reuse that fibre for left.
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raceQueues(fbr.joinWithNever, resultQR.take)
2171+
.flatMap[(Option[Stream[F2, O2]], QueuesState)] {
2172+
case (None, Left(fbr)) =>
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readNext(RightDone(fbr.some))
2174+
case (None, Right(fbr)) =>
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readNext(LeftDone(fbr.some))
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case (Some(s), fbr) =>
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F.pure(s.some -> BothActive(fbr.some))
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}
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// Left queue is done, but, it's possible we retrieve an active fibre for right.
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case LeftDone(fbr) =>
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fbr
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.map(_.joinWithNever) // join the incoming fibre if given
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.getOrElse(resultQR.take) // ordinary take() if no fibre has been given
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.map {
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case None =>
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None -> BothDone
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case os =>
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os -> LeftDone(None)
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}
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// mirror case of above
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case RightDone(fbr) =>
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fbr
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.map(_.joinWithNever)
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.getOrElse(resultQL.take)
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.map {
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case None =>
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None -> BothDone
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case os =>
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os -> RightDone(None)
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}
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2204+
// this should never happen, but we need to make the compiler happy
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case BothDone =>
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F.pure(None -> BothDone)
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}
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// readNext() returns None in _1 if and only if both queues are done
2210+
readNext(s).map {
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case (None, _) =>
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None // finish the stream (unfoldEval)
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case (Some(s), st) =>
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(s -> st).some // emit element and new state (unfoldEval)
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}
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}
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.flatten // we have Stream[F2, Stream[F2, O2]] and flatten that to Stream[F2, O2]
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val atRunEnd: F2[Unit] =
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for {
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_ <- signalInterruption // interrupt so the upstreams have chance to complete
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left <- resultL.get
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right <- resultR.get
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r <- F.fromEither(CompositeFailure.fromResults(left, right))
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} yield r
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val runStreams =
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runStream(this, resultL, resultQL).start >> runStream(that, resultR, resultQR).start
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2230+
Stream.bracket(runStreams)(_ => atRunEnd) >> watchInterrupted(pumpFromQueue)
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}
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Stream.eval(fstream).flatten
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}
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/** Given two sorted streams emits a single sorted stream, like in merge-sort.
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* For entries that are considered equal by the Order, left stream element is emitted first.
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* Note: both this and another streams MUST BE ORDERED already

‎core/shared/src/test/scala/fs2/StreamSuite.scala‎

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Original file line numberDiff line numberDiff line change
@@ -401,6 +401,12 @@ class StreamSuite extends Fs2Suite {
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}
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}
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test("mergePreferred") {
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testCancelation {
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constantStream.mergePreferred(constantStream)
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}
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}
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test("parJoin") {
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testCancelation {
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Stream(constantStream, constantStream).parJoin(2)

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