feat(download): judge a queued slskd peer by its queue position

No bytes for ten minutes usually means a peer has queued us, and the
stall timer could not tell position 2 from position 400: the first was
abandoned while it was about to start, the second was waited on for ten
minutes for nothing.

While a requested file is "Queued, Remotely", the grab asks slskd for
its place (GET .../downloads/{user}/{id}/position, which asks the peer)
once a minute. A place that improved counts as progress and restarts
the stall clock. A place beyond 50 twice running gives the peer up at
once, and the manager moves to the next copy; two readings because
slskd documents the figure as possibly inaccurate. Waiting in a queue
has an overall ceiling of an hour without a byte, since a queue moving
one place an hour would otherwise hold the grab all day. As with a
stall, files that already arrived still go forward.

Closes #275

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017HJiuc3ZZhxsPXz3ozTirT

backend/download/provider_slskd.go

@@ -95,7 +95,8 @@ const (
 	// it covers a peer that queues us and never starts as well as one
 	// that starts and stops.  Ten minutes is long enough for a short
 	// queue ahead of us to clear and short enough that one unresponsive
-	// peer does not hold slskd's single transfer slot for an evening.
+	// peer does not hold a transfer slot for an evening.  A queue whose
+	// position improves restarts it (see queueWatch).
 	slskdStallAfter = 10 * time.Minute
 
 	// slskdAbsentGrace is how long a requested file may be missing from
@@ -104,6 +105,23 @@ const (
 	// a few polls was refused.
 	slskdAbsentGrace = 30 * time.Second
 
+	// slskdPositionPoll is how often a peer that has queued us is asked
+	// where we are in its queue.  Each ask is a message to the peer, so
+	// it is far slower than the transfer poll.
+	slskdPositionPoll = time.Minute
+
+	// slskdMaxQueuePosition is the queue position past which a peer is
+	// not worth waiting for: at a few minutes a track, fifty albums
+	// ahead of us is days.  slskd warns the figure can be inaccurate, so
+	// it takes two readings in a row to act on (see awaitTransfers).
+	slskdMaxQueuePosition = 50
+
+	// slskdQueueCeiling is the longest a grab waits in a peer's queue
+	// without a byte arriving, however steadily the queue moves.  An
+	// improving position restarts the stall clock, so without this a
+	// queue moving one place an hour would hold the grab all day.
+	slskdQueueCeiling = time.Hour
+
 	// slskdCancelTimeout bounds the cleanup that cancels abandoned
 	// transfers.
 	slskdCancelTimeout = 15 * time.Second
@@ -168,6 +186,8 @@ type slskd struct {
 	transferPoll time.Duration
 	stallAfter   time.Duration
 	absentGrace  time.Duration
+	positionPoll time.Duration
+	queueCeiling time.Duration
 
 	// batches is whether the daemon takes batch downloads, which is
 	// how a grab gets a folder of its own (see enqueue).
@@ -228,6 +248,8 @@ func newSlskd(
 		transferPoll:  slskdTransferPoll,
 		stallAfter:    slskdStallAfter,
 		absentGrace:   slskdAbsentGrace,
+		positionPoll:  slskdPositionPoll,
+		queueCeiling:  slskdQueueCeiling,
 	}, nil
 }
 
@@ -300,6 +322,12 @@ type slskdTransfer struct {
 	BytesTransferred int64  `json:"bytesTransferred"`
 }
 
+// remotelyQueued reports whether the peer has accepted the request and
+// put it in its upload queue, where it waits for a slot.
+func (t slskdTransfer) remotelyQueued() bool {
+	return strings.Contains(t.State, "Queued") && strings.Contains(t.State, "Remotely")
+}
+
 // done reports whether the transfer reached a terminal state, and
 // whether it succeeded.  slskd reports compound states such as
 // "Completed, Succeeded" and "Completed, Errored".
@@ -1028,11 +1056,12 @@ func (s *slskd) terminalTransferIDs(
 // state, the transfer stalls, or the caller gives up.
 //
 // Soulseek queues are measured in hours, so there is no deadline on the
-// transfer as a whole — but there is one on *progress*.  slskd's
-// transfer limit is one, so a peer that holds us in its queue without
-// sending a byte is not only failing this download, it is holding every
-// other Soulseek download behind it.  After stallAfter with nothing
-// moving the peer is given up on, and the manager tries another.
+// transfer as a whole — but there is one on *progress*.  A peer that
+// holds us without sending a byte is failing this download and holding
+// one of the daemon's few transfer slots.  After stallAfter with nothing
+// moving the peer is given up on, and the manager tries another; a peer
+// that has queued us is judged by its queue position as well (see
+// queueWatch).
 //
 // Whatever way this ends short of every file finishing, the transfers
 // still live in slskd are cancelled there.  Returning without doing so
@@ -1055,6 +1084,7 @@ func (s *slskd) awaitTransfers(
 		lastProgress = started
 		lastBytes    int64
 		live         []slskdTransfer
+		queue        queueWatch
 	)
 
 	for {
@@ -1096,6 +1126,28 @@ func (s *slskd) awaitTransfers(
 			lastProgress = time.Now()
 		}
 
+		if err := queue.observe(ctx, s, username, tally.live, &lastProgress); err != nil {
+			s.cancelTransfers(username, live)
+
+			// As with a stall: what already arrived goes forward.
+			if tally.done > 0 {
+				s.logger.Info("slskd queue too long; keeping what arrived", "error", err)
+
+				return nil
+			}
+
+			return err
+		}
+
+		if tally.bytes == 0 && time.Since(started) >= s.queueCeiling {
+			s.cancelTransfers(username, live)
+
+			return fmt.Errorf(
+				"%w: %s sent nothing in %s",
+				ErrSlskdTimeout, username, s.queueCeiling,
+			)
+		}
+
 		if onProgress != nil {
 			onProgress(Progress{
 				Current: tally.bytes,
@@ -1148,6 +1200,77 @@ func (s *slskd) awaitTransfers(
 	}
 }
 
+// queueWatch follows our place in a peer's upload queue while nothing is
+// arriving (#275).
+//
+// No bytes for stallAfter usually means the peer has queued us, and the
+// timer alone cannot tell position 2 from position 400.  So a queued
+// grab asks where it stands every positionPoll: a place that improved is
+// progress and restarts the stall clock, and a place past
+// slskdMaxQueuePosition twice running gives the peer up at once, so the
+// manager moves to the next copy without waiting out the timer.
+type queueWatch struct {
+	lastAsk   time.Time
+	lastPlace int
+	far       int
+}
+
+func (q *queueWatch) observe(
+	ctx context.Context,
+	s *slskd,
+	username string,
+	live []slskdTransfer,
+	lastProgress *time.Time,
+) error {
+	var queued *slskdTransfer
+
+	for i := range live {
+		if live[i].remotelyQueued() && live[i].ID != "" {
+			queued = &live[i]
+
+			break
+		}
+	}
+
+	if queued == nil || time.Since(q.lastAsk) < s.positionPoll {
+		return nil
+	}
+
+	q.lastAsk = time.Now()
+
+	var place int
+
+	if err := s.client.get(
+		ctx, slskdDownloadsPath(username)+"/"+url.PathEscape(queued.ID)+"/position", &place,
+	); err != nil {
+		// The peer may simply not answer; the stall timer still applies.
+		s.logger.Debug("slskd queue position unavailable", "peer", username, "error", err)
+
+		return nil
+	}
+
+	if q.lastPlace > 0 && place > 0 && place < q.lastPlace {
+		*lastProgress = time.Now()
+	}
+
+	q.lastPlace = place
+
+	if place > slskdMaxQueuePosition {
+		q.far++
+	} else {
+		q.far = 0
+	}
+
+	if q.far >= 2 {
+		return fmt.Errorf(
+			"%w: %s has us at position %d in its queue",
+			ErrSlskdTimeout, username, place,
+		)
+	}
+
+	return nil
+}
+
 // transferTally is one poll's reading of the files a grab asked for.
 type transferTally struct {
 	done, failed int