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/slskd_queue_test.go

@@ -0,0 +1,132 @@
+package download
+
+import (
+	"context"
+	"errors"
+	"strings"
+	"testing"
+	"time"
+)
+
+// A peer that has queued us is judged by where we are in its queue, not
+// only by a timer (#275).
+
+func queuedThen(polls int, final string) [][]slskdTransfer {
+	queued := []slskdTransfer{
+		{ID: "t0", Filename: `\s\Album\01 A.flac`, State: "Queued, Remotely"},
+		{ID: "t1", Filename: `\s\Album\02 B.flac`, State: "Queued, Remotely"},
+	}
+
+	out := make([][]slskdTransfer, 0, polls+1)
+	for range polls {
+		out = append(out, queued)
+	}
+
+	return append(out, []slskdTransfer{
+		{ID: "t0", Filename: `\s\Album\01 A.flac`, State: final, BytesTransferred: 500},
+		{ID: "t1", Filename: `\s\Album\02 B.flac`, State: final, BytesTransferred: 500},
+	})
+}
+
+func descending(from int) []int {
+	out := make([]int, 0, from)
+	for p := from; p >= 1; p-- {
+		out = append(out, p)
+	}
+
+	return out
+}
+
+// Two readings far back in the queue give the peer up at once, rather
+// than after the stall timer.
+func TestSlskdGivesUpOnALongQueue(t *testing.T) {
+	t.Parallel()
+
+	stub := newSlskdStub(t)
+	stub.transfers = queuedThen(100_000, "Completed, Succeeded")
+	stub.positions = []int{400}
+
+	s, _ := newStubSlskd(t, stub)
+	s.stallAfter = time.Hour
+
+	started := time.Now()
+
+	_, err := s.Grab(context.Background(), slskdAlbum(t, stub, "peer"), t.TempDir(), nil)
+	if !errors.Is(err, ErrSlskdTimeout) || !strings.Contains(err.Error(), "position 400") {
+		t.Fatalf("Grab = %v, want a queue-position give-up", err)
+	}
+
+	if time.Since(started) > 5*time.Second {
+		t.Error("the give-up waited on something other than the position")
+	}
+
+	if len(stub.cancelledURIs()) == 0 {
+		t.Error("the queued transfers were not cancelled")
+	}
+}
+
+// One far reading is not enough: slskd says the figure can be wildly
+// wrong.
+func TestSlskdOneBadPositionIsNotEnough(t *testing.T) {
+	t.Parallel()
+
+	stub := newSlskdStub(t)
+	stub.transfers = queuedThen(20, "Completed, Succeeded")
+	stub.positions = []int{400, 3}
+
+	s, _ := newStubSlskd(t, stub)
+	s.positionPoll = 0
+
+	if _, err := s.Grab(
+		context.Background(),
+		slskdAlbum(t, stub, "peer", "01 A.flac", "02 B.flac"),
+		t.TempDir(), nil,
+	); err != nil {
+		t.Fatalf("Grab: %v", err)
+	}
+}
+
+// A queue that is moving is progress: the grab outlives the stall timer
+// while its position improves.
+func TestSlskdAMovingQueueIsProgress(t *testing.T) {
+	t.Parallel()
+
+	stub := newSlskdStub(t)
+	// Near enough to wait for, with more improving readings (45) than
+	// there are queued polls (30), so the queue outlives the stall timer
+	// (30 polls of at least 2 ms against 40 ms) while still improving,
+	// however slowly the machine runs the loop.
+	stub.transfers = queuedThen(30, "Completed, Succeeded")
+	stub.positions = descending(slskdMaxQueuePosition - 5)
+
+	s, _ := newStubSlskd(t, stub)
+	s.transferPoll = 2 * time.Millisecond
+	s.positionPoll = 0
+	s.stallAfter = 40 * time.Millisecond
+
+	if _, err := s.Grab(
+		context.Background(),
+		slskdAlbum(t, stub, "peer", "01 A.flac", "02 B.flac"),
+		t.TempDir(), nil,
+	); err != nil {
+		t.Fatalf("Grab: %v; a moving queue was treated as a stall", err)
+	}
+}
+
+// However steadily the queue moves, waiting in it has a ceiling.
+func TestSlskdQueueHasACeiling(t *testing.T) {
+	t.Parallel()
+
+	stub := newSlskdStub(t)
+	stub.transfers = queuedThen(100_000, "Completed, Succeeded")
+	stub.positions = descending(100_000)
+
+	s, _ := newStubSlskd(t, stub)
+	s.stallAfter = time.Hour
+	s.queueCeiling = 50 * time.Millisecond
+
+	_, err := s.Grab(context.Background(), slskdAlbum(t, stub, "peer"), t.TempDir(), nil)
+	if !errors.Is(err, ErrSlskdTimeout) {
+		t.Fatalf("Grab = %v, want the queue ceiling", err)
+	}
+}