download: free the -j slot once a torrent starts seeding

download/download.go

@@ -64,3 +64,14 @@ type Download interface {
 	Run(ctx context.Context) error
 	Stat() Stat
 }
+
+// Seeder is an optional capability of a Download whose work continues after its
+// content is fully downloaded — a torrent that keeps running to seed.
+// SeedingStarted is closed at that download->seed transition. The engine uses it
+// to free the concurrency slot the moment a download stops downloading: a
+// seed-only torrent is no longer a download, so it must not count against
+// --max-concurrent-downloads (-j) and block a queued download. HTTP downloads do
+// not implement this, so they hold their slot until they finish.
+type Seeder interface {
+	SeedingStarted() <-chan struct{}
+}

download/engine.go

@@ -60,7 +60,24 @@ func (e *Engine) Run(ctx context.Context, downloads []Download) []Result {
 				results <- Result{Name: d.Name(), Index: i, Err: ctx.Err(), Stat: d.Stat()}
 				return
 			}
-			defer func() { <-slots }()
+			// Release the slot exactly once — whether the download finishes or, for
+			// a torrent, first detaches into seed-only. A seeding torrent is no
+			// longer downloading, so it stops counting against -j and lets a queued
+			// download start; this goroutine lives on to seed.
+			var once sync.Once
+			release := func() { once.Do(func() { <-slots }) }
+			defer release()
+			if s, ok := d.(Seeder); ok {
+				done := make(chan struct{})
+				defer close(done)
+				go func() {
+					select {
+					case <-s.SeedingStarted():
+						release()
+					case <-done:
+					}
+				}()
+			}
 
 			err := d.Run(ctx)
 			results <- Result{Name: d.Name(), Index: i, Err: err, Stat: d.Stat()}

download/engine_test.go

@@ -0,0 +1,107 @@
+package download
+
+import (
+	"context"
+	"testing"
+	"time"
+)
+
+// fakeDownload is a Download that blocks in Run until released, and (optionally)
+// announces a download->seed transition so the engine can free its -j slot.
+type fakeDownload struct {
+	name    string
+	started chan struct{} // closed when Run begins
+	seeding chan struct{} // SeedingStarted signal
+	release chan struct{} // Run returns once this is closed
+	detach  bool          // if set, Run signals seeding right after starting
+}
+
+func newFake(name string, detach bool) *fakeDownload {
+	return &fakeDownload{
+		name:    name,
+		started: make(chan struct{}),
+		seeding: make(chan struct{}),
+		release: make(chan struct{}),
+		detach:  detach,
+	}
+}
+
+func (f *fakeDownload) Name() string                    { return f.name }
+func (f *fakeDownload) Stat() Stat                      { return Stat{Name: f.name} }
+func (f *fakeDownload) SeedingStarted() <-chan struct{} { return f.seeding }
+
+func (f *fakeDownload) Run(ctx context.Context) error {
+	close(f.started)
+	if f.detach {
+		close(f.seeding)
+	}
+	select {
+	case <-f.release:
+		return nil
+	case <-ctx.Done():
+		return ctx.Err()
+	}
+}
+
+func started(f *fakeDownload, d time.Duration) bool {
+	select {
+	case <-f.started:
+		return true
+	case <-time.After(d):
+		return false
+	}
+}
+
+// A torrent that has finished downloading and is now seeding must not keep
+// occupying a -j slot: with -j 1, a seeding download has to let a queued one run.
+func TestEngineSeedingFreesSlot(t *testing.T) {
+	eng := NewEngine(1)
+	a := newFake("a", true)
+	b := newFake("b", true)
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	done := make(chan []Result, 1)
+	go func() { done <- eng.Run(ctx, []Download{a, b}) }()
+
+	// Neither is ever released, yet both must start: each frees the single slot
+	// the instant it detaches into seeding.
+	if !started(a, time.Second) || !started(b, time.Second) {
+		t.Fatal("both downloads should start once seeding frees the -j slot")
+	}
+	close(a.release)
+	close(b.release)
+	<-done
+}
+
+// Control: without a seeding transition, -j 1 still serializes — the second
+// download waits for the first to finish before it starts.
+func TestEngineLimitsConcurrentDownloads(t *testing.T) {
+	eng := NewEngine(1)
+	a := newFake("a", false)
+	b := newFake("b", false)
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	done := make(chan []Result, 1)
+	go func() { done <- eng.Run(ctx, []Download{a, b}) }()
+
+	var first, second *fakeDownload
+	select {
+	case <-a.started:
+		first, second = a, b
+	case <-b.started:
+		first, second = b, a
+	case <-time.After(time.Second):
+		t.Fatal("one download should start")
+	}
+	if started(second, 100*time.Millisecond) {
+		t.Fatal("second download must not start while the only slot is held")
+	}
+	close(first.release) // first finishes, freeing the slot
+	if !started(second, time.Second) {
+		t.Fatal("second download should start after the first frees the slot")
+	}
+	close(second.release)
+	<-done
+}