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

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()}