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

bt/bt.go

@@ -203,12 +203,17 @@ type Download struct {
 	// t is published once Run adds the torrent to the client. It is atomic so
 	// Stat stays lock-free, exactly like httpdl.Stat and bt's own name field.
 	t atomic.Pointer[torrent.Torrent]
+
+	// seeding is closed once Run finishes downloading and transitions to
+	// seed-only, so the engine can drop this torrent from the -j concurrency
+	// count. Created in New, closed exactly once in Run.
+	seeding chan struct{}
 }
 
 // New builds a torrent download on the shared client. source is a .torrent path
 // when isFile is true, otherwise a magnet URI.
 func New(client *torrent.Client, source string, isFile bool, opts Options) *Download {
-	d := &Download{client: client, source: source, isFile: isFile, opts: opts}
+	d := &Download{client: client, source: source, isFile: isFile, opts: opts, seeding: make(chan struct{})}
 	if isFile {
 		d.setName(strings.TrimSuffix(filepath.Base(source), ".torrent"))
 	} else {
@@ -233,6 +238,12 @@ func (d *Download) setStatus(s download.Status) {
 	atomic.StoreInt32(&d.status, int32(s))
 }
 
+// SeedingStarted is closed when this torrent finishes downloading and switches
+// to seed-only. The engine watches it to release the -j concurrency slot at that
+// moment: seeding is not downloading, so a seed-only torrent must not hold a slot
+// that a queued download could use. Implements download.Seeder.
+func (d *Download) SeedingStarted() <-chan struct{} { return d.seeding }
+
 // id is the process-unique identity for this download: the infohash once known,
 // otherwise the source string (a pre-metadata magnet has no infohash yet, and
 // two such magnets would otherwise collide on Name).
@@ -329,6 +340,7 @@ func (d *Download) Run(ctx context.Context) (err error) {
 	}
 
 	d.setStatus(download.Seeding)
+	close(d.seeding) // free the -j slot: a seed-only torrent no longer downloads
 	d.seed(ctx, t)
 	d.setStatus(download.Complete)
 	return nil