config: make reaping configurable

This allows for configurable reaping rates and changes the default to
what many public trackers use in the wild.

config/config.go

@@ -81,6 +81,8 @@ type TrackerConfig struct {
 	PurgeInactiveTorrents bool     `json:"purge_inactive_torrents"`
 	Announce              Duration `json:"announce"`
 	MinAnnounce           Duration `json:"min_announce"`
+	ReapInterval          Duration `json:"reap_interval"`
+	ReapRatio             float64  `json:"reap_ratio"`
 	NumWantFallback       int      `json:"default_num_want"`
 	TorrentMapShards      int      `json:"torrent_map_shards"`
 
@@ -121,6 +123,8 @@ var DefaultConfig = Config{
 		PurgeInactiveTorrents: true,
 		Announce:              Duration{30 * time.Minute},
 		MinAnnounce:           Duration{15 * time.Minute},
+		ReapInterval:          Duration{60 * time.Second},
+		ReapRatio:             1.25,
 		NumWantFallback:       50,
 		TorrentMapShards:      1,
 

example_config.json

@@ -5,6 +5,8 @@
   "purge_inactive_torrents": true,
   "announce": "30m",
   "min_announce": "15m",
+  "reap_interval": "60s",
+  "reap_ratio": "1.25",
   "default_num_want": 50,
   "torrent_map_shards": 1,
   "allow_ip_spoofing": true,

http/announce_test.go

@@ -84,9 +84,8 @@ func TestTorrentPurging(t *testing.T) {
 
 func TestStalePeerPurging(t *testing.T) {
 	cfg := config.DefaultConfig
-	cfg.Announce = config.Duration{
-		Duration: 10 * time.Millisecond,
-	}
+	cfg.MinAnnounce = config.Duration{10 * time.Millisecond}
+	cfg.ReapInterval = config.Duration{10 * time.Millisecond}
 
 	srv, err := setupTracker(&cfg)
 	if err != nil {
@@ -110,7 +109,7 @@ func TestStalePeerPurging(t *testing.T) {
 	// Add a leecher.
 	peer2 := makePeerParams("peer2", false)
 	expected := makeResponse(1, 1, peer1)
-	expected["interval"] = int64(0)
+	expected["min interval"] = int64(0)
 	checkAnnounce(peer2, expected, srv, t)
 
 	// Let them both expire.

tracker/tracker.go

@@ -49,8 +49,8 @@ func New(cfg *config.Config) (*Tracker, error) {
 
 	go tkr.purgeInactivePeers(
 		cfg.PurgeInactiveTorrents,
-		cfg.Announce.Duration*2,
-		cfg.Announce.Duration,
+		time.Duration(float64(cfg.MinAnnounce.Duration)*cfg.ReapRatio),
+		cfg.ReapInterval.Duration,
 	)
 
 	if cfg.ClientWhitelistEnabled {
@@ -86,13 +86,6 @@ type Writer interface {
 
 // purgeInactivePeers periodically walks the torrent database and removes
 // peers that haven't announced recently.
-//
-// The default threshold is 2x the announce interval, which gives delayed
-// peers a chance to stay alive, while ensuring the majority of responses
-// contain active peers.
-//
-// The default interval is equal to the announce interval, since this is a
-// relatively expensive operation.
 func (tkr *Tracker) purgeInactivePeers(purgeEmptyTorrents bool, threshold, interval time.Duration) {
 	for _ = range time.NewTicker(interval).C {
 		before := time.Now().Add(-threshold)