Send tracker stopped event from the tracker scraper routine

Fixes potential blocking on the announce while the client lock is held, as well as differing arguments to the other announces introduced by #321.

torrent.go

@@ -666,9 +666,6 @@ func (t *Torrent) numPiecesCompleted() (num int) {
 }
 
 func (t *Torrent) close() (err error) {
-	for _, ta := range t.trackerAnnouncers {
-		ta.Stop()
-	}
 	t.closed.Set()
 	t.tickleReaders()
 	if t.storage != nil {

tracker_scraper.go

@@ -9,16 +9,13 @@ import (
 	"time"
 
 	"github.com/anacrolix/dht/krpc"
-	"github.com/anacrolix/missinggo"
 	"github.com/anacrolix/torrent/tracker"
 )
 
 // Announces a torrent to a tracker at regular intervals, when peers are
 // required.
 type trackerScraper struct {
-	u url.URL
-	// Causes the trackerScraper to stop running.
-	stop         missinggo.Event
+	u            url.URL
 	t            *Torrent
 	lastAnnounce trackerAnnounceResult
 }
@@ -109,6 +106,7 @@ func (me *trackerScraper) announce(event tracker.AnnounceEvent) (ret trackerAnno
 	me.t.cl.lock()
 	req := me.t.announceRequest(event)
 	me.t.cl.unlock()
+	//log.Printf("announcing %s %s to %q", me.t, req.Event, me.u.String())
 	res, err := tracker.Announce{
 		HTTPProxy:  me.t.cl.config.HTTPProxy,
 		UserAgent:  me.t.cl.config.HTTPUserAgent,
@@ -131,6 +129,7 @@ func (me *trackerScraper) announce(event tracker.AnnounceEvent) (ret trackerAnno
 }
 
 func (me *trackerScraper) Run() {
+	defer me.announceStopped()
 	// make sure first announce is a "started"
 	e := tracker.Started
 	for {
@@ -159,8 +158,6 @@ func (me *trackerScraper) Run() {
 		select {
 		case <-me.t.closed.LockedChan(me.t.cl.locker()):
 			return
-		case <-me.stop.LockedChan(me.t.cl.locker()):
-			return
 		case <-wantPeers:
 			goto wait
 		case <-time.After(time.Until(ar.Completed.Add(interval))):
@@ -168,15 +165,6 @@ func (me *trackerScraper) Run() {
 	}
 }
 
-func (me *trackerScraper) Stop() {
-	req := me.t.announceRequest(tracker.Stopped)
-	ip, _ := me.getIp()
-	tracker.Announce{
-		HTTPProxy:  me.t.cl.config.HTTPProxy,
-		UserAgent:  me.t.cl.config.HTTPUserAgent,
-		TrackerUrl: me.trackerUrl(ip),
-		Request:    req,
-		HostHeader: me.u.Host,
-		ServerName: me.u.Hostname(),
-	}.Do()
+func (me *trackerScraper) announceStopped() {
+	me.announce(tracker.Stopped)
 }