Rework HTTP announce timeouts

Use Request.Context to implement timeouts, set the default to 3s for announces from the Client.

tracker/http.go

@@ -113,17 +113,18 @@ func announceHTTP(opt Announce, _url *url.URL) (ret AnnounceResponse, err error)
 		req = req.WithContext(opt.Context)
 	}
 	resp, err := (&http.Client{
-		Timeout: time.Second * 15,
+		//Timeout: time.Second * 15,
 		Transport: &http.Transport{
-			Dial: (&net.Dialer{
-				Timeout: 15 * time.Second,
-			}).Dial,
-			Proxy:               opt.HTTPProxy,
-			TLSHandshakeTimeout: 15 * time.Second,
+			//Dial: (&net.Dialer{
+			//	Timeout: 15 * time.Second,
+			//}).Dial,
+			Proxy: opt.HTTPProxy,
+			//TLSHandshakeTimeout: 15 * time.Second,
 			TLSClientConfig: &tls.Config{
 				InsecureSkipVerify: true,
 				ServerName:         opt.ServerName,
 			},
+			// This is for S3 trackers that hold connections open.
 			DisableKeepAlives: true,
 		},
 	}).Do(req)

tracker/tracker.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"net/http"
 	"net/url"
+	"time"
 
 	"github.com/anacrolix/dht/v2/krpc"
 )
@@ -70,6 +71,13 @@ func (me Announce) Do() (res AnnounceResponse, err error) {
 	if err != nil {
 		return
 	}
+	if me.Context == nil {
+		// This is just to maintain the old behaviour that should be a timeout of 15s. Users can
+		// override it by providing their own Context.
+		ctx, cancel := context.WithTimeout(context.Background(), 15*time.Second)
+		defer cancel()
+		me.Context = ctx
+	}
 	switch _url.Scheme {
 	case "http", "https":
 		return announceHTTP(me, _url)

tracker_scraper.go

@@ -2,6 +2,7 @@ package torrent
 
 import (
 	"bytes"
+	"context"
 	"errors"
 	"fmt"
 	"net"
@@ -119,8 +120,11 @@ func (me *trackerScraper) announce(event tracker.AnnounceEvent) (ret trackerAnno
 	me.t.cl.rLock()
 	req := me.t.announceRequest(event)
 	me.t.cl.rUnlock()
+	ctx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
+	defer cancel()
 	me.t.logger.WithDefaultLevel(log.Debug).Printf("announcing to %q: %#v", me.u.String(), req)
 	res, err := tracker.Announce{
+		Context:    ctx,
 		HTTPProxy:  me.t.cl.config.HTTPProxy,
 		UserAgent:  me.t.cl.config.HTTPUserAgent,
 		TrackerUrl: me.trackerUrl(ip),