Modify HTTP request before sending (#787)

* set up custom request headers

* apply headers defined in torrent client config

* add error handling

* provide better name for method

* update error message

* only apply HTTPRequestDirector if not nil

config.go

@@ -102,6 +102,9 @@ type ClientConfig struct {
 	LookupTrackerIp func(*url.URL) ([]net.IP, error)
 	// HTTPUserAgent changes default UserAgent for HTTP requests
 	HTTPUserAgent string
+	// HTTPRequestDirector modifies the request before it's sent.
+	// Useful for adding authentication headers, for example
+	HTTPRequestDirector func(*http.Request) error
 	// Updated occasionally to when there's been some changes to client
 	// behaviour in case other clients are assuming anything of us. See also
 	// `bep20`.

tracker/http/http.go

@@ -80,6 +80,7 @@ type AnnounceOpt struct {
 	HostHeader          string
 	ClientIp4           net.IP
 	ClientIp6           net.IP
+	HTTPRequestDirector func(*http.Request) error
 }
 
 type AnnounceRequest = udp.AnnounceRequest
@@ -95,6 +96,15 @@ func (cl Client) Announce(ctx context.Context, ar AnnounceRequest, opt AnnounceO
 	if userAgent != "" {
 		req.Header.Set("User-Agent", userAgent)
 	}
+
+	if opt.HTTPRequestDirector != nil {
+		err = opt.HTTPRequestDirector(req)
+		if err != nil {
+			err = fmt.Errorf("error modifying HTTP request: %s", err)
+			return
+		}
+	}
+
 	req.Host = opt.HostHeader
 	resp, err := cl.hc.Do(req)
 	if err != nil {

tracker/tracker.go

@@ -39,6 +39,7 @@ type Announce struct {
 	Request             AnnounceRequest
 	HostHeader          string
 	HTTPProxy           func(*http.Request) (*url.URL, error)
+	HTTPRequestDirector func(*http.Request) error
 	DialContext         func(ctx context.Context, network, addr string) (net.Conn, error)
 	ListenPacket        func(network, addr string) (net.PacketConn, error)
 	ServerName          string
@@ -83,5 +84,6 @@ func (me Announce) Do() (res AnnounceResponse, err error) {
 		HostHeader:          me.HostHeader,
 		ClientIp4:           me.ClientIp4.IP,
 		ClientIp6:           me.ClientIp6.IP,
+		HTTPRequestDirector: me.HTTPRequestDirector,
 	})
 }

tracker_scraper.go

@@ -158,6 +158,7 @@ func (me *trackerScraper) announce(ctx context.Context, event tracker.AnnounceEv
 	res, err := tracker.Announce{
 		Context:             ctx,
 		HTTPProxy:           me.t.cl.config.HTTPProxy,
+		HTTPRequestDirector: me.t.cl.config.HTTPRequestDirector,
 		DialContext:         me.t.cl.config.TrackerDialContext,
 		ListenPacket:        me.t.cl.config.TrackerListenPacket,
 		UserAgent:           me.t.cl.config.HTTPUserAgent,