Add MaxRedirects option

Add `MaxRedirects` option to set maximum redirection-followings allowed.
The option is only valid when `FollowRedirects` is `true`.

Being able to limit the amount of redirections is helpful in order to
avoid possible loops of redirections or just too long round trips.

imageproxy_test.go

@@ -17,6 +17,8 @@ import (
 	"net/url"
 	"os"
 	"reflect"
+	"regexp"
+	"strconv"
 	"strings"
 	"testing"
 )
@@ -349,7 +351,17 @@ func (t testTransport) RoundTrip(req *http.Request) (*http.Response, error) {
 
 		raw = fmt.Sprintf("HTTP/1.1 200 OK\nContent-Length: %d\nContent-Type: image/png\n\n%s", len(img.Bytes()), img.Bytes())
 	default:
-		raw = "HTTP/1.1 404 Not Found\n\n"
+		redirectRegexp := regexp.MustCompile(`/redirects-(\d+)`)
+		if redirectRegexp.MatchString(req.URL.Path) {
+			redirectsLeft, _ := strconv.ParseUint(redirectRegexp.FindStringSubmatch(req.URL.Path)[1], 10, 8)
+			if redirectsLeft == 0 {
+				raw = "HTTP/1.1 200 OK\n\n"
+			} else {
+				raw = fmt.Sprintf("HTTP/1.1 302\nLocation: /http://redirect.test/redirects-%d\n\n", redirectsLeft-1)
+			}
+		} else {
+			raw = "HTTP/1.1 404 Not Found\n\n"
+		}
 	}
 
 	buf := bufio.NewReader(bytes.NewBufferString(raw))
@@ -414,6 +426,34 @@ func TestProxy_ServeHTTP_is304(t *testing.T) {
 	}
 }
 
+func TestProxy_ServeHTTP_maxRedirects(t *testing.T) {
+	p := &Proxy{
+		Client: &http.Client{
+			Transport: testTransport{},
+		},
+		FollowRedirects: true,
+	}
+
+	tests := []struct {
+		url  string
+		code int
+	}{
+		{"/http://redirect.test/redirects-0", http.StatusOK},
+		{"/http://redirect.test/redirects-2", http.StatusOK},
+		{"/http://redirect.test/redirects-11", http.StatusInternalServerError}, // too many redirects
+	}
+
+	for _, tt := range tests {
+		req, _ := http.NewRequest("GET", "http://localhost"+tt.url, nil)
+		resp := httptest.NewRecorder()
+		p.ServeHTTP(resp, req)
+
+		if got, want := resp.Code, tt.code; got != want {
+			t.Errorf("ServeHTTP(%v) returned status %d, want %d", req, got, want)
+		}
+	}
+}
+
 func TestProxy_log(t *testing.T) {
 	var b strings.Builder