Properly follow relative links when listing tags

The previous code assumed that the link returned when listing tags was
always absolute. However, some registries, such as quay.io, return the
link as a relative link (e.g. the second page for the quay.io/coreos/etcd
image is /v2/coreos/etcd/tags/list?next_page=<truncated>&n=50). Because
the relative link was retrieved directly, the fetch failed (with the
error `unsupported protocol scheme ""`).

Signed-off-by: Kevin Lin <kevin@kelda.io>

registry/client/repository.go

@@ -205,13 +205,18 @@ type tags struct {
 func (t *tags) All(ctx context.Context) ([]string, error) {
 	var tags []string
 
-	u, err := t.ub.BuildTagsURL(t.name)
+	listURLStr, err := t.ub.BuildTagsURL(t.name)
+	if err != nil {
+		return tags, err
+	}
+
+	listURL, err := url.Parse(listURLStr)
 	if err != nil {
 		return tags, err
 	}
 
 	for {
-		resp, err := t.client.Get(u)
+		resp, err := t.client.Get(listURL.String())
 		if err != nil {
 			return tags, err
 		}
@@ -231,7 +236,13 @@ func (t *tags) All(ctx context.Context) ([]string, error) {
 			}
 			tags = append(tags, tagsResponse.Tags...)
 			if link := resp.Header.Get("Link"); link != "" {
-				u = strings.Trim(strings.Split(link, ";")[0], "<>")
+				linkURLStr := strings.Trim(strings.Split(link, ";")[0], "<>")
+				linkURL, err := url.Parse(linkURLStr)
+				if err != nil {
+					return tags, err
+				}
+
+				listURL = listURL.ResolveReference(linkURL)
 			} else {
 				return tags, nil
 			}

registry/client/repository_test.go

@@ -1136,13 +1136,27 @@ func TestManifestTagsPaginated(t *testing.T) {
 			queryParams["n"] = []string{"1"}
 			queryParams["last"] = []string{tagsList[i-1]}
 		}
+
+		// Test both relative and absolute links.
+		relativeLink := "/v2/" + repo.Name() + "/tags/list?n=1&last=" + tagsList[i]
+		var link string
+		switch i {
+		case 0:
+			link = relativeLink
+		case len(tagsList) - 1:
+			link = ""
+		default:
+			link = s.URL + relativeLink
+		}
+
 		headers := http.Header(map[string][]string{
 			"Content-Length": {fmt.Sprint(len(body))},
 			"Last-Modified":  {time.Now().Add(-1 * time.Second).Format(time.ANSIC)},
 		})
-		if i < 2 {
-			headers.Set("Link", "<"+s.URL+"/v2/"+repo.Name()+"/tags/list?n=1&last="+tagsList[i]+`>; rel="next"`)
+		if link != "" {
+			headers.Set("Link", fmt.Sprintf(`<%s>; rel="next"`, link))
 		}
+
 		m = append(m, testutil.RequestResponseMapping{
 			Request: testutil.Request{
 				Method:      "GET",