Iterate through group membership http response

Follows the nextLink in http response to iterate through
paginated group membership response in order to obtain all
groups that the user is a member of.

Also, checks cache to make sure we don't add empty groups
to the cache.

authn/authenticate.go

@@ -52,7 +52,8 @@ type Group struct {
 
 // Groups holds a slice of Group for marshalling from JSON
 type Groups struct {
-	Groups []Group `json:"value"`
+	NextLink string  `json:"@odata.nextLink"`
+	Groups   []Group `json:"value"`
 }
 
 // Auth holds state, configuration, and utilities needed for authentication.
@@ -240,25 +241,39 @@ func (a *Auth) Redirect(w http.ResponseWriter, r *http.Request) {
 func (a *Auth) getGroups(token string) ([]Group, error) {
 	var groups Groups
 
-	g, ok := a.groupsCache[token]
-	if ok && (time.Now().Sub(g.cacheTime) < a.cacheTTL) {
-		return g.groups, nil
+	gc, ok := a.groupsCache[token]
+	if ok && (time.Now().Sub(gc.cacheTime) < a.cacheTTL) && len(gc.groups) > 0 {
+		return gc.groups, nil
 	}
 
-	req, err := http.NewRequest("GET", a.groupEndpoint, nil)
-	if err != nil {
-		return groups.Groups, errors.Wrap(err, "creating new request to group endpoint")
+	nextLink := a.groupEndpoint
+	for nextLink != "" {
+		req, err := http.NewRequest("GET", nextLink, nil)
+		if err != nil {
+			return nil, errors.Wrap(err, "creating new request to group endpoint")
+		}
+
+		req.Header.Add("Authorization", fmt.Sprintf("Bearer %s", token))
+		response, err := http.DefaultClient.Do(req)
+		if err != nil {
+			return nil, errors.Wrap(err, "getting group membership info")
+		}
+		if response.StatusCode != http.StatusOK {
+			return nil, fmt.Errorf("getting group membership info: %s", response.Status)
+		}
+
+		var g Groups
+		if err = json.NewDecoder(response.Body).Decode(&g); err != nil {
+			return groups.Groups, errors.Wrap(err, "failed unmarshalling group membership response")
+		}
+
+		response.Body.Close()
+		groups.Groups = append(groups.Groups, g.Groups...)
+		nextLink = g.NextLink
 	}
 
-	req.Header.Add("Authorization", fmt.Sprintf("Bearer %s", token))
-	response, err := http.DefaultClient.Do(req)
-	if err != nil {
-		return groups.Groups, errors.Wrap(err, "getting group membership info")
-	}
-
-	defer response.Body.Close()
-	if err = json.NewDecoder(response.Body).Decode(&groups); err != nil {
-		return groups.Groups, errors.Wrap(err, "failed unmarshalling group membership response")
+	if len(groups.Groups) == 0 {
+		return nil, fmt.Errorf("no groups found")
 	}
 
 	a.groupsCache[token] = cachedGroups{

authn/authenticate_internal_test.go

@@ -352,19 +352,36 @@ func TestGetGroups(t *testing.T) {
 			cacheTime: time.Now(),
 			groups: []Group{
 				{
-					GroupID:   "i feel it in the water",
-					GroupName: "i feel it in the earth",
+					GroupID:   "a han noston ned wilith",
+					GroupName: "I smell it in the air",
 				},
 			},
 		},
 	}
-	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+	srvNext := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		body, err := json.Marshal(
 			Groups{
 				Groups: []Group{
 					{
-						GroupID:   "much that once was is lost",
-						GroupName: "for none now live who remember it",
+						GroupID:   "han mathon ne chae",
+						GroupName: "I feel it in the earth",
+					},
+				},
+			},
+		)
+		if err != nil {
+			t.Fatalf("unexpected error marshalling groups response: %v", err)
+		}
+		fmt.Fprintf(w, "%s", body)
+	}))
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		body, err := json.Marshal(
+			Groups{
+				NextLink: srvNext.URL,
+				Groups: []Group{
+					{
+						GroupID:   "han mathon ne nen",
+						GroupName: "i feel it in the water",
 					},
 				},
 			},
@@ -384,8 +401,8 @@ func TestGetGroups(t *testing.T) {
 			token: "the world is changed",
 			groups: []Group{
 				{
-					GroupID:   "i feel it in the water",
-					GroupName: "i feel it in the earth",
+					GroupID:   "a han noston ned wilith",
+					GroupName: "I smell it in the air",
 				},
 			},
 		},
@@ -393,8 +410,12 @@ func TestGetGroups(t *testing.T) {
 			token: "i smell it in the air",
 			groups: []Group{
 				{
-					GroupID:   "much that once was is lost",
-					GroupName: "for none now live who remember it",
+					GroupID:   "han mathon ne nen",
+					GroupName: "i feel it in the water",
+				},
+				{
+					GroupID:   "han mathon ne chae",
+					GroupName: "I feel it in the earth",
 				},
 			},
 		},