internal/authutil: treat an empty scope in a token response as absent

internal/authutil/scopes.go

@@ -27,10 +27,16 @@ func UnionScopes(existing, challenged []string) []string {
 
 // ScopesFromToken extracts the granted scopes from an OAuth2 token response.
 // Per RFC 6749 §5.1, the scope parameter is optional; returns nil if absent.
+// An empty scope names no scope-token, so it is treated as absent too:
+// callers take nil to mean the requested scopes were granted.
 func ScopesFromToken(token *oauth2.Token) []string {
 	scope, ok := token.Extra("scope").(string)
 	if !ok {
 		return nil
 	}
-	return strings.Fields(scope)
+	scopes := strings.Fields(scope)
+	if len(scopes) == 0 {
+		return nil
+	}
+	return scopes
 }

internal/authutil/scopes_test.go

@@ -4,10 +4,12 @@
 package authutil
 
 import (
+	"net/url"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
 	"github.com/google/go-cmp/cmp/cmpopts"
+	"golang.org/x/oauth2"
 )
 
 func TestUnionScopes(t *testing.T) {
@@ -70,3 +72,59 @@ func TestUnionScopes(t *testing.T) {
 		})
 	}
 }
+
+func TestScopesFromToken(t *testing.T) {
+	tests := []struct {
+		name  string
+		token *oauth2.Token
+		want  []string
+	}{
+		{
+			name:  "no scope",
+			token: &oauth2.Token{AccessToken: "t"},
+			want:  nil,
+		},
+		{
+			name:  "single scope",
+			token: (&oauth2.Token{AccessToken: "t"}).WithExtra(map[string]any{"scope": "read"}),
+			want:  []string{"read"},
+		},
+		{
+			name:  "space-delimited scopes",
+			token: (&oauth2.Token{AccessToken: "t"}).WithExtra(map[string]any{"scope": "read  write admin"}),
+			want:  []string{"read", "write", "admin"},
+		},
+		{
+			name:  "form-encoded response",
+			token: (&oauth2.Token{AccessToken: "t"}).WithExtra(url.Values{"scope": {"read write"}}),
+			want:  []string{"read", "write"},
+		},
+		{
+			name:  "non-string scope",
+			token: (&oauth2.Token{AccessToken: "t"}).WithExtra(map[string]any{"scope": 42.0}),
+			want:  nil,
+		},
+		// An empty scope names no scope-token (RFC 6749 section 3.3), so it
+		// carries no more information than an absent one. Callers rely on nil
+		// to fall back to the requested scopes.
+		{
+			name:  "empty scope",
+			token: (&oauth2.Token{AccessToken: "t"}).WithExtra(map[string]any{"scope": ""}),
+			want:  nil,
+		},
+		{
+			name:  "whitespace-only scope",
+			token: (&oauth2.Token{AccessToken: "t"}).WithExtra(map[string]any{"scope": " \t "}),
+			want:  nil,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := ScopesFromToken(tt.token)
+			if diff := cmp.Diff(tt.want, got); diff != "" {
+				t.Errorf("ScopesFromToken() mismatch (-want +got):\n%s", diff)
+			}
+		})
+	}
+}