http servers: allow CORS to be set with --allow-origin flag - fixes #5078

Some changes about test cases:
Because MiddlewareCORS will return early on OPTIONS request,
this middleware should only be used once at NewServer function.
Test cases should pass AllowOrigin config instead of adding
this middleware again.

A new test case was added to test CORS preflight request with
an authenticator. Preflight request should always return 200 OK
regardless of autentications.

Co-authored-by: yuudi <yuudi@users.noreply.github.com>

lib/http/middleware.go

@@ -181,6 +181,13 @@ func MiddlewareCORS(allowOrigin string) Middleware {
 			w.Header().Add("Access-Control-Request-Method", "POST, OPTIONS, GET, HEAD")
 			w.Header().Add("Access-Control-Allow-Headers", "authorization, Content-Type")
 
+			if r.Method == "OPTIONS" {
+				w.WriteHeader(http.StatusOK)
+				return
+				// Because CORS preflight OPTIONS requests are not authenticated,
+				// and require a 200 OK response, we will return early here.
+			}
+
 			next.ServeHTTP(w, r)
 		})
 	}

lib/http/middleware_test.go

@@ -329,23 +329,22 @@ var _testCORSHeaderKeys = []string{
 
 func TestMiddlewareCORS(t *testing.T) {
 	servers := []struct {
-		name   string
-		http   Config
-		origin string
+		name string
+		http Config
 	}{
 		{
 			name: "EmptyOrigin",
 			http: Config{
-				ListenAddr: []string{"127.0.0.1:0"},
+				ListenAddr:  []string{"127.0.0.1:0"},
+				AllowOrigin: "",
 			},
-			origin: "",
 		},
 		{
 			name: "CustomOrigin",
 			http: Config{
-				ListenAddr: []string{"127.0.0.1:0"},
+				ListenAddr:  []string{"127.0.0.1:0"},
+				AllowOrigin: "http://test.rclone.org",
 			},
-			origin: "http://test.rclone.org",
 		},
 	}
 
@@ -357,8 +356,6 @@ func TestMiddlewareCORS(t *testing.T) {
 				require.NoError(t, s.Shutdown())
 			}()
 
-			s.Router().Use(MiddlewareCORS(ss.origin))
-
 			expected := []byte("data")
 			s.Router().Mount("/", testEchoHandler(expected))
 			s.Serve()
@@ -384,8 +381,69 @@ func TestMiddlewareCORS(t *testing.T) {
 			}
 
 			expectedOrigin := url
-			if ss.origin != "" {
-				expectedOrigin = ss.origin
+			if ss.http.AllowOrigin != "" {
+				expectedOrigin = ss.http.AllowOrigin
+			}
+			require.Equal(t, expectedOrigin, resp.Header.Get("Access-Control-Allow-Origin"), "allow origin should match")
+		})
+	}
+}
+
+func TestMiddlewareCORSWithAuth(t *testing.T) {
+	authServers := []struct {
+		name string
+		http Config
+		auth AuthConfig
+	}{
+		{
+			name: "ServerWithAuth",
+			http: Config{
+				ListenAddr:  []string{"127.0.0.1:0"},
+				AllowOrigin: "http://test.rclone.org",
+			},
+			auth: AuthConfig{
+				Realm:     "test",
+				BasicUser: "test_user",
+				BasicPass: "test_pass",
+			},
+		},
+	}
+
+	for _, ss := range authServers {
+		t.Run(ss.name, func(t *testing.T) {
+			s, err := NewServer(context.Background(), WithConfig(ss.http))
+			require.NoError(t, err)
+			defer func() {
+				require.NoError(t, s.Shutdown())
+			}()
+
+			expected := []byte("data")
+			s.Router().Mount("/", testEchoHandler(expected))
+			s.Serve()
+
+			url := testGetServerURL(t, s)
+
+			client := &http.Client{}
+			req, err := http.NewRequest("OPTIONS", url, nil)
+			require.NoError(t, err)
+
+			resp, err := client.Do(req)
+			require.NoError(t, err)
+			defer func() {
+				_ = resp.Body.Close()
+			}()
+
+			require.Equal(t, http.StatusOK, resp.StatusCode, "OPTIONS should return ok even if not authenticated")
+
+			testExpectRespBody(t, resp, []byte{})
+
+			for _, key := range _testCORSHeaderKeys {
+				require.Contains(t, resp.Header, key, "CORS headers should be sent even if not authenticated")
+			}
+
+			expectedOrigin := url
+			if ss.http.AllowOrigin != "" {
+				expectedOrigin = ss.http.AllowOrigin
 			}
 			require.Equal(t, expectedOrigin, resp.Header.Get("Access-Control-Allow-Origin"), "allow origin should match")
 		})

lib/http/server.go

@@ -109,6 +109,7 @@ type Config struct {
 	TLSKeyBody         []byte        // TLS PEM Private key body, ignores TLSKey
 	ClientCA           string        // Client certificate authority to verify clients with
 	MinTLSVersion      string        // MinTLSVersion contains the minimum TLS version that is acceptable.
+	AllowOrigin        string        // AllowOrigin sets the Access-Control-Allow-Origin header
 }
 
 // AddFlagsPrefix adds flags for the httplib
@@ -122,6 +123,7 @@ func (cfg *Config) AddFlagsPrefix(flagSet *pflag.FlagSet, prefix string) {
 	flags.StringVarP(flagSet, &cfg.ClientCA, prefix+"client-ca", "", cfg.ClientCA, "Client certificate authority to verify clients with")
 	flags.StringVarP(flagSet, &cfg.BaseURL, prefix+"baseurl", "", cfg.BaseURL, "Prefix for URLs - leave blank for root")
 	flags.StringVarP(flagSet, &cfg.MinTLSVersion, prefix+"min-tls-version", "", cfg.MinTLSVersion, "Minimum TLS version that is acceptable")
+	flags.StringVarP(flagSet, &cfg.AllowOrigin, prefix+"allow-origin", "", cfg.AllowOrigin, "Origin which cross-domain request (CORS) can be executed from")
 }
 
 // AddHTTPFlagsPrefix adds flags for the httplib
@@ -236,6 +238,8 @@ func NewServer(ctx context.Context, options ...Option) (*Server, error) {
 		return nil, err
 	}
 
+	s.mux.Use(MiddlewareCORS(s.cfg.AllowOrigin))
+
 	s.initAuth()
 
 	for _, addr := range s.cfg.ListenAddr {

lib/http/server_test.go

@@ -82,8 +82,6 @@ func TestNewServerUnix(t *testing.T) {
 
 	require.Empty(t, s.URLs(), "unix socket should not appear in URLs")
 
-	s.Router().Use(MiddlewareCORS(""))
-
 	expected := []byte("hello world")
 	s.Router().Mount("/", testEchoHandler(expected))
 	s.Serve()