internal handlers: support for response modifiers

Co-authored-by: Julien Salleyron <julien@containo.us>
Co-authored-by: Romain <rtribotte@users.noreply.github.com>
Co-authored-by: Jean-Baptiste Doumenjou <jb.doumenjou@gmail.com>

integration/fixtures/headers/secure.toml

@@ -2,6 +2,9 @@
   checkNewVersion = false
   sendAnonymousUsage = false
 
+[api]
+  insecure = true
+
 [log]
   level = "DEBUG"
 
@@ -24,6 +27,11 @@
     rule = "Host(`test2.localhost`)"
     service = "service1"
 
+  [http.routers.router3]
+    rule = "Host(`internal.localhost`)"
+    middlewares = ["secure"]
+    service = "api@internal"
+
 [http.middlewares]
   [http.middlewares.secure.headers]
     featurePolicy = "vibrate 'none';"

integration/headers_test.go

@@ -134,6 +134,7 @@ func (s *HeadersSuite) TestSecureHeadersResponses(c *check.C) {
 		desc            string
 		expected        http.Header
 		reqHost         string
+		internalReqHost string
 	}{
 		{
 			desc: "Feature-Policy Set",
@@ -141,6 +142,7 @@ func (s *HeadersSuite) TestSecureHeadersResponses(c *check.C) {
 				"Feature-Policy": {"vibrate 'none';"},
 			},
 			reqHost:         "test.localhost",
+			internalReqHost: "internal.localhost",
 		},
 	}
 
@@ -149,7 +151,14 @@ func (s *HeadersSuite) TestSecureHeadersResponses(c *check.C) {
 		c.Assert(err, checker.IsNil)
 		req.Host = test.reqHost
 
-		err = try.Request(req, 500*time.Millisecond, try.HasHeaderStruct(test.expected))
+		err = try.Request(req, 500*time.Millisecond, try.StatusCodeIs(http.StatusOK), try.HasHeaderStruct(test.expected))
+		c.Assert(err, checker.IsNil)
+
+		req, err = http.NewRequest(http.MethodGet, "http://127.0.0.1:8000/api/rawdata", nil)
+		c.Assert(err, checker.IsNil)
+		req.Host = test.internalReqHost
+
+		err = try.Request(req, 500*time.Millisecond, try.StatusCodeIs(http.StatusOK), try.HasHeaderStruct(test.expected))
 		c.Assert(err, checker.IsNil)
 	}
 }

pkg/middlewares/headers/headers.go

@@ -54,13 +54,13 @@ func New(ctx context.Context, next http.Handler, cfg dynamic.Headers, name strin
 	nextHandler := next
 
 	if hasSecureHeaders {
-		logger.Debug("Setting up secureHeaders from %v", cfg)
+		logger.Debugf("Setting up secureHeaders from %v", cfg)
 		handler = newSecure(next, cfg)
 		nextHandler = handler
 	}
 
 	if hasCustomHeaders || hasCorsHeaders {
-		logger.Debug("Setting up customHeaders/Cors from %v", cfg)
+		logger.Debugf("Setting up customHeaders/Cors from %v", cfg)
 		handler = NewHeader(nextHandler, cfg)
 	}
 

pkg/responsemodifiers/response_modifier.go

@@ -19,6 +19,7 @@ type Builder struct {
 }
 
 // Build Builds the response modifier.
+// It returns nil if there is no modifier to apply.
 func (f *Builder) Build(ctx context.Context, names []string) func(*http.Response) error {
 	var modifiers []func(*http.Response) error
 
@@ -60,5 +61,5 @@ func (f *Builder) Build(ctx context.Context, names []string) func(*http.Response
 		}
 	}
 
-	return func(response *http.Response) error { return nil }
+	return nil
 }

pkg/responsemodifiers/response_modifier_test.go

@@ -184,6 +184,9 @@ func TestBuilderBuild(t *testing.T) {
 			builder := NewBuilder(rtConf.Middlewares)
 
 			rm := builder.Build(context.Background(), test.middlewares)
+			if rm == nil {
+				return
+			}
 
 			resp := test.buildResponse(test.conf)
 

pkg/server/service/internalhandler.go

@@ -8,6 +8,7 @@ import (
 	"strings"
 
 	"github.com/containous/traefik/v2/pkg/config/runtime"
+	"github.com/containous/traefik/v2/pkg/log"
 )
 
 type serviceManager interface {
@@ -42,13 +43,87 @@ func NewInternalHandlers(api func(configuration *runtime.Configuration) http.Han
 	}
 }
 
-// BuildHTTP builds an HTTP handler.
+type responseModifier struct {
-func (m *InternalHandlers) BuildHTTP(rootCtx context.Context, serviceName string, responseModifier func(*http.Response) error) (http.Handler, error) {
+	r *http.Request
-	if strings.HasSuffix(serviceName, "@internal") {
+	w http.ResponseWriter
-		return m.get(serviceName)
+
+	headersSent bool // whether headers have already been sent
+	code        int  // status code, must default to 200
+
+	modifier    func(*http.Response) error // can be nil
+	modified    bool                       // whether modifier has already been called for the current request
+	modifierErr error                      // returned by modifier call
+}
+
+// modifier can be nil.
+func newResponseModifier(w http.ResponseWriter, r *http.Request, modifier func(*http.Response) error) *responseModifier {
+	return &responseModifier{
+		r:        r,
+		w:        w,
+		modifier: modifier,
+		code:     http.StatusOK,
+	}
+}
+
+func (w *responseModifier) WriteHeader(code int) {
+	if w.headersSent {
+		return
+	}
+	defer func() {
+		w.code = code
+		w.headersSent = true
+	}()
+
+	if w.modifier == nil || w.modified {
+		w.w.WriteHeader(code)
+		return
 	}
 
-	return m.serviceManager.BuildHTTP(rootCtx, serviceName, responseModifier)
+	resp := http.Response{
+		Header:  w.w.Header(),
+		Request: w.r,
+	}
+
+	if err := w.modifier(&resp); err != nil {
+		w.modifierErr = err
+		// we are propagating when we are called in Write, but we're logging anyway,
+		// because we could be called from another place which does not take care of
+		// checking w.modifierErr.
+		log.Errorf("Error when applying response modifier: %v", err)
+		w.w.WriteHeader(http.StatusInternalServerError)
+		return
+	}
+
+	w.modified = true
+	w.w.WriteHeader(code)
+}
+
+func (w *responseModifier) Header() http.Header {
+	return w.w.Header()
+}
+
+func (w *responseModifier) Write(b []byte) (int, error) {
+	w.WriteHeader(w.code)
+	if w.modifierErr != nil {
+		return 0, w.modifierErr
+	}
+
+	return w.w.Write(b)
+}
+
+// BuildHTTP builds an HTTP handler.
+func (m *InternalHandlers) BuildHTTP(rootCtx context.Context, serviceName string, respModifier func(*http.Response) error) (http.Handler, error) {
+	if !strings.HasSuffix(serviceName, "@internal") {
+		return m.serviceManager.BuildHTTP(rootCtx, serviceName, respModifier)
+	}
+
+	internalHandler, err := m.get(serviceName)
+	if err != nil {
+		return nil, err
+	}
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		internalHandler.ServeHTTP(newResponseModifier(w, r, respModifier), r)
+	}), nil
 }
 
 func (m *InternalHandlers) get(serviceName string) (http.Handler, error) {