Don't panic with undefined middleware

pkg/responsemodifiers/response_modifier.go

@@ -23,6 +23,11 @@ func (f *Builder) Build(ctx context.Context, names []string) func(*http.Response
 
 	for _, middleName := range names {
 		if conf, ok := f.configs[middleName]; ok {
+			if conf == nil || conf.Middleware == nil {
+				getLogger(ctx, middleName, "undefined").Error("Invalid Middleware configuration (ResponseModifier)")
+				continue
+			}
+
 			if conf.Headers != nil {
 				getLogger(ctx, middleName, "Headers").Debug("Creating Middleware (ResponseModifier)")
 

pkg/responsemodifiers/response_modifier_test.go

@@ -47,7 +47,7 @@ func TestBuilderBuild(t *testing.T) {
 			assertResponse: func(t *testing.T, resp *http.Response) {
 				t.Helper()
 
-				assert.Equal(t, resp.Header.Get("X-Foo"), "foo")
+				assert.Equal(t, "foo", resp.Header.Get("X-Foo"))
 			},
 		},
 		{
@@ -85,7 +85,7 @@ func TestBuilderBuild(t *testing.T) {
 			assertResponse: func(t *testing.T, resp *http.Response) {
 				t.Helper()
 
-				assert.Equal(t, resp.Header.Get("Referrer-Policy"), "no-referrer")
+				assert.Equal(t, "no-referrer", resp.Header.Get("Referrer-Policy"))
 			},
 		},
 		{
@@ -107,8 +107,8 @@ func TestBuilderBuild(t *testing.T) {
 			assertResponse: func(t *testing.T, resp *http.Response) {
 				t.Helper()
 
-				assert.Equal(t, resp.Header.Get("X-Foo"), "foo")
-				assert.Equal(t, resp.Header.Get("X-Bar"), "bar")
+				assert.Equal(t, "foo", resp.Header.Get("X-Foo"))
+				assert.Equal(t, "bar", resp.Header.Get("X-Bar"))
 			},
 		},
 		{
@@ -130,7 +130,7 @@ func TestBuilderBuild(t *testing.T) {
 			assertResponse: func(t *testing.T, resp *http.Response) {
 				t.Helper()
 
-				assert.Equal(t, resp.Header.Get("X-Foo"), "foo")
+				assert.Equal(t, "foo", resp.Header.Get("X-Foo"))
 			},
 		},
 		{
@@ -157,9 +157,18 @@ func TestBuilderBuild(t *testing.T) {
 			assertResponse: func(t *testing.T, resp *http.Response) {
 				t.Helper()
 
-				assert.Equal(t, resp.Header.Get("X-Foo"), "foo")
+				assert.Equal(t, "foo", resp.Header.Get("X-Foo"))
 			},
 		},
+		{
+			desc:          "nil middleware",
+			middlewares:   []string{"foo"},
+			buildResponse: stubResponse,
+			conf: map[string]*dynamic.Middleware{
+				"foo": nil,
+			},
+			assertResponse: func(t *testing.T, resp *http.Response) {},
+		},
 	}
 
 	for _, test := range testCases {

pkg/server/middleware/middlewares.go

@@ -102,6 +102,10 @@ func checkRecursion(ctx context.Context, middlewareName string) (context.Context
 // it is the responsibility of the caller to make sure that b.configs[middlewareName].Middleware exists
 func (b *Builder) buildConstructor(ctx context.Context, middlewareName string) (alice.Constructor, error) {
 	config := b.configs[middlewareName]
+	if config == nil || config.Middleware == nil {
+		return nil, fmt.Errorf("invalid middleware %q configuration", middlewareName)
+	}
+
 	var middleware alice.Constructor
 	badConf := errors.New("cannot create middleware: multi-types middleware not supported, consider declaring two different pieces of middleware instead")
 
@@ -314,7 +318,7 @@ func (b *Builder) buildConstructor(ctx context.Context, middlewareName string) (
 	}
 
 	if middleware == nil {
-		return nil, errors.New("middleware does not exist")
+		return nil, fmt.Errorf("middleware %q does not exist", middlewareName)
 	}
 
 	return tracing.Wrap(ctx, middleware), nil

pkg/server/middleware/middlewares_test.go

@@ -369,7 +369,6 @@ func TestBuilder_buildConstructor(t *testing.T) {
 			t.Parallel()
 
 			constructor, err := middlewaresBuilder.buildConstructor(context.Background(), test.middlewareID)
-
 			require.NoError(t, err)
 
 			middleware, err2 := constructor(http.HandlerFunc(func(_ http.ResponseWriter, _ *http.Request) {}))