fix: stripPrefix middleware with empty resulting path.

pkg/middlewares/stripprefix/strip_prefix.go

@@ -20,18 +20,20 @@ const (
 
 // stripPrefix is a middleware used to strip prefix from an URL request.
 type stripPrefix struct {
-	next     http.Handler
-	prefixes []string
-	name     string
+	next       http.Handler
+	prefixes   []string
+	forceSlash bool // TODO Must be removed (breaking), the default behavior must be forceSlash=false
+	name       string
 }
 
 // New creates a new strip prefix middleware.
 func New(ctx context.Context, next http.Handler, config dynamic.StripPrefix, name string) (http.Handler, error) {
 	log.FromContext(middlewares.GetLoggerCtx(ctx, name, typeName)).Debug("Creating middleware")
 	return &stripPrefix{
-		prefixes: config.Prefixes,
-		next:     next,
-		name:     name,
+		prefixes:   config.Prefixes,
+		forceSlash: config.ForceSlash,
+		next:       next,
+		name:       name,
 	}, nil
 }
 
@@ -42,9 +44,9 @@ func (s *stripPrefix) GetTracingInformation() (string, ext.SpanKindEnum) {
 func (s *stripPrefix) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 	for _, prefix := range s.prefixes {
 		if strings.HasPrefix(req.URL.Path, prefix) {
-			req.URL.Path = getPrefixStripped(req.URL.Path, prefix)
+			req.URL.Path = s.getPrefixStripped(req.URL.Path, prefix)
 			if req.URL.RawPath != "" {
-				req.URL.RawPath = getPrefixStripped(req.URL.RawPath, prefix)
+				req.URL.RawPath = s.getPrefixStripped(req.URL.RawPath, prefix)
 			}
 			s.serveRequest(rw, req, strings.TrimSpace(prefix))
 			return
@@ -59,10 +61,25 @@ func (s *stripPrefix) serveRequest(rw http.ResponseWriter, req *http.Request, pr
 	s.next.ServeHTTP(rw, req)
 }
 
-func getPrefixStripped(s, prefix string) string {
-	return ensureLeadingSlash(strings.TrimPrefix(s, prefix))
+func (s *stripPrefix) getPrefixStripped(urlPath, prefix string) string {
+	if s.forceSlash {
+		// Only for compatibility reason with the previous behavior,
+		// but the previous behavior is wrong.
+		// This needs to be removed in the next breaking version.
+		return "/" + strings.TrimPrefix(strings.TrimPrefix(urlPath, prefix), "/")
+	}
+
+	return ensureLeadingSlash(strings.TrimPrefix(urlPath, prefix))
 }
 
 func ensureLeadingSlash(str string) string {
-	return "/" + strings.TrimPrefix(str, "/")
+	if str == "" {
+		return str
+	}
+
+	if str[0] == '/' {
+		return str
+	}
+
+	return "/" + str
 }

pkg/middlewares/stripprefix/strip_prefix_test.go

@@ -31,6 +31,17 @@ func TestStripPrefix(t *testing.T) {
 			expectedStatusCode: http.StatusOK,
 			expectedPath:       "/noprefixes",
 		},
+		{
+			desc: "wildcard (.*) requests (ForceSlash)",
+			config: dynamic.StripPrefix{
+				Prefixes:   []string{"/"},
+				ForceSlash: true,
+			},
+			path:               "/",
+			expectedStatusCode: http.StatusOK,
+			expectedPath:       "/",
+			expectedHeader:     "/",
+		},
 		{
 			desc: "wildcard (.*) requests",
 			config: dynamic.StripPrefix{
@@ -38,9 +49,20 @@ func TestStripPrefix(t *testing.T) {
 			},
 			path:               "/",
 			expectedStatusCode: http.StatusOK,
-			expectedPath:       "/",
+			expectedPath:       "",
 			expectedHeader:     "/",
 		},
+		{
+			desc: "prefix and path matching (ForceSlash)",
+			config: dynamic.StripPrefix{
+				Prefixes:   []string{"/stat"},
+				ForceSlash: true,
+			},
+			path:               "/stat",
+			expectedStatusCode: http.StatusOK,
+			expectedPath:       "/",
+			expectedHeader:     "/stat",
+		},
 		{
 			desc: "prefix and path matching",
 			config: dynamic.StripPrefix{
@@ -48,9 +70,20 @@ func TestStripPrefix(t *testing.T) {
 			},
 			path:               "/stat",
 			expectedStatusCode: http.StatusOK,
-			expectedPath:       "/",
+			expectedPath:       "",
 			expectedHeader:     "/stat",
 		},
+		{
+			desc: "path prefix on exactly matching path (ForceSlash)",
+			config: dynamic.StripPrefix{
+				Prefixes:   []string{"/stat/"},
+				ForceSlash: true,
+			},
+			path:               "/stat/",
+			expectedStatusCode: http.StatusOK,
+			expectedPath:       "/",
+			expectedHeader:     "/stat/",
+		},
 		{
 			desc: "path prefix on exactly matching path",
 			config: dynamic.StripPrefix{
@@ -58,7 +91,7 @@ func TestStripPrefix(t *testing.T) {
 			},
 			path:               "/stat/",
 			expectedStatusCode: http.StatusOK,
-			expectedPath:       "/",
+			expectedPath:       "",
 			expectedHeader:     "/stat/",
 		},
 		{
@@ -101,6 +134,17 @@ func TestStripPrefix(t *testing.T) {
 			expectedPath:       "/us",
 			expectedHeader:     "/stat",
 		},
+		{
+			desc: "later prefix matching (ForceSlash)",
+			config: dynamic.StripPrefix{
+				Prefixes:   []string{"/mismatch", "/stat"},
+				ForceSlash: true,
+			},
+			path:               "/stat",
+			expectedStatusCode: http.StatusOK,
+			expectedPath:       "/",
+			expectedHeader:     "/stat",
+		},
 		{
 			desc: "later prefix matching",
 			config: dynamic.StripPrefix{
@@ -108,7 +152,7 @@ func TestStripPrefix(t *testing.T) {
 			},
 			path:               "/stat",
 			expectedStatusCode: http.StatusOK,
-			expectedPath:       "/",
+			expectedPath:       "",
 			expectedHeader:     "/stat",
 		},
 		{
@@ -162,12 +206,15 @@ func TestStripPrefix(t *testing.T) {
 			assert.Equal(t, test.expectedRawPath, actualRawPath, "Unexpected raw path.")
 			assert.Equal(t, test.expectedHeader, actualHeader, "Unexpected '%s' header.", ForwardedPrefixHeader)
 
-			expectedURI := test.expectedPath
+			expectedRequestURI := test.expectedPath
 			if test.expectedRawPath != "" {
 				// go HTTP uses the raw path when existent in the RequestURI
-				expectedURI = test.expectedRawPath
+				expectedRequestURI = test.expectedRawPath
 			}
-			assert.Equal(t, expectedURI, requestURI, "Unexpected request URI.")
+			if test.expectedPath == "" {
+				expectedRequestURI = "/"
+			}
+			assert.Equal(t, expectedRequestURI, requestURI, "Unexpected request URI.")
 		})
 	}
 }