Retry middleware : store headers per attempts and propagate them when responding.

middlewares/retry/retry.go

@@ -73,8 +73,7 @@ func (r *retry) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 
 	attempts := 1
 	for {
-		attemptsExhausted := attempts >= r.attempts
+		shouldRetry := attempts < r.attempts
-		shouldRetry := !attemptsExhausted
 		retryResponseWriter := newResponseWriter(rw, shouldRetry)
 
 		// Disable retries when the backend already received request data
@@ -118,6 +117,7 @@ type responseWriter interface {
 func newResponseWriter(rw http.ResponseWriter, shouldRetry bool) responseWriter {
 	responseWriter := &responseWriterWithoutCloseNotify{
 		responseWriter: rw,
+		headers:        make(http.Header),
 		shouldRetry:    shouldRetry,
 	}
 	if _, ok := rw.(http.CloseNotifier); ok {
@@ -130,6 +130,7 @@ func newResponseWriter(rw http.ResponseWriter, shouldRetry bool) responseWriter
 
 type responseWriterWithoutCloseNotify struct {
 	responseWriter http.ResponseWriter
+	headers        http.Header
 	shouldRetry    bool
 }
 
@@ -142,10 +143,7 @@ func (r *responseWriterWithoutCloseNotify) DisableRetries() {
 }
 
 func (r *responseWriterWithoutCloseNotify) Header() http.Header {
-	if r.ShouldRetry() {
+	return r.headers
-		return make(http.Header)
-	}
-	return r.responseWriter.Header()
 }
 
 func (r *responseWriterWithoutCloseNotify) Write(buf []byte) (int, error) {
@@ -168,6 +166,16 @@ func (r *responseWriterWithoutCloseNotify) WriteHeader(code int) {
 	if r.ShouldRetry() {
 		return
 	}
+
+	// In that case retry case is set to false which means we at least managed
+	// to write headers to the backend : we are not going to perform any further retry.
+	// So it is now safe to alter current response headers with headers collected during
+	// the latest try before writing headers to client.
+	headers := r.responseWriter.Header()
+	for header, value := range r.headers {
+		headers[header] = value
+	}
+
 	r.responseWriter.WriteHeader(code)
 }
 

middlewares/retry/retry_test.go

@@ -2,8 +2,10 @@ package retry
 
 import (
 	"context"
+	"fmt"
 	"net/http"
 	"net/http/httptest"
+	"net/http/httptrace"
 	"strings"
 	"testing"
 
@@ -149,6 +151,50 @@ func TestRetryListeners(t *testing.T) {
 	}
 }
 
+func TestMultipleRetriesShouldNotLooseHeaders(t *testing.T) {
+	attempt := 0
+	expectedHeaderName := "X-Foo-Test-2"
+	expectedHeaderValue := "bar"
+
+	next := http.HandlerFunc(func(rw http.ResponseWriter, req *http.Request) {
+		headerName := fmt.Sprintf("X-Foo-Test-%d", attempt)
+		rw.Header().Add(headerName, expectedHeaderValue)
+		if attempt < 2 {
+			attempt++
+			return
+		}
+
+		// Request has been successfully written to backend
+		trace := httptrace.ContextClientTrace(req.Context())
+		trace.WroteHeaders()
+
+		// And we decide to answer to client
+		rw.WriteHeader(http.StatusNoContent)
+	})
+
+	retry, err := New(context.Background(), next, config.Retry{Attempts: 3}, &countingRetryListener{}, "traefikTest")
+	require.NoError(t, err)
+
+	responseRecorder := httptest.NewRecorder()
+	retry.ServeHTTP(responseRecorder, testhelpers.MustNewRequest(http.MethodGet, "http://test", http.NoBody))
+
+	headerValue := responseRecorder.Header().Get(expectedHeaderName)
+
+	// Validate if we have the correct header
+	if headerValue != expectedHeaderValue {
+		t.Errorf("Expected to have %s for header %s, got %s", expectedHeaderValue, expectedHeaderName, headerValue)
+	}
+
+	// Validate that we don't have headers from previous attempts
+	for i := 0; i < attempt; i++ {
+		headerName := fmt.Sprintf("X-Foo-Test-%d", i)
+		headerValue = responseRecorder.Header().Get("headerName")
+		if headerValue != "" {
+			t.Errorf("Expected no value for header %s, got %s", headerName, headerValue)
+		}
+	}
+}
+
 // countingRetryListener is a Listener implementation to count the times the Retried fn is called.
 type countingRetryListener struct {
 	timesCalled int

old/middlewares/retry.go

@@ -44,9 +44,7 @@ func (retry *Retry) ServeHTTP(rw http.ResponseWriter, r *http.Request) {
 
 	attempts := 1
 	for {
-		attemptsExhausted := attempts >= retry.attempts
+		shouldRetry := attempts < retry.attempts
-
-		shouldRetry := !attemptsExhausted
 		retryResponseWriter := newRetryResponseWriter(rw, shouldRetry)
 
 		// Disable retries when the backend already received request data
@@ -99,6 +97,7 @@ type retryResponseWriter interface {
 func newRetryResponseWriter(rw http.ResponseWriter, shouldRetry bool) retryResponseWriter {
 	responseWriter := &retryResponseWriterWithoutCloseNotify{
 		responseWriter: rw,
+		headers:        make(http.Header),
 		shouldRetry:    shouldRetry,
 	}
 	if _, ok := rw.(http.CloseNotifier); ok {
@@ -109,6 +108,7 @@ func newRetryResponseWriter(rw http.ResponseWriter, shouldRetry bool) retryRespo
 
 type retryResponseWriterWithoutCloseNotify struct {
 	responseWriter http.ResponseWriter
+	headers        http.Header
 	shouldRetry    bool
 }
 
@@ -121,10 +121,7 @@ func (rr *retryResponseWriterWithoutCloseNotify) DisableRetries() {
 }
 
 func (rr *retryResponseWriterWithoutCloseNotify) Header() http.Header {
-	if rr.ShouldRetry() {
+	return rr.headers
-		return make(http.Header)
-	}
-	return rr.responseWriter.Header()
 }
 
 func (rr *retryResponseWriterWithoutCloseNotify) Write(buf []byte) (int, error) {
@@ -147,6 +144,16 @@ func (rr *retryResponseWriterWithoutCloseNotify) WriteHeader(code int) {
 	if rr.ShouldRetry() {
 		return
 	}
+
+	// In that case retry case is set to false which means we at least managed
+	// to write headers to the backend : we are not going to perform any further retry.
+	// So it is now safe to alter current response headers with headers collected during
+	// the latest try before writing headers to client.
+	headers := rr.responseWriter.Header()
+	for header, value := range rr.headers {
+		headers[header] = value
+	}
+
 	rr.responseWriter.WriteHeader(code)
 }
 

old/middlewares/retry_test.go

@@ -1,8 +1,10 @@
 package middlewares
 
 import (
+	"fmt"
 	"net/http"
 	"net/http/httptest"
+	"net/http/httptrace"
 	"strings"
 	"testing"
 
@@ -258,3 +260,45 @@ func TestRetryWithFlush(t *testing.T) {
 		t.Errorf("Wrong body %q want %q", responseRecorder.Body.String(), "FULL DATA")
 	}
 }
+
+func TestMultipleRetriesShouldNotLooseHeaders(t *testing.T) {
+	attempt := 0
+	expectedHeaderName := "X-Foo-Test-2"
+	expectedHeaderValue := "bar"
+
+	next := http.HandlerFunc(func(rw http.ResponseWriter, req *http.Request) {
+		headerName := fmt.Sprintf("X-Foo-Test-%d", attempt)
+		rw.Header().Add(headerName, expectedHeaderValue)
+		if attempt < 2 {
+			attempt++
+			return
+		}
+
+		// Request has been successfully written to backend
+		trace := httptrace.ContextClientTrace(req.Context())
+		trace.WroteHeaders()
+
+		// And we decide to answer to client
+		rw.WriteHeader(http.StatusNoContent)
+	})
+
+	retry := NewRetry(3, next, &countingRetryListener{})
+	responseRecorder := httptest.NewRecorder()
+	retry.ServeHTTP(responseRecorder, &http.Request{})
+
+	headerValue := responseRecorder.Header().Get(expectedHeaderName)
+
+	// Validate if we have the correct header
+	if headerValue != expectedHeaderValue {
+		t.Errorf("Expected to have %s for header %s, got %s", expectedHeaderValue, expectedHeaderName, headerValue)
+	}
+
+	// Validate that we don't have headers from previous attempts
+	for i := 0; i < attempt; i++ {
+		headerName := fmt.Sprintf("X-Foo-Test-%d", i)
+		headerValue = responseRecorder.Header().Get("headerName")
+		if headerValue != "" {
+			t.Errorf("Expected no value for header %s, got %s", headerName, headerValue)
+		}
+	}
+}