Ensure remote write v2 headers cannot be returned on v1 requests

exp/api/remote/remote_api.go

@@ -513,8 +513,8 @@ func SnappyDecodeMiddleware(logger *slog.Logger) func(http.Handler) http.Handler
 	}
 }
 
-// NewWriteHandler returns HTTP handler that receives Remote Write 2.0
-// protocol https://prometheus.io/docs/specs/remote_write_spec_2_0/.
+// NewWriteHandler returns an HTTP handler that can receive the Remote Write 1.0 or Remote Write 2.0
+// (https://prometheus.io/docs/specs/remote_write_spec_2_0/) protocol.
 func NewWriteHandler(store writeStorage, acceptedMessageTypes MessageTypes, opts ...WriteHandlerOption) http.Handler {
 	o := writeHandlerOpts{
 		logger:      slog.New(nopSlogHandler{}),
@@ -603,8 +603,8 @@ func (h *writeHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		writeResponse = NewWriteResponse()
 	}
 
-	// Set required X-Prometheus-Remote-Write-Written-* response headers, in all cases, along with any user-defined headers.
-	writeResponse.writeHeaders(w)
+	// Set any necessary response headers.
+	writeResponse.writeHeaders(msgType, w)
 
 	if storeErr != nil {
 		if writeResponse.statusCode == 0 {

exp/api/remote/remote_api_test.go

@@ -65,6 +65,44 @@ func TestRetryAfterDuration(t *testing.T) {
 	}
 }
 
+type defaultResponseStore struct{}
+
+func (m *defaultResponseStore) Store(*http.Request, WriteMessageType) (*WriteResponse, error) {
+	return NewWriteResponse(), nil
+}
+
+func Test_WriteHandler_V1HandlingDoesNotAddV2Headers(t *testing.T) {
+	tLogger := slog.Default()
+
+	h := NewWriteHandler(&defaultResponseStore{}, MessageTypes{WriteV2MessageType, WriteV1MessageType}, WithWriteHandlerLogger(tLogger))
+
+	body := "test"
+	bodyBytes := snappy.Encode(nil, []byte(body))
+	req := httptest.NewRequest(http.MethodPost, "/", bytes.NewReader(bodyBytes))
+	req.Header.Set("Content-Type", "application/x-protobuf")
+	rec := httptest.NewRecorder()
+
+	h.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusNoContent {
+		t.Fatalf("expected status code 204, got %d with body %s", rec.Code, rec.Body.String())
+	}
+
+	samplesHeader := rec.Header().Get(writtenSamplesHeader)
+	exemplarsHeader := rec.Header().Get(writtenExemplarsHeader)
+	histogramHeader := rec.Header().Get(writtenHistogramsHeader)
+
+	if samplesHeader != "" {
+		t.Fatal("expected no written samples header, got", samplesHeader)
+	}
+	if exemplarsHeader != "" {
+		t.Fatal("expected no written exemplars header, got", exemplarsHeader)
+	}
+	if histogramHeader != "" {
+		t.Fatal("expected no written histograms header, got", histogramHeader)
+	}
+}
+
 type mockStorage struct {
 	v2Reqs []*writev2.Request
 	protos []WriteMessageType

exp/api/remote/remote_headers.go

@@ -132,11 +132,18 @@ func (w *WriteResponse) SetExtraHeader(key, value string) {
 
 // writeHeaders sets response headers in a given response writer.
 // Make sure to use it before http.ResponseWriter.WriteHeader and .Write.
-func (w *WriteResponse) writeHeaders(rw http.ResponseWriter) {
+func (w *WriteResponse) writeHeaders(msgType WriteMessageType, rw http.ResponseWriter) {
 	h := rw.Header()
-	h.Set(writtenSamplesHeader, strconv.Itoa(w.Samples))
-	h.Set(writtenHistogramsHeader, strconv.Itoa(w.Histograms))
-	h.Set(writtenExemplarsHeader, strconv.Itoa(w.Exemplars))
+
+	// TODO make it easier to indicate if the stats are valid before adding the headers. WriteResponseStats.confirmed
+	//  could be used if there was a reliable way for it to be set without parsing headers. For now ensure we don't
+	//  add stats headers for v1 messages which can cause confusion/false positive errors logs.
+	if msgType != WriteV1MessageType {
+		h.Set(writtenSamplesHeader, strconv.Itoa(w.Samples))
+		h.Set(writtenHistogramsHeader, strconv.Itoa(w.Histograms))
+		h.Set(writtenExemplarsHeader, strconv.Itoa(w.Exemplars))
+	}
+
 	for k, v := range w.extraHeaders {
 		for _, vv := range v {
 			h.Add(k, vv)

exp/api/remote/remote_headers_test.go

@@ -71,7 +71,25 @@ func TestWriteResponse(t *testing.T) {
 		}
 	})
 
-	t.Run("writeHeaders", func(t *testing.T) {
+	t.Run("writeHeaders v1", func(t *testing.T) {
+		resp := NewWriteResponse()
+		resp.SetExtraHeader("Custom-Header", "custom-value")
+
+		w := httptest.NewRecorder()
+		resp.writeHeaders(WriteV1MessageType, w)
+
+		expectedHeaders := map[string]string{
+			"Custom-Header": "custom-value",
+		}
+
+		for k, want := range expectedHeaders {
+			if got := w.Header().Get(k); got != want {
+				t.Errorf("header %q: want %q, got %q", k, want, got)
+			}
+		}
+	})
+
+	t.Run("writeHeaders v2", func(t *testing.T) {
 		resp := NewWriteResponse()
 		resp.Add(WriteResponseStats{
 			Samples:    10,
@@ -82,7 +100,7 @@ func TestWriteResponse(t *testing.T) {
 		resp.SetExtraHeader("Custom-Header", "custom-value")
 
 		w := httptest.NewRecorder()
-		resp.writeHeaders(w)
+		resp.writeHeaders(WriteV2MessageType, w)
 
 		expectedHeaders := map[string]string{
 			"Custom-Header": "custom-value",