exp/api: Add accepted msg type validation to handler

exp/api/remote/remote_api.go

@@ -377,8 +377,9 @@ type writeStorage interface {
 }
 
 type handler struct {
-	store writeStorage
-	opts  handlerOpts
+	store                writeStorage
+	acceptedMessageTypes MessageTypes
+	opts                 handlerOpts
 }
 
 type handlerOpts struct {
@@ -455,15 +456,20 @@ func SnappyDecompressorMiddleware(logger *slog.Logger) func(http.Handler) http.H
 
 // NewHandler returns HTTP handler that receives Remote Write 2.0
 // protocol https://prometheus.io/docs/specs/remote_write_spec_2_0/.
-func NewHandler(store writeStorage, opts ...HandlerOption) http.Handler {
+func NewHandler(store writeStorage, acceptedMessageTypes MessageTypes, opts ...HandlerOption) http.Handler {
 	o := handlerOpts{
 		logger:      slog.New(nopSlogHandler{}),
 		middlewares: []func(http.Handler) http.Handler{SnappyDecompressorMiddleware(slog.New(nopSlogHandler{}))},
 	}
 	for _, opt := range opts {
 		opt(&o)
 	}
-	h := &handler{opts: o, store: store}
+
+	h := &handler{
+		opts:                 o,
+		store:                store,
+		acceptedMessageTypes: acceptedMessageTypes,
+	}
 
 	// Apply all middlewares in order
 	var handler http.Handler = h
@@ -524,6 +530,13 @@ func (h *handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	if !h.acceptedMessageTypes.Contains(msgType) {
+		err := fmt.Errorf("%v protobuf message is not accepted by this server; only accepts %v", msgType, h.acceptedMessageTypes.String())
+		h.opts.logger.Error("Unaccepted message type", "msgType", msgType, "err", err)
+		http.Error(w, err.Error(), http.StatusUnsupportedMediaType)
+		return
+	}
+
 	writeResponse, storeErr := h.store.Store(r.Context(), msgType, r)
 
 	// Set required X-Prometheus-Remote-Write-Written-* response headers, in all cases, alongwith any user-defined headers.

exp/api/remote/remote_api_test.go

@@ -147,7 +147,7 @@ func TestRemoteAPI_Write_WithHandler(t *testing.T) {
 	t.Run("success", func(t *testing.T) {
 		tLogger := slog.Default()
 		mStore := &mockStorage{}
-		srv := httptest.NewServer(NewHandler(mStore, WithHandlerLogger(tLogger)))
+		srv := httptest.NewServer(NewHandler(mStore, MessageTypes{WriteV2MessageType}, WithHandlerLogger(tLogger)))
 		t.Cleanup(srv.Close)
 
 		client, err := NewAPI(srv.URL,
@@ -182,7 +182,7 @@ func TestRemoteAPI_Write_WithHandler(t *testing.T) {
 			mockErr:  errors.New("storage error"),
 			mockCode: &mockCode,
 		}
-		srv := httptest.NewServer(NewHandler(mStore, WithHandlerLogger(tLogger)))
+		srv := httptest.NewServer(NewHandler(mStore, MessageTypes{WriteV2MessageType}, WithHandlerLogger(tLogger)))
 		t.Cleanup(srv.Close)
 
 		client, err := NewAPI(srv.URL,

exp/api/remote/remote_headers.go

@@ -17,6 +17,7 @@ import (
 	"errors"
 	"fmt"
 	"net/http"
+	"slices"
 	"strconv"
 	"strings"
 )
@@ -59,24 +60,28 @@ func (n WriteMessageType) Validate() error {
 	case WriteV1MessageType, WriteV2MessageType:
 		return nil
 	default:
-		return fmt.Errorf("unknown type for remote write protobuf message %v, supported: %v", n, messageTypes{WriteV1MessageType, WriteV2MessageType}.String())
+		return fmt.Errorf("unknown type for remote write protobuf message %v, supported: %v", n, MessageTypes{WriteV1MessageType, WriteV2MessageType}.String())
 	}
 }
 
-type messageTypes []WriteMessageType
+type MessageTypes []WriteMessageType
 
-func (m messageTypes) Strings() []string {
+func (m MessageTypes) Strings() []string {
 	ret := make([]string, 0, len(m))
 	for _, typ := range m {
 		ret = append(ret, string(typ))
 	}
 	return ret
 }
 
-func (m messageTypes) String() string {
+func (m MessageTypes) String() string {
 	return strings.Join(m.Strings(), ", ")
 }
 
+func (m MessageTypes) Contains(mType WriteMessageType) bool {
+	return slices.Contains(m, mType)
+}
+
 var contentTypeHeaders = map[WriteMessageType]string{
 	WriteV1MessageType: appProtoContentType, // Also application/x-protobuf;proto=prometheus.WriteRequest but simplified for compatibility with 1.x spec.
 	WriteV2MessageType: appProtoContentType + ";proto=io.prometheus.write.v2.Request",