server: set latestResponse before sending response (#71)

By setting the latest response prior to sending the response we
guarantee that the map is updated by the time the corresponding request
arrives. This ensures that the nonce check correctly uses the most
recent response.

Signed-off-by: Snow Pettersen <[email protected]>

Author: snowp

server/src/main/java/io/envoyproxy/controlplane/server/DiscoveryServer.java

@@ -185,7 +185,7 @@ public void onNext(DiscoveryRequest request) {
                   ads,
                   request,
                   ackedResources.getOrDefault(typeUrl, Collections.emptySet()),
-                  r -> latestResponse.put(typeUrl, send(r, typeUrl)));
+                  r -> send(r, typeUrl));
             });
 
             return;
@@ -223,7 +223,7 @@ private void cancel() {
         watches.values().forEach(Watch::cancel);
       }
 
-      private DiscoveryResponse send(Response response, String typeUrl) {
+      private void send(Response response, String typeUrl) {
         String nonce = Long.toString(streamNonce.getAndIncrement());
 
         DiscoveryResponse discoveryResponse = DiscoveryResponse.newBuilder()
@@ -237,9 +237,11 @@ private DiscoveryResponse send(Response response, String typeUrl) {
 
         callbacks.onStreamResponse(streamId, response.request(), discoveryResponse);
 
+        // Store the latest response *before* we send the response. This ensures that by the time the request
+        // is processed the map is guaranteed to be updated. Doing it afterwards leads to a race conditions
+        // which may see the incoming request arrive before the map is updated, failing the nonce check erroneously.
+        latestResponse.put(typeUrl, discoveryResponse);
         responseObserver.onNext(discoveryResponse);
-
-        return discoveryResponse;
       }
     };
   }