xds: Preserve nonce when unsubscribing type

This fixes a regression introduced in 19c9b99.

b/374697875

Author: ejona86

xds/src/main/java/io/grpc/xds/client/ControlPlaneClient.java

@@ -155,9 +155,10 @@ void adjustResourceSubscription(XdsResourceType<?> resourceType) {
     }
     adsStream.sendDiscoveryRequest(resourceType, resources);
     if (resources.isEmpty()) {
-      // The resource type no longer has subscribing resources; clean up references to it
+      // The resource type no longer has subscribing resources; clean up references to it, except
+      // for nonces. If the resource type becomes used again the control plane can ignore requests
+      // for old/missing nonces. Old type's nonces are dropped when the ADS stream is restarted.
       versions.remove(resourceType);
-      adsStream.respNonces.remove(resourceType);
     }
   }
 
@@ -270,7 +271,10 @@ private class AdsStream implements EventHandler<DiscoveryResponse> {
     // Nonce in each response is echoed back in the following ACK/NACK request. It is
     // used for management server to identify which response the client is ACKing/NACking.
     // To avoid confusion, client-initiated requests will always use the nonce in
-    // most recently received responses of each resource type.
+    // most recently received responses of each resource type. Nonces are never deleted from the
+    // map; nonces are only discarded once the stream closes because xds_protocol says "the
+    // management server should not send a DiscoveryResponse for any DiscoveryRequest that has a
+    // stale nonce."
     private final Map<XdsResourceType<?>, String> respNonces = new HashMap<>();
     private final StreamingCall<DiscoveryRequest, DiscoveryResponse> call;
     private final MethodDescriptor<DiscoveryRequest, DiscoveryResponse> methodDescriptor =

xds/src/test/java/io/grpc/xds/GrpcXdsClientImplTestBase.java

@@ -2782,10 +2782,9 @@ public void edsCleanupNonceAfterUnsubscription() {
     verifySubscribedResourcesMetadataSizes(0, 0, 0, 0);
     call.verifyRequest(EDS, Arrays.asList(), VERSION_1, "0000", NODE);
 
-    // When re-subscribing, the version and nonce were properly forgotten, so the request is the
-    // same as the initial request
+    // When re-subscribing, the version was forgotten but not the nonce
     xdsClient.watchXdsResource(XdsEndpointResource.getInstance(), "A.1", edsResourceWatcher);
-    call.verifyRequest(EDS, "A.1", "", "", NODE, Mockito.timeout(2000).times(2));
+    call.verifyRequest(EDS, "A.1", "", "0000", NODE, Mockito.timeout(2000));
   }
 
   @Test