Fix javadoc to reflect current behavior of getSubscribedResources. …

…Use existing function rather than calculating active authorities and make that function return a set when there are large numbers of authorities.

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

@@ -413,8 +413,9 @@ void handleResourceResponse(
   public interface ResourceStore {
 
     /**
-     * Returns the collection of resources currently subscribing to with the specified authority
-     * or {@code null} if not subscribing to any resources for this authority for the given type.
+     * Returns the collection of resources currently subscribed to which have an authority matching
+     * one of those for which the ControlPlaneClient associated with the specified ServerInfo is
+     * the active one, or {@code null} if no such resources are currently subscribed to.
      *
      * <p>Note an empty collection indicates subscribing to resources of the given type with
      * wildcard mode.

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

@@ -185,10 +185,7 @@ public Collection<String> getSubscribedResources(
       return null;
     }
 
-    Set<String> authorities = activatedCpClients.entrySet().stream()
-        .filter(entry -> entry.getValue().contains(targetCpc))
-        .map(Map.Entry::getKey)
-        .collect(Collectors.toSet());
+    Collection<String> authorities = getActiveAuthorities(targetCpc);
 
     Map<String, ResourceSubscriber<? extends ResourceUpdate>> resources =
         resourceSubscribers.getOrDefault(type, Collections.emptyMap());
@@ -432,7 +429,7 @@ private Set<String> getResourceKeys(XdsResourceType<?> xdsResourceType) {
 
   // cpcForThisStream is null when doing shutdown
   private void cleanUpResourceTimers(ControlPlaneClient cpcForThisStream) {
-    List<String> authoritiesForCpc = getActiveAuthorities(cpcForThisStream);
+    Collection<String> authoritiesForCpc = getActiveAuthorities(cpcForThisStream);
 
     for (Map<String, ResourceSubscriber<?>> subscriberMap : resourceSubscribers.values()) {
       for (ResourceSubscriber<?> subscriber : subscriberMap.values()) {
@@ -964,7 +961,7 @@ public void handleStreamClosed(Status status, boolean shouldTryFallback) {
 
       Status error = status.isOk() ? Status.UNAVAILABLE.withDescription(CLOSED_BY_SERVER) : status;
 
-      List<String> authoritiesForClosedCpc = getActiveAuthorities(cpcClosed);
+      Collection<String> authoritiesForClosedCpc = getActiveAuthorities(cpcClosed);
       for (Map<String, ResourceSubscriber<? extends ResourceUpdate>> subscriberMap :
           resourceSubscribers.values()) {
         for (ResourceSubscriber<? extends ResourceUpdate> subscriber : subscriberMap.values()) {
@@ -1035,12 +1032,15 @@ private CpcWithFallbackState(ControlPlaneClient cpc, boolean didFallback) {
     }
   }
 
-  private List<String> getActiveAuthorities(ControlPlaneClient cpc) {
-    return activatedCpClients.entrySet().stream()
+  private Collection<String> getActiveAuthorities(ControlPlaneClient cpc) {
+    List<String> asList = activatedCpClients.entrySet().stream()
         .filter(entry -> !entry.getValue().isEmpty()
             && cpc == entry.getValue().get(entry.getValue().size() - 1))
         .map(Map.Entry::getKey)
         .collect(Collectors.toList());
+
+    // Since this is usually used for contains, use a set when the list is large
+    return (asList.size() < 100) ? asList : new HashSet<>(asList);
   }
 
 }