fix(redis): only remove a key if it wouldn't be overwritten (#529)

Fixes a potential race condition persisting user permissions where previously we would
remove the entire set of permissions for a resource, then write the current set. This
changes to only remove the permissions if the updated UserPermission is empty for that
resource type, and to write a new value and swap over the old value if there are
Permissions so that an API request to load that UserPermission that overlaps with
a refresh of the UserPermission in redis doesn't get less data than it should

fiat-roles/src/main/java/com/netflix/spinnaker/fiat/permissions/RedisPermissionsRepository.java

@@ -130,12 +130,13 @@ public RedisPermissionsRepository put(@NonNull UserPermission permission) {
 
             for (ResourceType r : ResourceType.values()) {
               String userResourceKey = userKey(userId, r);
-
-              pipeline.del(userResourceKey);
-
               Map<String, String> redisValue = resourceTypeToRedisValue.get(r);
+              String tempKey = UUID.randomUUID().toString();
               if (redisValue != null && !redisValue.isEmpty()) {
-                pipeline.hmset(userResourceKey, redisValue);
+                pipeline.hmset(tempKey, redisValue);
+                pipeline.rename(tempKey, userResourceKey);
+              } else {
+                pipeline.del(userResourceKey);
               }
             }
             pipeline.sync();