Add retry to claims list load

A NullPointerException reported via StackDriver appears to stem from trying to load the claims list right at the moment it was being updated. Since the update only happens once every 12 hours, retrying the load once should fix the problem, if this is really the cause.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=163732624

java/google/registry/model/tmch/ClaimsListShard.java

@@ -43,9 +43,12 @@ import google.registry.model.common.CrossTldSingleton;
 import google.registry.util.CollectionUtils;
 import google.registry.util.Concurrent;
 import google.registry.util.NonFinalForTesting;
+import google.registry.util.Retrier;
+import google.registry.util.SystemSleeper;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.concurrent.Callable;
 import javax.annotation.Nullable;
 import org.joda.time.DateTime;
 
@@ -90,14 +93,12 @@ public class ClaimsListShard extends ImmutableObject {
   @Ignore
   boolean isShard = false;
 
-  /**
-   * A cached supplier that fetches the claims list shards from Datastore and recombines them
-   * into a single {@link ClaimsListShard} object.
-   */
-  private static final Supplier<ClaimsListShard> CACHE =
-      memoizeWithShortExpiration(new Supplier<ClaimsListShard>() {
+  private static final Retrier LOADER_RETRIER = new Retrier(new SystemSleeper(), 2);
+
+  private static final Callable<ClaimsListShard> LOADER_CALLABLE =
+      new Callable<ClaimsListShard>() {
         @Override
-        public ClaimsListShard get() {
+        public ClaimsListShard call() throws Exception {
           // Find the most recent revision.
           Key<ClaimsListRevision> revisionKey = getCurrentRevision();
 
@@ -109,16 +110,26 @@ public class ClaimsListShard extends ImmutableObject {
                 ofy().load().type(ClaimsListShard.class).ancestor(revisionKey).keys().list();
 
             // Load all of the shards concurrently, each in a separate transaction.
-            List<ClaimsListShard> shards = Concurrent.transform(
-                shardKeys, new Function<Key<ClaimsListShard>, ClaimsListShard>() {
-                  @Override
-                  public ClaimsListShard apply(final Key<ClaimsListShard> key) {
-                    return ofy().transactNewReadOnly(new Work<ClaimsListShard>() {
+            List<ClaimsListShard> shards =
+                Concurrent.transform(
+                    shardKeys,
+                    new Function<Key<ClaimsListShard>, ClaimsListShard>() {
                       @Override
-                      public ClaimsListShard run() {
-                        return ofy().load().key(key).now();
-                      }});
-                  }});
+                      public ClaimsListShard apply(final Key<ClaimsListShard> key) {
+                        return ofy()
+                            .transactNewReadOnly(
+                                new Work<ClaimsListShard>() {
+                                  @Override
+                                  public ClaimsListShard run() {
+                                    ClaimsListShard claimsListShard = ofy().load().key(key).now();
+                                    checkState(
+                                        claimsListShard != null,
+                                        "Key not found when loading claims list shards.");
+                                    return claimsListShard;
+                                  }
+                                });
+                      }
+                    });
 
             // Combine the shards together and return the concatenated ClaimsList.
             if (!shards.isEmpty()) {
@@ -126,12 +137,27 @@ public class ClaimsListShard extends ImmutableObject {
               for (ClaimsListShard shard : shards) {
                 combinedLabelsToKeys.putAll(shard.labelsToKeys);
                 checkState(
-                    creationTime.equals(shard.creationTime), "Inconsistent creation times.");
+                    creationTime.equals(shard.creationTime),
+                    "Inconsistent claims list shard creation times.");
               }
             }
           }
           return create(creationTime, ImmutableMap.copyOf(combinedLabelsToKeys));
-        }});
+        }
+      };
+
+  /**
+   * A cached supplier that fetches the claims list shards from Datastore and recombines them into a
+   * single {@link ClaimsListShard} object.
+   */
+  private static final Supplier<ClaimsListShard> CACHE =
+      memoizeWithShortExpiration(
+          new Supplier<ClaimsListShard>() {
+            @Override
+            public ClaimsListShard get() {
+              return LOADER_RETRIER.callWithRetry(LOADER_CALLABLE, IllegalStateException.class);
+            }
+          });
 
   public DateTime getCreationTime() {
     return creationTime;