Add a "ReplaySpecializer" to fix certain replays (#989)

* Add a "ReplaySpecializer" to fix certain replays

Due to the fact that a given entity in either database type can map to
multiple entities in the other database, there are certain replication
scenarios that don't quite work.  Current known examples include:

- propagation of cascading deletes from datastore to SQL
- creation of datastore indexed entities for SQL entities (where indexes are a
  first-class concept)

This change introduces a ReplaySpecializer class, which allows us to declare
static method hooks at the entity class level that define any special
operations that need to be performed before or after replaying a mutation for
any given entity type.

Currently, "before SQL delete" is the only supported hook.  A change to
DomainContent demonstrating how this facility can be used to fix problems in
cascading delete propagation will be sent as a subsequent PR.

* Throw exception on beforeSqlDelete failures

* Changes for review

core/src/main/java/google/registry/backup/ReplayCommitLogsToSqlAction.java

@@ -37,6 +37,7 @@ import google.registry.request.auth.Auth;
 import google.registry.schema.replay.DatastoreEntity;
 import google.registry.schema.replay.DatastoreOnlyEntity;
 import google.registry.schema.replay.NonReplicatedEntity;
+import google.registry.schema.replay.ReplaySpecializer;
 import google.registry.schema.replay.SqlReplayCheckpoint;
 import google.registry.util.RequestStatusChecker;
 import java.io.IOException;
@@ -179,6 +180,7 @@ public class ReplayCommitLogsToSqlAction implements Runnable {
     if (!NonReplicatedEntity.class.isAssignableFrom(entityClass)
         && !DatastoreOnlyEntity.class.isAssignableFrom(entityClass)
         && entityClass.getAnnotation(javax.persistence.Entity.class) != null) {
+      ReplaySpecializer.beforeSqlDelete(entityVKey);
       jpaTm().delete(entityVKey);
     }
   }

core/src/main/java/google/registry/model/ofy/ReplayQueue.java

@@ -24,6 +24,7 @@ import google.registry.config.RegistryEnvironment;
 import google.registry.model.UpdateAutoTimestamp;
 import google.registry.persistence.VKey;
 import google.registry.schema.replay.DatastoreEntity;
+import google.registry.schema.replay.ReplaySpecializer;
 import java.util.HashMap;
 import java.util.Map;
 import java.util.concurrent.ConcurrentLinkedQueue;
@@ -100,7 +101,9 @@ public class ReplayQueue {
                     .forEach(
                         entry -> {
                           if (entry.getValue().equals(TransactionInfo.Delete.SENTINEL)) {
-                            jpaTm().delete(VKey.from(entry.getKey()));
+                            VKey<?> vkey = VKey.from(entry.getKey());
+                            ReplaySpecializer.beforeSqlDelete(vkey);
+                            jpaTm().delete(vkey);
                           } else {
                             ((DatastoreEntity) entry.getValue())
                                 .toSqlEntity()

core/src/main/java/google/registry/schema/replay/ReplaySpecializer.java

@@ -0,0 +1,50 @@
+// Copyright 2021 The Nomulus Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package google.registry.schema.replay;
+
+import com.google.common.flogger.FluentLogger;
+import google.registry.persistence.VKey;
+import java.lang.reflect.InvocationTargetException;
+import java.lang.reflect.Method;
+
+/**
+ * Applies class-specific functions for model objects during transaction replays.
+ *
+ * <p>There are certain cases where changes to an entity require changes to other entities that are
+ * not directly present in the other database. This class allows us to do that by using reflection
+ * to invoke special class methods if they are present.
+ */
+public class ReplaySpecializer {
+  private static final FluentLogger logger = FluentLogger.forEnclosingClass();
+
+  public static void beforeSqlDelete(VKey<?> key) {
+    try {
+      Method method = key.getKind().getMethod("beforeSqlDelete", VKey.class);
+      method.invoke(null, new Object[] {key});
+    } catch (NoSuchMethodException e) {
+      // Ignore, this just means that the class doesn't need this hook.
+    } catch (IllegalAccessException e) {
+      throw new RuntimeException(
+          "beforeSqlDelete() method is defined for class "
+              + key.getKind().getName()
+              + " but is not public.",
+          e);
+    } catch (InvocationTargetException e) {
+      throw new RuntimeException(
+          "beforeSqlDelete() method for class " + key.getKind().getName() + " threw an exception.",
+          e);
+    }
+  }
+}