Add SqlEntity and DatastoreEntity interfaces (#562)

* Add SqlEntity and DatastoreEntity interfaces

These will be used when replaying transactions from either the Datastore
commit logs or the SQL Transaction objects.

When Datastore is the primary database, we will read in the
Datastore commit logs, convert each saved entity to however many SQL
entities, then save those SQL entities in SQL.

When SQL is the primary database, we will read in the SQL objects from a
yet-to-be-created SQL table, convert them to however many Datastore
entities, then save those Datastore entities in Datastore.

This PR includes a couple simple examples of how this will work for entities that are
saveable in both SQL and Datastore (the simple case).

* Add 1-1 mapping between entity annotations and interfaces

core/src/test/java/google/registry/schema/replay/EntityTest.java

@@ -0,0 +1,65 @@
+// Copyright 2020 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 static com.google.common.collect.ImmutableSet.toImmutableSet;
+import static com.google.common.truth.Truth.assertThat;
+
+import com.google.common.collect.ImmutableSet;
+import io.github.classgraph.ClassGraph;
+import io.github.classgraph.ClassInfo;
+import io.github.classgraph.ClassInfoList;
+import io.github.classgraph.ScanResult;
+import org.junit.Ignore;
+import org.junit.Test;
+
+/**
+ * Test to verify classes implement {@link SqlEntity} and {@link DatastoreEntity} when they should.
+ */
+public class EntityTest {
+
+  @Test
+  @Ignore("This won't be done until b/152410794 is done, since it requires many entity changes")
+  public void testSqlEntityPersistence() {
+    try (ScanResult scanResult =
+        new ClassGraph().enableAnnotationInfo().whitelistPackages("google.registry").scan()) {
+      // All javax.persistence entities must implement SqlEntity and vice versa
+      ImmutableSet<String> javaxPersistenceClasses =
+          getClassNames(
+              scanResult.getClassesWithAnnotation(javax.persistence.Entity.class.getName()));
+      ImmutableSet<String> sqlEntityClasses =
+          getClassNames(scanResult.getClassesImplementing(SqlEntity.class.getName()));
+      assertThat(javaxPersistenceClasses).isEqualTo(sqlEntityClasses);
+
+      // All com.googlecode.objectify.annotation.Entity classes must implement DatastoreEntity and
+      // vice versa
+      ImmutableSet<String> objectifyClasses =
+          getClassNames(
+              scanResult.getClassesWithAnnotation(
+                  com.googlecode.objectify.annotation.Entity.class.getName()));
+      ImmutableSet<String> datastoreEntityClasses =
+          getClassNames(scanResult.getClassesImplementing(DatastoreEntity.class.getName()));
+      assertThat(objectifyClasses).isEqualTo(datastoreEntityClasses);
+    }
+  }
+
+  private ImmutableSet<String> getClassNames(ClassInfoList classInfoList) {
+    return classInfoList.stream()
+        .filter(ClassInfo::isStandardClass)
+        .map(ClassInfo::loadClass)
+        .map(Class::getName)
+        .collect(toImmutableSet());
+  }
+}