Maintain only one row in the TmchCrl table at all times (#869)

* Maintain only one row in the TmchCrl table at all times

core/src/main/java/google/registry/model/tmch/TmchCrl.java

@@ -77,7 +77,16 @@ public final class TmchCrl extends CrossTldSingleton implements DatastoreEntity,
               tmchCrl.crl = checkNotNull(crl, "crl");
               tmchCrl.url = checkNotNull(url, "url");
               ofyTm().transactNew(() -> ofyTm().putWithoutBackup(tmchCrl));
-              jpaTm().transactNew(() -> jpaTm().putWithoutBackup(tmchCrl));
+              jpaTm()
+                  .transactNew(
+                      () -> {
+                        // Delete the old one and insert the new one
+                        jpaTm()
+                            .getEntityManager()
+                            .createQuery("DELETE FROM TmchCrl")
+                            .executeUpdate();
+                        jpaTm().putWithoutBackup(tmchCrl);
+                      });
             });
   }
 

core/src/test/java/google/registry/model/tmch/TmchCrlTest.java

@@ -47,6 +47,23 @@ public class TmchCrlTest extends EntityTestCase {
     assertThat(loadFromSql()).isEqualTo(expected);
   }
 
+  @Test
+  void testMultipleWrites() {
+    TmchCrl.set("first", "https://first.cat");
+    assertThat(TmchCrl.get().get().getCrl()).isEqualTo("first");
+    TmchCrl.set("second", "https://second.cat");
+    assertThat(TmchCrl.get().get().getCrl()).isEqualTo("second");
+    jpaTm()
+        .transact(
+            () ->
+                assertThat(
+                        jpaTm()
+                            .getEntityManager()
+                            .createQuery("SELECT COUNT(*) FROM TmchCrl", Long.class)
+                            .getSingleResult())
+                    .isEqualTo(1L));
+  }
+
   private static TmchCrl loadFromSql() {
     return jpaTm()
         .transact(