Adding synthetic source ID to BillingEvent.OneTime

Synthetic BillingEvent.OneTimes need to have a pointer to their source Recurring event, in order to match up properly against Cancellations.

Also improving the exception message for mismatched Flag.SYNTHETIC and syntheticCreationTime.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=122155636

java/google/registry/model/billing/BillingEvent.java

@@ -225,6 +225,13 @@ public abstract class BillingEvent extends ImmutableObject
     @Index
     DateTime syntheticCreationTime;
 
+    /**
+     * For {@link Flag#SYNTHETIC} events, the {@link BillingEvent} from which this OneTime was
+     * created. This is needed in order to properly match billing events against
+     * {@link Cancellation}s.
+     */
+    Long cancellationTargetId;
+
     public Money getCost() {
       return cost;
     }
@@ -241,6 +248,10 @@ public abstract class BillingEvent extends ImmutableObject
       return syntheticCreationTime;
     }
 
+    public Long getCancellationTargetId() {
+      return cancellationTargetId;
+    }
+
     @Override
     public Builder asBuilder() {
       return new Builder(clone(this));
@@ -277,6 +288,11 @@ public abstract class BillingEvent extends ImmutableObject
         return this;
       }
 
+      public Builder setCancellationTargetId(Long cancellationTargetId) {
+        getInstance().cancellationTargetId = cancellationTargetId;
+        return this;
+      }
+
       @Override
       public OneTime build() {
         OneTime instance = getInstance();
@@ -291,7 +307,11 @@ public abstract class BillingEvent extends ImmutableObject
         checkState(
             instance.getFlags().contains(Flag.SYNTHETIC)
                 == (instance.syntheticCreationTime != null),
-            "Billing events with SYNTHETIC flag set must have a synthetic creation time.");
+            "Synthetic creation time must be set if and only if the SYNTHETIC flag is set.");
+        checkState(
+            instance.getFlags().contains(Flag.SYNTHETIC)
+                == (instance.cancellationTargetId != null),
+            "Cancellation target ID must be set if and only if the SYNTHETIC flag is set.");
         return super.build();
       }
     }

javatests/google/registry/model/billing/BillingEventTest.java

@@ -91,6 +91,7 @@ public class BillingEventTest extends EntityTestCase {
             .setReason(Reason.CREATE)
             .setFlags(ImmutableSet.of(BillingEvent.Flag.ANCHOR_TENANT, BillingEvent.Flag.SYNTHETIC))
             .setSyntheticCreationTime(now.plusDays(10))
+            .setCancellationTargetId(1L)
             .setPeriodYears(2)
             .setCost(Money.of(USD, 1))
             .setEventTime(now)
@@ -179,7 +180,7 @@ public class BillingEventTest extends EntityTestCase {
   public void testFailure_syntheticFlagWithoutCreationTime() {
     thrown.expect(
         IllegalStateException.class,
-        "Billing events with SYNTHETIC flag set must have a synthetic creation time");
+        "Synthetic creation time must be set if and only if the SYNTHETIC flag is set.");
     oneTime.asBuilder()
         .setFlags(ImmutableSet.of(BillingEvent.Flag.SYNTHETIC))
         .build();
@@ -189,12 +190,32 @@ public class BillingEventTest extends EntityTestCase {
   public void testFailure_syntheticCreationTimeWithoutFlag() {
     thrown.expect(
         IllegalStateException.class,
-        "Billing events with SYNTHETIC flag set must have a synthetic creation time");
+        "Synthetic creation time must be set if and only if the SYNTHETIC flag is set");
     oneTime.asBuilder()
         .setSyntheticCreationTime(now.plusDays(10))
         .build();
   }
 
+  @Test
+  public void testFailure_syntheticFlagWithoutCancellationTargetId() {
+    thrown.expect(
+        IllegalStateException.class,
+        "Synthetic creation time must be set if and only if the SYNTHETIC flag is set.");
+    oneTime.asBuilder()
+        .setFlags(ImmutableSet.of(BillingEvent.Flag.SYNTHETIC))
+        .build();
+  }
+
+  @Test
+  public void testFailure_cancellationTargetIdWithoutFlag() {
+    thrown.expect(
+        IllegalStateException.class,
+        "Cancellation target ID must be set if and only if the SYNTHETIC flag is set");
+    oneTime.asBuilder()
+        .setCancellationTargetId(2L)
+        .build();
+  }
+
   @Test
   public void testSuccess_cancellation_forGracePeriod_withOneTime() {
     BillingEvent.Cancellation newCancellation = BillingEvent.Cancellation.forGracePeriod(