fix: copy backup issue when backup is done across different instance IDs

google-cloud-spanner/src/main/java/com/google/cloud/spanner/BackupInfo.java

@@ -156,9 +156,6 @@ public Builder setVersionTime(Timestamp versionTime) {
 
     @Override
     public Builder setDatabase(DatabaseId database) {
-      Preconditions.checkArgument(
-          database.getInstanceId().equals(id.getInstanceId()),
-          "The instance of the source database must be equal to the instance of the backup.");
       this.database = Preconditions.checkNotNull(database);
       return this;
     }

google-cloud-spanner/src/test/java/com/google/cloud/spanner/DatabaseAdminClientImplTest.java

@@ -73,6 +73,7 @@
 public class DatabaseAdminClientImplTest {
   private static final String PROJECT_ID = "my-project";
   private static final String INSTANCE_ID = "my-instance";
+  private static final String INSTANCE_ID_2 = "my-instance-2";
   private static final String INSTANCE_NAME = "projects/my-project/instances/my-instance";
   private static final String DB_ID = "my-db";
   private static final String DB_NAME = "projects/my-project/instances/my-instance/databases/my-db";
@@ -591,6 +592,41 @@ public void copyBackupWithBackupObject() throws ExecutionException, InterruptedE
     assertThat(op.get().getId().getName()).isEqualTo(BK_NAME);
   }
 
+  @Test
+  public void copyBackupWithBackupObject_onDifferentInstances()
+      throws ExecutionException, InterruptedException {
+    Backup testProto =
+        Backup.newBuilder()
+            .setName(BK_NAME)
+            .setDatabase("projects/my-project/instances/my-instance-2/databases/my-db")
+            .setState(Backup.State.READY)
+            .build();
+    final OperationFuture<Backup, CopyBackupMetadata> rawOperationFuture =
+        OperationFutureUtil.immediateOperationFuture(
+            "copyBackup", testProto, CopyBackupMetadata.getDefaultInstance());
+    final Timestamp expireTime =
+        Timestamp.ofTimeMicroseconds(
+            TimeUnit.MILLISECONDS.toMicros(System.currentTimeMillis())
+                + TimeUnit.HOURS.toMicros(28));
+    final Timestamp versionTime =
+        Timestamp.ofTimeMicroseconds(
+            TimeUnit.MILLISECONDS.toMicros(System.currentTimeMillis()) - TimeUnit.DAYS.toMicros(2));
+    final com.google.cloud.spanner.Backup requestBackup =
+        client
+            .newBackupBuilder(BackupId.of(PROJECT_ID, INSTANCE_ID_2, BK_ID))
+            .setExpireTime(expireTime)
+            .setVersionTime(versionTime)
+            .build();
+    BackupId sourceBackupId = BackupId.of(PROJECT_ID, INSTANCE_ID, BK_ID);
+
+    when(rpc.copyBackup(sourceBackupId, requestBackup)).thenReturn(rawOperationFuture);
+
+    final OperationFuture<com.google.cloud.spanner.Backup, CopyBackupMetadata> op =
+        client.copyBackup(sourceBackupId, requestBackup);
+    assertThat(op.isDone()).isTrue();
+    assertThat(op.get().getId().getName()).isEqualTo(BK_NAME);
+  }
+
   @Test
   public void copyEncryptedBackup() throws ExecutionException, InterruptedException {
     final OperationFuture<Backup, CopyBackupMetadata> rawOperationFuture =