test case and handle in-place migration correctly

parity/migration.rs

@@ -201,19 +201,23 @@ fn migrate_database(version: u32, db_path: PathBuf, mut migrations: MigrationMan
 	// migrate old database to the new one
 	let temp_path = migrations.execute(&db_path, version)?;
 
-	// create backup
-	fs::rename(&db_path, &backup_path)?;
+	// completely in-place migration leads to the paths being equal.
+	// in that case, no need to shuffle directories.
+	if temp_path != db_path {
+		// create backup
+		fs::rename(&db_path, &backup_path)?;
 
-	// replace the old database with the new one
-	if let Err(err) = fs::rename(&temp_path, &db_path) {
-		// if something went wrong, bring back backup
-		fs::rename(&backup_path, &db_path)?;
-		return Err(err.into());
+		// replace the old database with the new one
+		if let Err(err) = fs::rename(&temp_path, &db_path) {
+			// if something went wrong, bring back backup
+			fs::rename(&backup_path, &db_path)?;
+			return Err(err.into());
+		}
+
+		// remove backup
+		fs::remove_dir_all(&backup_path)?;
 	}
 
-	// remove backup
-	fs::remove_dir_all(&backup_path)?;
-
 	Ok(())
 }
 

util/src/migration/tests.rs

@@ -226,3 +226,24 @@ fn pre_columns() {
 	// short of the one before it.
 	manager.execute(&db_path, 0).unwrap();
 }
+
+#[test]
+fn change_columns() {
+	use kvdb::DatabaseConfig;
+
+	let mut manager = Manager::new(Config::default());
+	manager.add_migration(::migration::ChangeColumns {
+		pre_columns: None,
+		post_columns: Some(4),
+		version: 1,
+	}).unwrap();
+
+	let dir = RandomTempPath::create_dir();
+	let db_path = db_path(dir.as_path());
+
+	let new_path = manager.execute(&db_path, 0).unwrap();
+
+	let config = DatabaseConfig::with_columns(Some(4));
+	let db = Database::open(&config, new_path.to_str().unwrap()).unwrap();
+	assert_eq!(db.num_columns(), 4);
+}