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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"[Spanner] TransactionManager.close() not idempotent; API doc example is broken","articleBody":"I am using the new `TransactionManager` API and am getting mysterious `\"rollback can only be called if the transaction is in progress\"` exceptions.\r\n\r\nI think I've tracked down the problem...\r\n\r\nThe API docs example shown with `DatabaseClient.transactionManager()` is this:\r\n```\r\n long singerId = my_singer_id;\r\n try (TransactionManager manager = dbClient.transactionManager()) {\r\n   TransactionContext txn = manager.begin();\r\n   while (true) {\r\n     String column = \"FirstName\";\r\n     Struct row = txn.readRow(\"Singers\", Key.of(singerId), Collections.singleton(column));\r\n     String name = row.getString(column);\r\n     txn.buffer(\r\n         Mutation.newUpdateBuilder(\"Singers\").set(column).to(name.toUpperCase()).build());\r\n     try {\r\n       manager.commit();\r\n       break;\r\n     } catch (AbortedException e) {\r\n       Thread.sleep(e.getRetryDelayInMillis() / 1000);\r\n       txn = manager.resetForRetry();\r\n     }\r\n   }\r\n }\r\n```\r\nHowever, this example usage is actually wrong.\r\n\r\nThe reason is that the implementation returned from `DatabaseClient.transactionManager()` is actually an `AutoClosingTransactionManager` (in `SessionPool.java`), which automatically `close()`s the transaction if `commit()` or `rollback()` is invoked. Sounds like a good idea, right?\r\n\r\nThe problem is that `AutoClosingTransactionManager.close()` is not idempotent:\r\n```\r\n    @Override\r\n    public void close() {\r\n      try {\r\n        delegate.close();\r\n      } finally {\r\n        session.close();\r\n      }\r\n    }\r\n```\r\nIf the transaction is closed twice, then the second time around the `session` is closed _again_, even though it has already been returned to the pool and possibly reassigned to another transaction. If so, this in turn causes that other transaction to fail on `commit()` with an exception `\"rollback can only be called if the transaction is in progress\"`.\r\n\r\nOne fix for this is to document that a `TransactionManager` must only be closed exactly once, and fix your API javadoc example accordingly.\r\n\r\nA much better and more robust fix would be to fix `AutoClosingTransactionManager.close()` to be idempotent. `TransactionManager` could then implement `Closeable` instead of `AutoCloseable` to denote this (if desired).","author":{"url":"https://github.com/archiecobbs","@type":"Person","name":"archiecobbs"},"datePublished":"2018-04-19T18:56:42.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":4},"url":"https://github.com/3172/google-cloud-java/issues/3172"}