Investigate Concurrency Issues

[andrewkchan]

https://github.com/codebase-berkeley/CodeDoor/blob/908787266a95e2c61131885ee6a6aeb403a78fd8/codebank/utils/transaction.py#L39

transaction.atomic() does not lock the entire table, it only journals the queries so they can be rolled back safely. On the other hand select_for_update() locks the selected row. In the current mode send_codebucks can cause a deadlock if 2 users try to send codebucks to each other, each request could acquire the sender lock and wait on the recipient lock forever. Postgres will detect this and raise an exception, and the fix would be to just restart one of the transactions until the deadlock goes away.

There may be more edge cases possible when using row-level locks that I haven't thought of. Should investigate these issues sometime, and consider locking the entire table (slow!).