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Code monitors: fix transaction rollbacks

Administrator requested to merge cc/fix-cm-transaction-rollback into main

Created by: camdencheek

This is a fun one.

I was seeing an issue where, when I create a code monitor, even if it failed, the monitor would still be created. We supposedly have transactional semantics in the creation code, so the transaction should be rolled back if there is a failure, but apparently that wasn't happening. Scary 👻

Turns out the issue was that err was being declared in the function body, and the defer func() { err = tx.Done(err) } was referring to that declared err variable. This would be fine, except we make a new scope on line 150, which makes the declaration on line 151 create a new variable. This means when we assign to it, we aren't assigning to the variable that is being read by tx.Done().

I fixed this by declaring the variable in the method signature so that the error referenced by tx.Done() will always be whatever error is returned with return.

Test plan

Added a test to make sure that a failed create actually rolls back the transaction.

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