You're seeing just the callback handle_rollback, go back to DBConnection module for more information.
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handle_rollback(opts, state)

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handle_rollback(opts :: Keyword.t(), state :: any()) ::
  {:ok, result(), new_state :: any()}
  | {status(), new_state :: any()}
  | {:disconnect, Exception.t(), new_state :: any()}

Handle rolling back a transaction. Return {:ok, result, state} on successfully rolling back transaction, {status, state} to notify caller that the transaction can not rollback due to the transaction status status, {:error, exception, state} (deprecated) to error and no longer be inside transaction, or {:disconnect, exception, state} to error and disconnect.

A callback implementation should only return status if it can determine the database' transaction status without side effect.

This callback is called in the client and connection process.