Export architecture improvement spike

[eloquence]

For the 9/19-10/9 sprint, we've agreed to a time-boxed, 4 hour improvement spike to the current export tooling (I've increased the estimate to 8 total, to account for review and integration). The current architecture was a first pass, and a targeted improvement spike will likely identify several areas where it should be refactored or reorganized to increase maintainability and resilience.

This ticket exists to log initial observations, and PRs without corresponding issues can be tracked against it.

[sssoleileraaa]

One big architecture improvement would be to return status codes (e.g. USB_CONNECTED, ERROR_USB_MOUNT, ERROR_USB_WRITE, ERROR_PRINT, ERROR_PRINTER_URI) instead of writing these codes to stderr. There are a lot of subprocess calls in the export code that sometimes results in a lot of extra text in stderr, which the client is expected to use to check status codes. For example, I've seen:

chown: cannot read directory `/media/....
device-mapper: remove ioctl on encrypted...
device-mapper: remove ioctl on encrypted...
device-mapper: remove ioctl on encrypted...
device-mapper: remove ioctl on encrypted...
device-mapper: remove ioctl on encrypted...
device-mapper: remove ioctl on encrypted...
device-mapper: remove ioctl on encrypted...
ERROR_USB_MOUNT

It would actually be useful for the client to show the ERROR_USB_MOUNT code to the user when we say to contact their admin, so a clean return code here is preferred.

[emkll]

The intent is that all exceptions should be caught and return a defined, documented error code. If there's stderr output or any other message, that means an exception that should be been caught was not caught.

The error you mention in #15 (comment) is due to the uncaught subprocess.check_call in the except block here: https://github.com/freedomofpress/securedrop-export/blob/master/securedrop_export/export.py#L242

[eloquence]

For the 10/23-11/6 sprint period, we've agreed to finalize the round of changes made in the 15-sprint-39-improvements branch, and to land them in master.

[sssoleileraaa]

see related discussion about device management for export