what happens if a "tahoe backup" process is in progress when another one is started?

[zooko]

How does the "tahoe backup" process handle the situation that another "tahoe backup" process is running at the same time, accessing the same backupdb?

To answer this question, start here: docs/backupdb.rst​ and src/allmydata/scripts/backupdb.py​.

This is relevant to #2053, because a simple, easy improvement for #2053 is just to make sure that backup processes don't corrupt each other's state if they run concurrently, and that they make good quick progress, and take advantage of one another's (complete, consistent) results for faster progress.

[daira]

See also #2285.