make "tahoe backup" avoid "piling up" if the backup job takes longer than the period

[zooko]

<myers> the backup cronjob should be built to not pile up as I'm guessing it's going to take more than 24 hours for this first run to finish

<myers> and if you have that built in you should put that in the documents

[daira]

The tricky design decision here is how to detect that another backup is running (since they will be in different processes, currently). I can think of several possibilities:

1. Move the backup job into the gateway.
2. Have a longer-running process (perhaps the gateway but not necessarily) schedule backups and ensure mutual exclusion.
3. Have the backup process acquire a lock (perhaps just by opening the backupdb in exclusive mode) while it runs.

3 seems simplest starting from where we are.

[zooko]

See also #2062.

[zooko]

We've been assuming that the user is backing up the same directory multiple times. In that case it makes sense to abort or delay the second backup until the first one completes. But, in #2285, a real live user is hitting a bug because he's running tahoe backup concurrently on two different directories. That use case, which we hadn't apparently previously considered, should probably not cause one of the backups to wait until the other backup is completely finished before it begins!

[daira]

Well, it's quite possible that the directory trees overlap (and if we follow symlinks then we won't know this in advance of starting the second backup).