Limit the number of files deleted in a single tarsnap call to 500

[jeffwidman]

I had a misconfiguration preventing tarsnapper from deleting
files several years. When I realized it and fixed it, I discovered I
now have 200K+ tarsnap archives to delete. However, when tarsnapper
tries to call tarsnap with 200K+ arguments, it results in
OSError: [Errno 7] Argument list too long. The root cause is bash
complaining that the bytes length of the entire command exceeds
ARG_MAX.

So this limits the maximum number of files deleted in a single call to
tarsnap to 500.

500 is a somewhat arbitrary batch size. The actual restriction is
typically a bytes limit on the size of the command that the shell will
accept, which can vary widely across OS flavors. Because it's a bytes
limit, this is also dependent on the length of the backup filenames. So
rather than deal with all this complexity, 500 was chosen as a
reasonable balance between safety and speed.

[jeffwidman]

An alternative strategy for dealing with this issue would be to run loop that optimistically tries the full command, catches the Python exception, removes N files from it, then tries the command again... while this would generate the optimal result, it could be a little slow, for example in my case to drop from 100K files down to 1K or so would require 99K failures... Plus it makes for a really slow delete call... doing a couple of batches of 500 or another reasonable number still seems simplest.

[miracle2k]

Oh my. Is there a UX improvement that can be done to prevent this kind of misconfiguration?

As for the PR, thanks. I don't think there is a need for testing the length, a fixed value is fine.

[jeffwidman]

Thanks for merging!

Is there a UX improvement that can be done to prevent this kind of misconfiguration?

My problem was #18 and it was entirely my fault for not checking logs for errors, only improvement possible is #52