Support bagging to a destination other than the source

[finoradin]

Currently the module only allows one to do what the LOC Java library calls "bag in place". It would be very useful to have built-in the ability to specify one or more payloads as the "source" and to then specify a "destination" where the bag containing the payloads will be created.

Minor but important note – the hashes in the manifest should be generated from the source payloads, not the copied files in the bag.

[finoradin]

bump

[acdha]

@finoradin Thanks – I'll work on a pull request for this. Strong +1 on checksumming the source files — the way I typically do things like that would be streaming reads at some large block size since that's also good for performance on network filesystems and it's a natural extension to feed each block into the hashers as well.

[runderwood]

I'm willing to take this on. Would it be agreeable to add a --destination option, which, when present, tells bagit.py to draw on the bag_dir args as sources and create the new bag in the destination path?

I would, of course, checksum the files as described above.

[acdha]

@runderwood +1 – that should cover the most common use-cases

[runderwood]

@acdha Clarification: With multiple sources, should:

1. ...multiple bags be generated?
2. ...one bag be generated with the sources merged?
3. ...one bag be generated with directories for each source?

[johnscancella]

I would vote for choice 3, one bag with each directory for each source

[runderwood]

@johnscancella I can see that being useful, but I also find it counter-intuitive, since directories in bagit world are usually transmuted into payloads at the root of data/.

EDIT: I also should mention that merging directories seems consistent w/ some of the use cases that have come up for me. But that's also a bit weird, I suppose -- though maybe no weirder than in-place bagging.