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Started implementing [[transfer_control]]. Although I'm currently calling
the configuration for it "preferred content expressions". (What a mouthful!)
I was mostly able to reuse the Limit code (used to handle parameters like
--not --in otherrepo), so it can already build Matchers for preferred content
expressions in my little Domain Specific Language.
Preferred content expressions can be edited with `git annex vicfg`, which
checks that they parse properly.
The plan is that the first place to use them is not going to be inside the
assistant, but in commands that use the `--auto` parameter, which will use
them as an additional constraint, in addition to the numcopies setting
already used. Once I get it working there, I'll add it to the assistant.
Let's say a repo has a preferred content setting of
"(not copies=trusted:2) and (not in=usbdrive)"
* `git annex get --auto` will get files that have less than 2 trusted
copies, and are not in the usb drive.
* `git annex drop --auto` will drop files that have 2 or more trusted
copies, and are not in the usb drive (assuming numcopies allows dropping
them of course).
* `git annex copy --auto --to thatrepo` run from another repo
will only copy files that have less than 2 trusted copies. (And if that
was run on the usb drive, it'd never copy anything!)
There is a complication here.. What if the repo with that preferred content
setting is itself trusted? Then when it gets a file, its number of
trusted copies increases, which will make it be dropped again. :-/
This is a nuance that the numcopies code already deals with, but it's
much harder to deal with it in these complicated expressions. I need to think
about this; the three ideas I'm working on are:
1. Leave it to whoever/whatever writes these expressions to write ones
that avoid such problems. Which is ok if I'm the only one writing
pre-canned ones, in practice..
2. Transform expressions into ones that avoid such problems. (For example,
replace "not copies=trusted:2" with "not (copies=trusted:2 or (in=here and
trusted=here and copies=trusted:3))"
3. Have some of the commands (mostly drop I think) pretend the drop
has already happened, and check if it'd then want to get the file back
again.
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