I've designed these to work well together, I hope. If I get it wrong,
I can just change the code in one place, since these expressions
won't be stored in the git-annex branch.
Solves the issue with preferred content expressions and dropping that
I mentioned yesterday. My solution was to add a parameter to specify a set
of repositories where content should be assumed not to be present. When
deciding whether to drop, it can put the current repository in, and then
if the expression fails to match, the content can be dropped.
Using yesterday's example "(not copies=trusted:2) and (not in=usbdrive)",
when the local repo is one of the 2 trusted copies, the drop check will
see only 1 trusted copy, so the expression matches, and so the content will
not be dropped.
This includes a full parser for the boolean expressions in the log,
that compiles them into Matchers. Those matchers are not used yet.
A complication is that matching against an expression should never
crash git-annex with an error. Instead, vicfg checks that the expressions
parse. If a bad expression (or an expression understood by some future
git-annex version) gets into the log, it'll be ignored.
Most of the code in Limit couldn't fail anyway, but I did have to make
limitCopies check its parameter first, and return an error if it's bad,
rather than erroring at runtime.
