I'm not convinced (trying to) preserve UID or GID is a good idea; these IDs will often vary between computers (even if the very same human set them up). Even if the same human did set them up, they'd probably want the symbolic names preserved/set (i.e. "alice" or "a_group" rather than the 1000 or whatever).
I *think* trying to preserve mtime should work, but we should double-check that magic-folder doesn't depend on these when searching for updates. *Can* we actually can mess with ctime directly..?
A different reason people might look at the timestamps could be to answer the question "when did magic-folder update this file" rather than "when did Alice update this file"; do we have any real-user feedback indicating which one is least surprising?
Consider an example: Alice adds a new file to her magic-folder that hasn't been modified for a year (but she happens to "cp" it into her magic directory). Thus, from Linux's perspective, the mtime will be "now" (while Alice might *except* it to be "a year ago" -- because she forgot to do "cp -a"). Which one does Bob want to see as "mtime"? (One of three choices: when his Tahoe client downloaded it; Alice's "now"; or "a year ago").
We can't actually choose "a year ago" in the above example, because Alice accidentally destroyed that metadata. So, presuming she didn't (by using cp -a) then we could chose that as "the" answer.
