Magic Folder: account properly for NFC normalization

[daira]

Tahoe-LAFS maps filenames to their NFC-normalized form. When a local path relative to the magic folder directory is not NFC, the corresponding encoded filename in the client's DMD will still be mapped to NFC. Then when the client reads its own DMD at start-up, it may be confused by the decoded path not matching the local db entry.

This should probably be fixed by mapping the db paths to NFC both on creation and lookup.

Note that the issue here isn't the fact that there can be distinct paths with the same NFC normalization (which can be expected to be a rare case); it's that the db may fail to do its job across restarts for filenames that are not NFC-normalized.

[daira]

Now that I think about this, I'm confused as to why a client scans its own DMD at start-up. Maybe it doesn't need to.

[daira]

We should have tests for non-NFC directory and file names.

[exarkun]

magic-folder has been split out into a separate project - ​https://github.com/leastauthority/magic-folder