Arvados

Add a new TrashDirectory class. It has two subdirectories: home, a ProjectDirectory for the current user, and shared, a SharedDirectory. That structure is similar to what you get in the default mount, but then differences from here:

• The only collections listed in any of these directories should be expiring collections (ones whose expires_at attribute is not null).
• The TrashDirectory itself and everything under it are read-only, regardless of the --read-write setting. This helps avoid surprising users by letting them add files to a collection that will expire soon. (If they really want to do that, they can still access the specific collection by its UUID.)

In --all mode, the TrashDirectory appears under the mount root, named trash.

Add a --trash switch to make the TrashDirectory the mount root.

Add a --mount-trash switch to specify a path where the TrashDirectory is mounted under the mount root (akin to the other --mount-* flags).

Notes:

• We don't need a by_id subdirectory in the TrashDirectory. You can already access expiring collections through the regular MagicDirectory. This is also how you can write to an expiring collection.