don't re-use metadata from earlier snapshots, in a "tahoe backup"

[zooko]

Currently if you run tahoe backup on a directory, and the contents of the children of that directory have not changed, then tahoe backup will reuse the already-created immutable LAFS-directory. Therefore, the metadata on the children, such as the timestamps, owner information, etc., will be re-used. This can lead to a snapshot which says "Here's the state of the directory at time T", and then it shows the children, with their metadata from an earlier snapshot. This is very misleading, and only a very sophisticated user would be able to figure out that the metadata was actually re-used from a previous snapshot, and would be able to figure out in what cases metadata gets re-used vs. gets read afresh from the filesystem.

To fix this, make it so that if any of the metadata of any of the children has changed, then we make a new LAFS-directory to hold the current metadata of all the children, even if the contents of the children (and therefore their immutable file read-caps) haven't changed. (Excluding st_atime, which we do not record anyway, and which would cause intolerable thrashing if we did.)

[zooko]

See related tickets:

• #897 ("tahoe backup" thinks "ctime" means "creation time")
• #1946 (consider removing some st_* fields from metadata)

[daira]

Agreed.

[warner]

Milestone renamed

[warner]

renaming milestone

[exarkun]

Moving open issues out of closed milestones.

[meejah]

Ticket retargeted after milestone closed