ping @ibuildthecloud @cpuguy83 PR for the downgrade issue

Downgrades have never worked

@crosbymichael Is that to say that we shouldn't even consider this PR?

Agreed on IRC that we should close this PR.

Reopening in light of the recent conversations on #13695. Does this fully resolve the issue?

There is some data migration handling that I can fix here.

This changes the local storage layout a little bit to allow Downgrade Docker 1.7 to Docker 1.6, only if a volume was created with Docker < 1.7.

History:

Pre 1.7m, volume information was stored in two separated directories:

1. The volume configuration was in ROOT/volumes/VOLUME_ID/config.json.
2. The data stored in the volumes was in ROOT/vfs/dir/VOLUME_ID.

Post 1.7, volume information is stored in one place, ROOT/volumes/VOLUME_ID/_data.

The docker daemon takes care of migrating volumes from 1.6 to 1.7 by creating a symlink from ROOT/vfs/dir/VOLUME_ID to ROOT/volumes/VOLUME_ID/_data. The file config.json is preserved, so you can boot the container with Docker 1.6 and all the information is in the same place that it was before booting Docker 1.7.

Unfortunately, the two previous RCs for 1.7 didn't follow that migration, so there is a special migration for volumes created with Docker 1.7 RC1 and Docker 1.7 RC2. This is a very small corner case, I don't expect anyone running Docker 1.7 RC in production, except us, ouch.

Thanks @calavera! Is an additional test needed to check if docker rm -v properly cleans up all locations and the symlinks?

good catch @thaJeztah, fixed.

Thanks! ❤️

I'm not sure we should bother with previous RC migration paths. Nevertheless, LGTM: I'm not 100% confident because the change is quite significant, but reading several times through it it seems good (and thanks for the code comments btw).

@tiborvass I think the comments are more clear now, let me know what you think.

I was concerned about using prefix test on paths at first, but considering that filepath.EvalSymLink does a filepath.Clean before returning I think we're safe.

Still LGTM.

LGTM

cherry-picked