The cache used to serve a hit by committing the layer during Prepare and
returning ErrAlreadyExists, which is incompatible with parallel unpacking:
the "rebase" capability defers the parent to Commit time, so a
commit-at-Prepare layer ends up parentless. The erofs snapshotter therefore
had to disable rebase whenever the cache was enabled, making every cache
*miss* fall back to a fully sequential (slower-than-cold) pull.
Introduce snapshots.ErrAlreadyStaged, returned from Prepare to mean "the
active snapshot's content is staged; skip the layer download and apply, but
still Commit it" (where the parent is applied). Unlike ErrAlreadyExists it
does not end the layer's lifecycle. The unpacker handles it by emitting a
status whose bottom half runs the normal serialized commit (rebasing the
parent in), and the metadata snapshotter threads it through Prepare like a
normal active snapshot. The erofs cache hit now stages the blob and returns
ErrAlreadyStaged instead of committing, so it can advertise "rebase"
unconditionally: hits skip download+conversion and misses stay parallel.
Also promote the "rebase" capability string to snapshots.RebaseCap (shared by
the unpacker, erofs, and overlay).
Follow up: fetch is range-based, so only a fully cached contiguous
prefix skips downloads; a miss in a lower layer still pulls everything above
it. Per-layer fetch-skip is left as a follow-up.
Signed-off-by: Maksym Pavlenko <pavlenko.maksym@gmail.com>
Fix issue 13388
```
[...]
May 12 16:57:23 kind-control-plane kubelet[257]: failed to extract layer (application/vnd.docker.image.rootfs
.diff.tar sha256:6f1cdceb6a3146f0ccb986521156bef8a422cdbb0863396f7f751f575ba308f4) to overlayfs as "extract-920875437
-7QPF sha256:31e64620332e54e3e4fb246d8325ed2c9f1c2cc64a95f0bb23b4b7e82834c95a": failed to mount /var/lib/containerd/t
mpmounts/containerd-mount2180142388: mount source: "overlay", target: "/var/lib/containerd/tmpmounts/containerd-mount
2180142388", fstype: overlay, flags: 0, data: "upperdir=/var/lib/containerd/io.containerd.snapshotter.v1.overlayfs/sn
apshots/279/fs", err: invalid argument
[...]
```
This was a regression introduced in PR 13115.
Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>
Some of the snapshotters that allow you to change their root location
were already doing this, this just makes all of them follow the same
pattern.
Signed-off-by: Danny Canter <danny@dcantah.dev>
Maksym Pavlenko