[pull] main from containerd:main - #444
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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>
Replace the snapshots.ErrAlreadyStaged sentinel error with a read-only-mounts signal: a snapshotter (e.g. erofs serving a layer content cache hit) now returns Prepare mounts normally, with no error, when the layer content is already staged into the active snapshot. The unpacker's isStaged helper checks the last mount's ReadOnly() to decide whether to skip fetch+apply and just commit. Signed-off-by: Maksym Pavlenko <pavlenko.maksym@gmail.com>
erofs: enable parallel unpack with content cache
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