With this PR, we introduce the concept of image handlers. They support
walking a tree of image resource descriptors for doing various tasks
related to processing them. Handlers can be dispatched sequentially or
in parallel and can be stacked for various effects.
The main functionality we introduce here is parameterized fetch without
coupling format resolution to the process itself. Two important
handlers, `remotes.FetchHandler` and `image.ChildrenHandler` can be
composed to implement recursive fetch with full status reporting. The
approach can also be modified to filter based on platform or other
constraints, unlocking a lot of possibilities.
This also includes some light refactoring in the fetch command, in
preparation for submission of end to end pull.
Signed-off-by: Stephen J Day <stephen.day@docker.com>
The service can use the snapshotter directly to get the rootfs.
Removed debug line for mount response.
Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)
Also remove the comment fields in the vndr script, they are too hard to
keep up to date and add little value when we have the git commit
timestamps
Signed-off-by: Michael Crosby <crosbymichael@gmail.com>
After receiving feedback during containerd summit walk through of the
pull POC, we found that the resolution flow for names was out of place.
We could see this present in awkward places where we were trying to
re-resolve whether something was a digest or a tag and extra retries to
various endpoints.
By centering this problem around, "what do we write in the metadata
store?", the following interface comes about:
```
Resolve(ctx context.Context, ref string) (name string, desc ocispec.Descriptor, fetcher Fetcher, err error)
```
The above takes an "opaque" reference (we'll get to this later) and
returns the canonical name for the object, a content description of the
object and a `Fetcher` that can be used to retrieve the object and its
child resources. We can write `name` into the metadata store, pointing
at the descriptor. Descisions about discovery, trust, provenance,
distribution are completely abstracted away from the pulling code.
A first response to such a monstrosity is "that is a lot of return
arguments". When we look at the actual, we can see that in practice, the
usage pattern works well, albeit we don't quite demonstrate the utility
of `name`, which will be more apparent later. Designs that allowed
separate resolution of the `Fetcher` and the return of a collected
object were considered. Let's give this a chance before we go
refactoring this further.
With this change, we introduce a reference package with helps for
remotes to decompose "docker-esque" references into consituent
components, without arbitrarily enforcing those opinions on the backend.
Utlimately, the name and the reference used to qualify that name are
completely opaque to containerd. Obviously, implementors will need to
show some candor in following some conventions, but the possibilities
are fairly wide. Structurally, we still maintain the concept of the
locator and object but the interpretation is up to the resolver.
For the most part, the `dist` tool operates exactly the same, except
objects can be fetched with a reference:
```
dist fetch docker.io/library/redis:latest
```
The above should work well with a running containerd instance. I
recommend giving this a try with `fetch-object`, as well. With
`fetch-object`, it is easy for one to better understand the intricacies
of the OCI/Docker image formats.
Ultimately, this serves the main purpose of the elusive "metadata
store".
Signed-off-by: Stephen J Day <stephen.day@docker.com>
