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>
With the rename of fetch to fetch-object, we now introduce the `fetch`
command. It will fetch all of the resources required for an image into
the content store. We'll still need to follow this up with metadata
registration but this is a good start.
Signed-off-by: Stephen J Day <stephen.day@docker.com>
To make using the `fetch-object` for demonstrations much easier, the
mediatypes are defaulted when a non-digest object identifier is
provided. We also add support for OCI mediatypes, although they are
mostly unavailable.
Signed-off-by: Stephen J Day <stephen.day@docker.com>
To allow us to differentiate from fetching an image, fetch a part of an
image and pulling an image, we now call the `fetch` command the
`fetch-object` command. We can now introduce a command that does the
complete image fetch without creating snapshots, allowing `pull` to
perform the entire process.
Signed-off-by: Stephen J Day <stephen.day@docker.com>
That is, e55af28bae.
Since f77ece9cb5
the license files regex is case insensitive which picks up one more file.
Signed-off-by: Ian Campbell <ian.campbell@docker.com>
The message was defined but the method was returning empty, plumb through the
result from the shim layer.
Compile tested only.
Signed-off-by: Ian Campbell <ian.campbell@docker.com>
To make restarting after failed pull less racy, we define `Truncate(size
int64) error` on `content.Writer` for the zero offset. Truncating a
writer will dump any existing data and digest state and start from the
beginning. All subsequent writes will start from the zero offset.
For the service, we support this by defining the behavior for a write
that changes the offset. To keep this narrow, we only support writes out
of order at the offset 0, which causes the writer to dump existing data
and reset the local hash.
This makes restarting failed pulls much smoother when there was a
previously encountered error and the source doesn't support arbitrary
seeks or reads at arbitrary offsets. By allowing this to be done while
holding the write lock on a ref, we can restart the full download
without causing a race condition.
Once we implement seeking on the `io.Reader` returned by the fetcher,
this will be less useful, but it is good to ensure that our protocol
properly supports this use case for when streaming is the only option.
Signed-off-by: Stephen J Day <stephen.day@docker.com>
Allow deletion of content over the GRPC interface. For now, we are going
with a model that conducts reference management outside of the content
store, in the metadata store but this design is valid either way.
Signed-off-by: Stephen J Day <stephen.day@docker.com>
Two test cases added
- Prepare, Stat on new layer, should return Active layer.
- Prepare & Commit , Stat on new layer, should return Committed layer
Signed-off-by: Kunal Kushwaha <kushwaha_kunal_v7@lab.ntt.co.jp>
