oras push: support streamed uploading from stdin

[qweeah]

Are there any plans to implement this also for the input so that something like the following will be possible:

command-a | command-b | oras push localhost:5000/json-artifact:v1 -

...

oras pull localhost:5000/json-artifact:v1 --output - | jq

Originally posted by @ProbstDJakob in #346 (comment)

[qweeah]

Regarding the user experience, here are some to-be-settled designs we need to think of:

1. How should user provide the name of the blob from stdin?
2. Besides from tar -c ...., what are the possible commands that can generate the to-be-uploaded stdin? (Asking since oras push <artifact> <folder> can already natively tar and upload the <folder> without invoking tar)

[ProbstDJakob]

The following script is a real world example where streaming could come in handy.

Background

We fully manage the life cycle of an OpenShift cluster via a GitLab Pipeline. When creating a cluster with the openshift-install tool some files like terraform state and kube-configs will be created. Those files are needed during the whole life cycle of the cluster (not only in the current pipeline), thus they need to be stored persistently. In our case we use the existing GitLab registry and oras to create an image.

Current way to push the artifacts to the registry

#!/usr/bin/env sh
set -eu

# [...] some preparations

git ls-tree --name-only -r --full-tree HEAD "$CI_PROJECT_DIR/openshift-install" \
  | tar -czC "$CI_PROJECT_DIR" --exclude-from=- openshift-install \
  | sops --encrypt --input-type binary --output-type binary --output "$CI_PROJECT_DIR/openshift-install-artifacts.tar.gz.enc" /dev/stdin

cd "$CI_PROJECT_DIR"
oras push "$ENCRYPTED_OPENSHIFT_INSTALL_ARTIFACTS_IMAGE" openshift-install-artifacts.tar.gz.enc

Possible way to push the artifacts to the registry with pipelining

#!/usr/bin/env sh
set -eu

# [...] some preparations

git ls-tree --name-only -r --full-tree HEAD "$CI_PROJECT_DIR/openshift-install" \
  | tar -czC "$CI_PROJECT_DIR" --exclude-from=- openshift-install \
  | sops --encrypt --input-type binary --output-type binary /dev/stdin \
  | oras push "$ENCRYPTED_OPENSHIFT_INSTALL_ARTIFACTS_IMAGE" -

This way there is no need to create a temporary file and make sure to be in the correct directory in order to include the file without an absolute path which would make the image unable to unpack in other environments (OSs).

Counterpart

See #346 (comment)