oras manifest push removes a manifest from index.json file in OCI layout

[qweeah]

What happened in your environment?

I pushed a manifest file via oras manifest push without tagging and the manifest disappears from the layout index.json file.

What did you expect to happen?

The manifest should still be in the index.json file in the OCI layout.

How can we reproduce it?

Reproduce script:

$ cat manifest.json
{"schemaVersion":2,"mediaType":"application/vnd.oci.image.manifest.v1+json","config":{"mediaType":"application/vnd.oci.image.config.v1+json","digest":"sha256:44136fa355b3678a1146ad16f7e8649e94fb4fc21fe77e8310c060f61caaff8a","size":2},"layers":[]}

$ oras manifest push . --oci-layout ./manifest.json
Pushed [oci-layout] .
Digest: sha256:962b1ae83825c37b6eb3ee98dbe587461074a338cc0b838a635d274e2844096d

$ cat index.json| jq
{
  "schemaVersion": 2,
  "manifests": []
}

$ # But the manifest is actually in the layout
$ oras manifest fetch .@sha256:962b1ae83825c37b6eb3ee98dbe587461074a338cc0b838a635d274e2844096d --oci-layout
{"schemaVersion":2,"mediaType":"application/vnd.oci.image.manifest.v1+json","config":{"mediaType":"application/vnd.oci.image.config.v1+json","digest":"sha256:44136fa355b3678a1146ad16f7e8649e94fb4fc21fe77e8310c060f61caaff8a","size":2},"layers":[]}

What is the version of your ORAS CLI?

$ oras version
Version:        1.0.0-rc.2+unreleased
Go version:     go1.20
Git commit:     38409cc
Git tree state: dirty

What is your OS environment?

Ubuntu 20.04

Are you willing to submit PRs to fix it?

• Yes, I am willing to fix it.