Are empty blobs/layers supported in the distribution spec?

[vdice]

Question

Is the upload of an empty blob or layer supported -- or defined at all -- in the distribution spec?

Background/Context

I'm having trouble finding a definitive answer in the spec itself. I suppose the question usually wouldn't be relevant for 'classic' Docker/OCI images, as the client wouldn't create an empty layer locally to begin with. This would equate to no change to the file system and thus would be pointless to push.

However, my use case is to utilize OCI registries to host a different type of OCI object, one in which a layer may equate 1:1 to a file. A simple example would be a static site. Currently, my approach is to upload each static asset as its own individual layer. Which leads to the empty file (layer) scenarios: there happen to be use cases in this static site generator domain (and beyond) for empty files; think .nojeckyll, .gitkeep and others.

Implementation samples

The OCI client library I've been using, krustlet/oci-distribution, actually prevents uploading empty blobs/layers by default. Out of curiousity, I updated the code to indeed push empty blobs, sending an empty body on the request as well as header key/values of Content-Length: 0 and, in the case of chunked uploads, Content-Range: 0-0 (spoiler, perhaps this latter header should not be set when the upload is a one-chunk, 0 byte blob; see below).

It turns out, this leads to different behavior between a handful of distribution implementations surveyed:

• Docker's registry container implementation (using chunked blob uploads)
  • accepts the uploads with the request formulation as described above (both Content-Length and Content-Range headers included). The object is succesfully pushed and I've confirmed that the expected empty files are present in the live application.
• DockerHub (using chunked blob uploads):
  • returns an ErrCodeSizeInvalid when I send the Content-Range: 0-0 header/value (as it always performs a +1 as seen here)
  • however, when I omit the Content-Range altogether, the one-chunk, 0 byte upload is accepted (and the application has the expected empty files)
• AWS ECR (falls back to monolithic uploads):
  • returns a 500 internal server error when attempting to upload an empty/0 byte layer (I don't see any headers returned indicating a min layer size, alas).

[vdice]

A few updates:

• This image-spec section around empty descriptors was brought to my attention which mentions "While an empty blob (size of 0) may be preferable, practice has shown that not to be ubiquitously supported."
• In my testing, some registries also won't accept layers with 1 byte either. As far as I can tell, 2 bytes appear to be the minimum. The aforementioned link also mentions this as a minimum.