Provide seek-able interface to Client

[thundergolfer]

rust-oci-client has Client::pull_blob_stream but this is not seek-able. I'm looking for a seek-able interface because it allows me to read ranges of bytes from a blob and extract a Toc (Table-of-Contents) data structure from an eStargz image.

stargz-snapshotter uses containerd's seek-able ContentStore to achieve the same.

We at modal.com are open to upstreaming this functionality if suitable :)

[thomastaylor312]

Thanks for the issue @thundergolfer! Is there any reason what you're doing couldn't be achieved with using StreamExt (or TryStreamExt) methods like chunks or take?

Using something like AsyncSeek wouldn't work unless we buffered into memory (because we can't go backwards in the stream)

[thundergolfer]

Thanks for the reply @thomastaylor312. I think I ultimately need an interface which will materialize arbitrary HTTP range requests against the blob and though I don't get how chunks would do it I think skip + take may end up working.

I do see now that pull_blob_response accepts an offset which gets stuck into a range header. So if there was just an additional len: u64 or similar to do bytes={offset}-{offset+len} then I think I'd be done :)

[thomastaylor312]

Closed in #163