Would be great to have an rsync like protocol running in the webbrowser ideally based on webrtc purely P2P. No install needed. I hate all those custom clients for all those cloud storage services. Particularly was looking for an option for a client with a lot of IT restrictions to upload a large number of large files. Embarrassingly often we end up shipping encrypted hard drives.
rclone is basically rsync with cloud native endpoints, it's not browser based but does what needs to be done very well. It does have an experimental web GUI but it's not a feature I've tried personally. https://github.com/rclone/rclone-webui-react
The changes you would need to make for HTTP to support a rsync style pull protocol are pretty minimal. Two of them, really.
1) A checksum header in the HEAD response
2) Altering the HEAD method to optionally accept the same range request headers that a GET does, and have the checksum header return the checksum for that range.
I'm mildly surprised it hasn't happened. Pretty sure you could do it with vanilla php or the nginx lua module.
Like you already mentioned etag sucks, because you just get a guarantee X hasn't changed but no way to now how it was calculated. i.e. it is opaque.
SRI is only useful for everything outside of the original document(i.e. for <script> tags and the like.
Content-MD5 actually works.. except it's using the broken MD5 digest.. so it might work for the use case, depending on the details of the use-case. :)
2 is a no-go as far as I'm aware.
Though you could perhaps do an HTML document where you link to each "chunk" of the file using SRI.. though that would be a total bastardization of HTML and a total hack.. and a bad one at that :) Also the chunks would be decided well ahead of time that way, so not the same as a RANGE..
The first is already there in the form of a strong etag. Http conditional requests with if-range support etag validation but that is a document checksum not a chunk checksum.
Or stored it as opaque metadata on first retrieval, regardless of how it was computed, paired with the actual hash. When one is invalidated, the other is too.
