An HTTP digest for PATCHing?

Hi digest enthusiasts!

As part of some discussion in one of the resumable uploads issues, we stumbled upon a bit of and edge case that I forgot about since it was tangential to the primary issue. I'll attempt to summarise my understanding in an example scenario.

Imagine there's an existing resource of X bytes, with a client wishing to use PATCH to append Y bytes to it. The server accepts the request, performs the patch, and the resource is now X+Y bytes.

The client initiating the request can indicate the digest(s) of the patch document: content, repr, or identity. The server can indicate the digests(s) related to the resource and/or transfers related to it. This could be prior to or after patching. (ETags also play a role, if aspects of RFC 5789 are in play [1]). 

However, there's no digesty way for a client to articulate what it might expect the outcome of the patch to be. In other words, it can't say "If you apply patch document with bytes Y the hash of X+Y is foo. If it's not foo, the operation failed". This seems very niche and I don't have a personal use case. So throwing it out to the list in case there's other opinions.

Cheers,
Lucas

[1] https://httpwg.org/specs/rfc5789.html#rfc.section.2