feature request: tokio::io::AsyncSeek support for Reader

[rbtcollins]

Use case: I have zip files being added to a cacache instance, and zip is a non-streamable format - the index of a zip is at the end, so it is somewhat inefficient to read everything, then scan the index, then reopen and re-read to extract the actual content (using async_zip)

[zkat]

sure, this sounds good.

Ideally, we'll also have the async-std version of this done at the same time, though.

[baszalmstra]

What do you think the API would look like @zkat ? The Reader in cacache also performs integrity checks, if we could seek in the file this becomes much harder.

[zkat]

yeah I have no idea how that would work tbh

[rbtcollins]

I think you could read through to do the verification, then let the caller use seek to read the bits they need; the main point is to not have to buffer the entire file in RAM or make a temp copy of it, when using a non-streamable layer on top

[zkat]

Yeah. And we can have the unchecked and checked versions, in case you don't want to read the whole thing because, e.g. the file is enormous.

Ok.

I'll take a PR for this if anyone's interested, but it would need to be implemented for tokio, async-std, and sync. I don't want feature drift between the backends.