I had to dig a bit into the different QueryStream implementations and the only reason for !Send I could find was the builder function passed to the builder in the ouroboros context. The self_referencing macro generates a signature on the current release of ouroboros that allows !Send constructors:

https://github.com/joshua-maros/ouroboros/blob/main/ouroboros_macro/src/info_structures.rs#L265

I forked ouroboros and added the Send bound on the constructor future and replaced the crates.io dependency with one pointing at my branch. After fixing up the Send requirements on the TransactionStream, a few other places in this crate needed + Send bounds.

This PR is not meant to be merged, it's mostly for documentation / demonstration of what would be required to get streams working in tokio::spawn context. If you're interested in getting this PR into better shape, we'll have to get into touch with the ouroboros maintainers to figure out if the missing Send bound is intentional.

I just recognized that TransactionStream::build can use a synchronous builder since the builder closure doesn't await any futures. That means the change on ouroboros is not even required to enable Send streams.

The first commit here is a solution that keeps the async constructor, the second one just removes the unused future on build