FFI callback traits force non async

[xstoicunicornx]

Which crate is this feature request for (if any)?

payjoin-ffi

Question

Hey there,

As part of my BOSS 2026 project I am working on proving out the Javascript FFI bindings. My first step has been building out a plain node project that essentially recreates the payjoin-cli functionality just to get a feel for the bindings api (all this bindings stuff is totally foreign to me).

My question is in regards to the callback method within the CanBroadcast trait here:

rust-payjoin/payjoin-ffi/src/receive/mod.rs

Lines 631 to 634 in 6c3a3e4

	pub trait CanBroadcast: Send + Sync {
	fn callback(&self, tx: Vec<u8>) -> Result<bool, ForeignError>;
	}

When the bindings are generated in javascript it is forcing a non async function to be used as the callback. Is it realistic to expect this to be a non async function in javascript?

My current implementation was planning to call the bitcoin-cli testmempoolaccept to implement the callback method but I do not know of a non async way of doing this. Was there better recommended way of implementing callback?

Thanks!

[spacebear21]

Related discussion here: #816

tl;dr currently all callback accepting functions are sync-only. This is a known limitation that we get around in Rust by resolving the async calls inside a blocking runtime. I'm not sure if there's a similar workaround available in JS but would check if that's doable. In our JS integration tests we use an internal synchronous client wrapper.

A better, long-term solution would be the one proposed by Armin in that issue thread.

[xstoicunicornx]

thanks this is extremely informative!

feel free to close this as duplicate as I think the related discussion #816 comprehensively covers this question.

[spacebear21]

I'll keep this open because I think the FFI surface deserves its own discussion, thanks for bringing it up!